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{{short description|1979 nuclear accident in Pennsylvania}}
{{use mdy dates|date=March 2024}}{{Use American English|date=February 2025}}
{{Infobox news event
| image       = Three Mile Island (color)-2.jpg
| caption     = Three Mile Island nuclear facility, {{circa|1979}}
| date        = {{start date and age|1979|03|28|p=yes|br=yes}}
| time        = 04:00 ([[Eastern Time Zone]] UTC−5)
| place       = [[Londonderry Township, Dauphin County, Pennsylvania]], U.S.
|outcome = [[International Nuclear Event Scale|INES]] Level 5 (accident with wider consequences)
| notes       = {{designation list|embed=yes|designation1=Pennsylvania|designation1_date=March 25, 1999<ref name="PAHMDB">{{cite web |url=http://www.phmc.state.pa.us/apps/historical-markers.html |title=PHMC Historical Markers Search |website=Pennsylvania Historical and Museum Commission |access-date=January 25, 2014}}</ref>}}
}}
The '''Three Mile Island accident''' was a partial [[nuclear meltdown]] of the Unit 2 reactor (TMI-2) of the [[Three Mile Island Nuclear Generating Station]], located on the [[Susquehanna River]] in [[Londonderry Township, Dauphin County, Pennsylvania|Londonderry Township, Dauphin County]] near [[Harrisburg, Pennsylvania]]. The reactor accident began at 4:00&nbsp;a.m. on March&nbsp;28, 1979, and released [[Radioactive decay|radioactive]] gases and radioactive [[iodine-131|iodine]] into the environment.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. The problem was caused by a cooling problem and caused some of the core to melt in the 2nd reactor. Volume I |publisher=[[U.S. Nuclear Regulatory Commission]] |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |date=January 1980 |access-date=October 17, 2021 |page=3 |doi=10.2172/5395798 |osti=5395798 |ref=Rogovin1}}</ref><ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume II Part 2 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://tmi2kml.inl.gov/Documents/2b-Rogovin/NUREGCR-1250V2PT2,%20TMI,%20A%20Report%20To%20The%20Commissioners%20And%20To%20The%20Public%20(Rogovin%20Report)%20(1980-01).pdf |date=January 1980 |access-date=October 17, 2021 |page=309 |ref=Rogovin2-2}}</ref> It is the worst accident in U.S. commercial [[nuclear power plant]] history, although its small radioactive releases had no detectable health effects on plant workers or the public.<ref name="NRC Backgrounder">{{cite web |title=Backgrounder on the Three Mile Island Accident |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html |publisher=U.S. Nuclear Regulatory Commission |access-date=March 6, 2018}}</ref> On the seven-point [[Logarithmic scale|logarithmic]] [[International Nuclear Event Scale]], the TMI-2 reactor accident is rated Level{{nbs}}5, an "Accident with Wider Consequences".<ref>{{cite journal |last=Spiegelberg-Planer |first=Rejane |title=A Matter of Degree |url=https://www.iaea.org/sites/default/files/publications/magazines/bulletin/bull51-1/51102744649.pdf |quote=A revised International Nuclear and Radiological Event Scale (INES) extends its reach. |postscript=Three Mile Island is mentioned on page 47 |journal=IAEA Bulletin |volume=51 |issue=1 |page=46 |date=September 2009 |access-date=October 16, 2021 |publisher=Division of Public Information, International Atomic Energy Agency |location=Vienna, Austria}}</ref><ref>{{cite web |title=The International Nuclear and Radiological Event Scale |url=https://www.iaea.org/sites/default/files/ines.pdf |quote=Level 5: Accident with Wider Consequences; Three Mile Island, USA, 1979 – Severe damage to the reactor core. |website=INES |publisher=International Atomic Energy Agency |date=August 1, 2008 |access-date=October 16, 2021}}</ref>

The accident began with failures in the non-nuclear secondary system,<ref>{{cite web |title=Secondary system |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/secondary-system.html |quote=The steam generator tubes, steam turbine, condenser, and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation. Most commonly used in reference to pressurized water reactors. |publisher=U.S. Nuclear Regulatory Commission |date=March 9, 2021 |access-date=October 16, 2021}}</ref> followed by a stuck-open [[pilot-operated relief valve]] (PORV) in the primary system,<ref>{{cite web |title=Primary system |url=https://www.nrc.gov/reading-rm/basic-ref/students/for-educators/04.pdf |website=Reactor Concepts Manual, Pressurized Water Reactor |publisher=USNRC Technical Training Center |access-date=October 16, 2021 |page=4-3 |quote=The primary system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping that connects these components to the reactor vessel. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators. A second function is to contain any fission products that escape the fuel.}}</ref> which allowed large amounts of water to escape from the pressurized isolated coolant loop. The mechanical failures were compounded by the initial failure of [[plant operator]]s to recognize the situation as a [[loss-of-coolant accident]] (LOCA). TMI training and operating procedures left operators and management ill-prepared for the deteriorating situation caused by the LOCA. During the accident, those inadequacies were compounded by design flaws, such as poor control design, the use of multiple similar alarms, and a failure of the equipment to indicate either the coolant-inventory level or the position of the stuck-open PORV.<ref name="cacophony">{{cite book |last=Walker |first=J. Samuel |author-link=J. Samuel Walker |url=https://archive.org/details/threemileislandn00walk/page/211/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=[[University of California Press]] |isbn=0-520-23940-7 |location=Berkeley, California |pages=211 |language=en-us |quote=The commission concluded that Met Ed, GPU, [[Babcock & Wilcox]], and the NRC shared responsibility for the shortcomings in operator training. Those inadequacies were compounded by design flaws that undermined the efforts of the plant staff to deal with the accident. They included the cacophony of undifferentiated alarms, the inconvenient arrangement of instruments and controls, and the absence of clear indicators either of levels of water in the pressure vessel or of the position of the stuck-open PORV. |access-date=October 19, 2021}}</ref>

The accident heightened nuclear safety concerns among the general public and led to new regulations for the nuclear industry. It accelerated the decline of efforts to build new reactors.<ref>{{cite news |url=https://www.youtube.com/watch?v=uNeECUgNGk0 |archive-url=https://web.archive.org/web/20110415024111/http://www.youtube.com/watch?v=uNeECUgNGk0 |archive-date=April 15, 2011 |url-status=dead |title=Michael Levi on Nuclear Policy |newspaper=[[The Economist]] |date=March 31, 2011 |via=YouTube |access-date=April 6, 2011}}{{time needed|date=April 2019}}</ref> [[Anti-nuclear movement]] activists expressed worries about regional health effects from the accident.<ref name="Gofman John W. xvii">{{cite book |last1=Gofman |first1=John W. |url=http://www.ratical.org/radiation/CNR/PP/Foreward1979.html |title=Poisoned Power: The Case Against Nuclear Power Plants Before and After Three Mile Island |last2=Tamplin |first2=Arthur R. |date=1979 |publisher=Rodale Press |edition=Updated |location=Emmaus, Pennsylvania |page=xvii |language=en-us |quote=...we arrive at 333 fatal cancers or leukemias |access-date=October 1, 2013}}.</ref> Some [[epidemiological]] studies analyzing the [[List of countries by cancer rate|rate of cancer]] in and around the area since the accident did determine that there was a [[statistically significant]] increase in the rate of cancer, while other studies did not. Due to the nature of such studies, a [[causal]] connection linking the accident with cancer is difficult to prove.{{efn|Attributed to multiple sources: <ref name="hatch90">{{cite journal |last=Hatch |first=Maureen C. |display-authors=etal |title=Cancer near the Three Mile Island Nuclear Plant: Radiation Emissions |journal=American Journal of Epidemiology |volume=132 |pages=397–412 |issue=3 |year=1990 |pmid=2389745 |doi=10.1093/oxfordjournals.aje.a115673|doi-access=free }}</ref><ref name="ncbi.nlm.nih.gov">{{cite journal |last=Levin |first=R. J. |year=2008 |title=Incidence of thyroid cancer in residents surrounding the Three-Mile Island nuclear facility |journal=Laryngoscope |volume=118 |issue=4 |pages=618–628 |doi=10.1097/MLG.0b013e3181613ad2 |pmid=18300710 |s2cid=27337295 |quote=Thyroid cancer incidence has not increased in Dauphin County, the county in which TMI is located. York County demonstrated a trend toward increasing thyroid cancer incidence beginning in 1995, approximately 15 years after the TMI accident. Lancaster County showed a significant increase in thyroid cancer incidence beginning in 1990. These findings, however, do not provide a [[causal]] link to the TMI accident.}}</ref><ref name="Levin RJ; De Simone NF; Slotkin JF; Henson BL. 1238:2064–71">{{cite journal |last1=Levin |first1=R. J. |last2=De Simone |first2=N. F. |last3=Slotkin |first3=J. F. |last4=Henson |first4=B. L. |date=August 2013 |title=Incidence of thyroid cancer surrounding Three Mile Island nuclear facility: the 30-year follow-up |journal=Laryngoscope |volume=123 |issue=8 |pages=2064–2071 |doi=10.1002/lary.23953 |pmid=23371046 |s2cid=19495983}}</ref><ref name="Han YY; Youk AO; Sasser H; Talbott EO. 1230–5">{{cite journal |last1=Han |first1=Y. Y. |last2=Youk |first2=A. O. |last3=Sasser |first3=H. |last4=Talbott |first4=E. O. |date=November 2011 |title=Cancer incidence among residents of the Three Mile Island accident area: 1982–1995 |journal=Environ Res |volume=111 |issue=8 |pages=1230–1235 |bibcode=2011ER....111.1230H |doi=10.1016/j.envres.2011.08.005 |pmid=21855866}}</ref><ref name="Hatch MC, Wallenstein S, Beyea J, Nieves JW, Susser M 1991 719–724">{{cite journal |last1=Hatch |first1=M. C. |last2=Wallenstein |first2=S. |last3=Beyea |first3=J. |last4=Nieves |first4=J. W. |last5=Susser |first5=M. |date=June 1991 |title=Cancer rates after the Three Mile Island nuclear accident and proximity of residence to the plant |journal=American Journal of Public Health |volume=81 |issue=6 |pages=719–724 |doi=10.2105/AJPH.81.6.719 |pmc=1405170 |pmid=2029040}}</ref><ref>{{cite web |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html#effects |title=Backgrounder on the Three Mile Island Accident: Health Effects |publisher=U.S. Nuclear Regulatory Commission |access-date=January 13, 2018 |quote=The NRC conducted detailed studies of the accident's radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. Several independent groups also conducted studies. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X-ray is about 6 millirem and the area's natural radioactive background dose is about 100–125 millirem per year for the area. The accident's maximum dose to a person at the site boundary would have been less than 100 milligrams above the background. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. Comprehensive investigations and assessments by several well-respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.}}</ref><ref>{{cite journal |vauthors=Goldenberg, D, Russo, M, Houser, K, Crist, H, Derr JB, Walter V, Warrick JI, Sheldon KE, Broach J, Bann, DV |date=2017 |title=Altered molecular profile in thyroid cancers from patients affected by the Three Mile Island nuclear accident |journal=Laryngoscope |volume=127 supplement 3 |pages=S1–S9 |doi=10.1002/lary.26687 |pmid=28555940 |s2cid=40795419}} Findings were consistent with observations from other radiation-exposed populations. These data raise the possibility that radiation released from [Three Mile Island] may have altered the molecular profile of [thyroid cancer] in the population surrounding TMI.</ref>}} Cleanup at TMI-2 started in August 1979 and officially ended in December 1993, with a total cost of about $1&nbsp;billion (equivalent to ${{Inflation|US|1|1993}}&nbsp;billion in {{Inflation/year|US}}).<ref name="nytimes1993">{{cite news |url=https://www.nytimes.com/1993/08/15/us/14-year-cleanup-at-three-mile-island-concludes.html |title=14-Year Cleanup at Three Mile Island Concludes |newspaper=The New York Times |date=August 15, 1993 |access-date=March 28, 2011}}</ref> TMI-1 was restarted in 1985, then retired in 2019 due to operating losses. It is expected to go back into service in either 2027 or 2028 as part of a deal with [[Microsoft]] to power its data centers.<ref>{{cite news |url=https://www.bbc.com/news/articles/cx25v2d7zexo | title=Microsoft chooses infamous nuclear site for AI power | publisher=[[BBC News]] | first=Natalie | last=Sherman | date=September 20, 2024 | access-date=September 20, 2024 }}</ref><ref>{{Cite web |last=Lehman |first=Tom |date=2025-06-25 |title=Nuclear plant at TMI could reopen earlier than expected |url=https://www.wgal.com/article/pennsylvania-nuclear-plant-three-mile-island-could-open-2027/65192313 |access-date=2025-06-25 |website=WGAL |language=en}}</ref>

==Accident==
===Background===
[[File:Tmi-2 schematic.svg|thumb|upright=2.2|A simplified schematic diagram of the TMI-2 plant<ref name="FactSheet">{{cite web |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html |title=Fact Sheet on the Three Mile Island Accident |publisher=U.S. Nuclear Regulatory Commission |access-date=November 25, 2008}}</ref>]]
In the night hours before the incident, the TMI-2 reactor was running at 97% power while the companion TMI-1 reactor was shut down for refueling.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/71/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |page=71 |quote=TMI1 was not operating because it had been shut down for routine refueling. |access-date=October 18, 2021}}</ref> The main chain of events leading to the partial [[Nuclear meltdown|core meltdown]] on Wednesday, March 28, 1979, began at 4:00:36&nbsp;a.m. EST in TMI-2's secondary loop, one of the three main water/steam loops in a [[pressurized water reactor]].<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/72/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=72 |quote=The chain of events that set off the severe accident at TMI-2 and melted a substantial portion of its core began innocently enough at 4:00 A.M. |access-date=October 23, 2021}}</ref>

The initial cause of the accident happened 11 hours earlier, during an attempt by operators to fix a blockage in one of the eight [[condensate polisher]]s, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and other impurities in the water from accumulating in the steam generators and to decrease corrosion rates on the secondary side.

Blockages are common with these resin filters and are usually fixed easily, but in this case, the usual method of forcing the stuck resin out with compressed air did not succeed. The operators decided to blow compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument [[air line]]. This would eventually cause the [[feedwater pump]]s, condensate booster pumps, and condensate pumps to turn off around 4:00&nbsp;a.m., which would, in turn, cause a [[turbine trip]].

===Reactor overheating and malfunction of relief valve===
Given that the steam generators were no longer receiving feedwater, heat transfer from the reactor coolant system (RCS)<ref>{{cite web |title=Reactor coolant system |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/reactor-coolant-system.html |quote=The system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine. |website=U.S. NRC Glossary |publisher=U.S. Nuclear Regulatory Commission |date=March 9, 2021 |access-date=October 20, 2021}}</ref> was greatly reduced, and RCS temperature rose. The rapidly-heating coolant expanded and surged into the pressurizer,<ref>{{cite web |title=Pressurizer |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/pressurizer.html |quote=A tank or vessel that acts as a head tank (or surge volume) to control the pressure in a pressurized water reactor. |website=U.S. NRC Glossary |publisher=U.S. Nuclear Regulatory Commission |date=March 9, 2021 |access-date=October 21, 2021}}</ref><ref>{{cite book |last1=Kerlin |first1=Thomas W. |last2=Upadhyaya |first2=Belle R. |title=Dynamics and Control of Nuclear Reactors |date=October 2019 |publisher=Academic Press |location=London / San Diego, CA / Cambridge, MA / Oxford |isbn=9780128152614 |page=116 |edition=1 |url=https://www.sciencedirect.com/topics/engineering/pressuriser |access-date=October 27, 2021 |quote=A PWR pressurizer is a vessel with liquid water in the bottom section and saturated steam in the top section. A pressurizer is used to regulate the primary coolant pressure (≈ 150 bars) in PWRs and CANDU reactors. The pressurizer is connected to one of the hot leg pipings with a long surge line... Because of the contact between steam and liquid water, the water is also at the saturation temperature at steady state. Spray of cooler water enters from the top and electrical heaters at the bottom heat the liquid water. The steady state can be disturbed by water inflow or outflow, changes in inlet water temperature, changes in spray flow or changes in heater power. A PWR pressurizer control system can alter the pressure by modulating heater power and/or spray flow.}}</ref><ref>{{cite book |last1=Kerlin |first1=Thomas W. |url=https://www.sciencedirect.com/topics/engineering/pressuriser |title=Dynamics and Control of Nuclear Reactors |last2=Upadhyaya |first2=Belle R. |date=October 2019 |publisher=Academic Press |isbn=9780128152614 |edition=1 |location=London, UK; San Diego, California; Cambridge, Massachusetts; Oxford, UK |page=141 |language=en |quote=The purpose of the pressurizer is to control the pressure in the primary loop at a nominal coolant pressure of 2250 lb./in<sup>2</sup> (≈ 153 bars). The primary pressure is regulated by modulating heater power and spray flow from a cold leg… The water in the pressurizer is the only free surface in the primary coolant system. At full power the pressurizer contains 60% of its volume full of water. Changes in pressurizer water level are usually the result of water density changes caused by changes in average coolant temperature… A system called the… Makeup and Purification System controls the water level in the pressurizer… Water is injected into the primary coolant system to increase the pressurizer water level to the set point. A let down flow system decreases the water level… (other functions) are water purification using filters and demineralizers and controlling soluble poison concentration by adding or removing boric acid. |access-date=October 27, 2021}}</ref> compressing the steam bubble at the top. When RCS pressure rose to {{convert|2255|psi|bar|abbr=on}}, the [[pilot-operated relief valve]] (PORV) opened, relieving steam through piping to the reactor coolant drain tank<ref>{{cite web |date=October 2009 |title=Reactor Coolant System, Piping and Pressurizer |url=https://www.nrc.gov/docs/ML1122/ML11221A106.pdf |access-date=November 1, 2021 |website=Pressurized Water B&W Technology Crosstraining Course Manual |publisher=U.S. Nuclear Regulatory Commission, Human Resources Training and Development (HRTD) |pages=2.2-7, 2.2-8 |quote=The Reactor Coolant Drain Tank (RCDT) is designed to condense and cool the steam effluent from the pressurizer safety and relief valves if they should ever be actuated. The tank also serves as a collection point for the liquid waste disposal system... Steam discharged from the code safety valves and relief valves enters the tank through sparger nozzles and is condensed by water contained in the tank. Should the safety valves lift, 1,400,000 lb/hr of saturated steam at 490 psig would be discharged into the manifold of the tank. The steam flow in the tank is assumed to last 15 seconds. Peak pressure and temperature in the tank (outside the sparger manifold) would occur at the end of the steam blowdown and would be 30 psig and 200°F. Overpressure protection for the RCDT is provided by a relief valve with a setpoint of 90 psig and a rupture disc with a 100-psig setting.}}</ref> in the containment building basement. RCS pressure continued to rise, reaching the [[reactor protection system]] high-pressure trip [[Setpoint (control system)|setpoint]] of {{convert|2355|psi|bar|abbr=on}} eight seconds after the turbine trip. The reactor automatically [[Scram|tripped]], its [[control rods]] falling into the [[Nuclear reactor core|core]] under gravity, halting the [[nuclear chain reaction]] and stopping the heat generated by fission. However, the reactor continued to generate [[decay heat]], initially equivalent to approximately 6% of the pre-trip power level. Because steam was no longer being used by the turbine and feed was not being supplied to the steam generators, heat removal from the reactor's primary water loop was limited to steaming the small amount of water remaining in the secondary side of the steam generators to the condenser using turbine bypass valves.<ref>{{cite web |title=Turbine bypass System |url=https://www.nuclear-power.com/nuclear-power-plant/turbine-generator-power-conversion-system/turbine-bypass-system-turbine-steam-dump-system/ |website=Nuclear Power Com |publisher=Nuclear Power for Everybody |access-date=October 21, 2021 |quote=The function of the turbine bypass system is to remove excess energy from the reactor coolant system by discharging a stated percentage of rated main steam flow directly to the main condensers, i.e. by bypassing the turbine. This heat is rejected to the condenser through the steam dump valves.}}</ref><ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/73 |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=73 |quote=The closing of the secondary system caused heat and pressure to rise rapidly in the primary system, largely because the steam generators could no longer remove heat from the water that had come from the core. As a result, eight seconds after the polisher pumps tripped, the reactor scrammed automatically. The control rods entered the core and terminated the production of heat from nuclear fission. But the problem of dealing with decay heat remained... |access-date=October 18, 2021}}</ref><ref>{{cite book |last=Kemeny |first=John J. |url=https://www.hsdl.org/?view&did=769775 |title=Report of the Commission on The Accident at Three Mile Island—The Need For Change: The Legacy of TMI |date=October 1979 |publisher=[[U.S. Government Printing Office]] |isbn=978-1297534478 |location=Washington, D.C. |page=90 |language=en-us |chapter=Account of the Accident |quote=When the feedwater flow stopped, the temperature of the reactor coolant increased. The rapidly heating water expanded. The pressurizer level (the level of the water inside the pressurizer tank) rose and the steam in the top of the tank compressed. Pressure inside the pressurizer built to 2,255 pounds per square inch, 100 psi more than normal. Then a valve atop the pressurizer, called a pilot-operated relief valve, or PORV, opened – as it was designed to do – and steam and water began flowing out of the reactor coolant system through a drain pipe to a tank on the floor of the containment building. Pressure continued to rise, however, and 8 seconds after the first pump tripped, TMI-2's reactor – as it was designed to do – scrammed: its control rods automatically dropped down into the reactor core to halt its nuclear fission. |access-date=October 20, 2021}}</ref>

When the feedwater pumps tripped, three emergency feedwater pumps started automatically. An operator noted that the pumps were running but did not notice that a block valve was closed in each of the two emergency feedwater lines, blocking emergency feed flow to both steam generators. The valve position lights for one block valve were covered by a yellow maintenance tag. The reason why the operator missed the lights for the second valve is not known, although one theory is that his own large belly hid it from his view.<ref>{{cite news |last1=Omang |first1=Joanne |last2=Reid |first2=R. R. |date=May 18, 1979 |title=Nuclear Plant Operators Misread Data in Accident |url=https://www.washingtonpost.com/archive/politics/1979/05/18/nuclear-plant-operators-misread-data-in-accident/ac5da240-33d8-4f2c-9068-73dee5b12694/ |newspaper=[[The Washington Post]] |location=Washington, D.C. |issn=0190-8286 |oclc=1330888409}}</ref> The valves may have been left closed during a surveillance test two days earlier.<ref>{{cite book |last=Kemeny |first=John J |title=Report of the Commission on The Accident at Three Mile Island—The Need For Change: The Legacy of TMI |date=October 1979 |publisher=U.S. Government Printing Office |location=Washington, D.C. |isbn=978-1297534478 |pages=46, 47 |url=https://www.hsdl.org/?view&did=769775 |access-date=October 20, 2021 |chapter=Commission Findings |quote=(v) A 1978 revision in the TMI-2 surveillance procedure for the emergency feedwater block valves violated TMI-2's technical specifications, but no one realized it at the time. The approval of the revision in the surveillance procedure was not done according to Met Ed's own administrative procedures. <vi/>Performance of surveillance tests was not adequately verified to be sure that the procedures were followed correctly. On the day of the accident, emergency feedwater block valves which should have been open were closed. They may have been left closed during a surveillance test 2 days earlier.}}</ref><ref>{{cite book |last=Kemeny |first=John J. |url=https://www.hsdl.org/?view&did=769775 |title=Report of the Commission on The Accident at Three Mile Island—The Need For Change: The Legacy of TMI |date=October 1979 |publisher=U.S. Government Printing Office |isbn=978-1297534478 |location=Washington, D.C. |pages=90, 91 |language=en-us |chapter=Account of the Accident |quote= |access-date=October 20, 2021}}</ref> With the block valves closed, the system was unable to pump water. The closure of these valves was a violation of a key [[Nuclear Regulatory Commission]] (NRC) rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure.<ref name=postch1>{{cite news |title=A Pump Failure and Claxon Alert |url=https://www.washingtonpost.com/wp-srv/national/longterm/tmi/stories/ch1.htm |quote=Apparently the valves were closed for routine maintenance, in violation of one of the most stringent rules that the Nuclear Regulatory Commission has. The rule states simply that auxiliary feed pumps can never all be down for maintenance while the reactor is running. |newspaper=[[The Washington Post]] |year=1979 |access-date=September 4, 2016}}</ref>

After the reactor tripped, secondary system steam valves operated to reduce steam generator temperature and pressure, cooling the RCS and lowering RCS temperature, as designed, resulting in a [[Thermal expansion|contraction]] of the primary coolant. With the coolant contraction and loss of coolant through the open PORV, RCS pressure dropped as did pressurizer level after peaking 15 seconds after the turbine trip. Also, 15 seconds after the turbine trip, coolant pressure had dropped to {{convert|2205|psi|bar|abbr=on}}, the reset setpoint for the PORV. Electric power to the PORV's solenoid was automatically cut, but the relief valve was stuck open with coolant water continuing to be released.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/73 |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=73–74 |quote=At TMI-2, the PORV opened three seconds after the condensate pumps tripped, exactly as designed. Unfortunately, ten seconds later, after the temperature and pressure in the primary system had diminished, it failed to close as designed. The open relief valve allowed growing quantities of reactor coolant to escape. This was not the first time that the PORV had stuck open at TMI-2, and it was a chronic problem at Babcock & Wilcox plants. The same sequence of events had occurred at Davis–Besse in 1977. In that case, an operator recognized that the valve was open and immediately blocked it. |access-date=October 18, 2021}}</ref>

In post-accident investigations, the indication for the PORV was one of many design flaws identified in the operators' [[User interface|controls, instruments and alarms]].<ref name=cacophony /> There was no direct indication of the valve's actual position. A light on a control panel, installed after the PORV had stuck open during startup testing,<ref>{{cite book |last=Kemeny |first=John J. |url=https://www.hsdl.org/?view&did=769775 |title=Report of the Commission on The Accident at Three Mile Island—The Need For Change: The Legacy of TMI |date=October 1979 |publisher=U.S. Government Printing Office |isbn=978-1297534478 |location=Washington, D.C. |page=44 |language=en-us |chapter=Commission Findings |quote=After an incident at TMI-2 a year earlier during which the PORV stuck open, an indicator light was installed in the control room. That light showed only that a signal had been sent to close the valve—it did not show whether the valve was actually closed—and this contributed to the confusion during the accident. |access-date=October 19, 2021}}</ref> came on when the PORV opened.<ref>{{cite book |last=Norman |first=Donald |title=The Design of Everyday Things |date=1988 |publisher=Basic Books |location=New York |isbn=978-0-465-06710-7 |pages=43–44 |url=https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxkZHN1bmxhbXxneDplZGRkMGFiODdhNmJiY2I |access-date=October 18, 2021}}</ref> When that light—labeled ''Light on – RC-RV2 open''<ref>{{cite report |url=https://www.osti.gov/servlets/purl/5603680 |title=Human Factors Evaluation of Control Room Design and Operator Performance at Three Mile Island – 2 (NUREG/CR-1270) |last1=Malone |first1=T. B. |last2=Kirkpatrick |first2=M. |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |pages=12, 13 |quote=Invalid Information. The PORV status indicator is a single red light located on Panel 4. The light is designed to come on when an electrical signal is transmitted to the PORV to open, and go out when a signal is transmitted for the valve to close. As indicated in Figure 3 the light is labeled "Light on – RC-R V2 open." This design is a violation of basic HFE principles as referenced by the following provision of MIL-STD-1472B, paragraph 5.2.2.1.4-. "The absence or extinguishment of a signal or visual indication shall not be used to denote a 'go-ahead,' 'ready,' 'in-tolerance,' or completion condition ...Changes in display status shall signify changes in functional status rather than results of control actuation alone." |last3=Mallory |first3=K. |last4=Eike |first4=D. |last5=Johnson |first5=J. H. |last6=Walker |first6=R. W. |access-date=October 21, 2021 |others=The Essex Corporation}}</ref>—went out, the operators believed that the valve was closed. In fact, the light when on only indicated that the PORV pilot valve's solenoid was powered, not the actual status of the PORV.<ref>{{cite web |title=Pressurized Water Reactor B&W Technology Crosstraining Course Manual |url=https://www.nrc.gov/docs/ML1122/ML11221A325.pdf |website=USNRC HRDT 18-2 |publisher=U.S. Nuclear Regulatory Commission |access-date=October 20, 2021 |page=18-3 |date=July 2011 |quote=An indicator light in the control room shows when the PORV has been ordered to close—that is, when power to the valve opening solenoid is cut off—but does not show when the valve actually closes. It is now known that the valve did not, if fact, close as it was designed to do. The operators, however, had no direct means of knowing this.}}</ref> While the main relief valve was stuck open, the operators believed the unlighted lamp meant the valve was shut. As a result, they did not correctly diagnose the problem for several hours.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 17, 2021 |pages=14–15 |doi=10.2172/5395798 |osti=5395798 |quote=An indicator light has been installed in the control room, connecting to the opening-and-closing mechanism in the relief valve: when electric power is passed to the solenoid, permitting the valve to open, the light goes on; when diminished pressure in the pressurizer cuts off power to the solenoid, permitting the valve to shut, the light goes off. Unfortunately, the light is proof only that power is reaching the valve-opening mechanism; it is only circumstantial evidence of the actual state of the valve itself, i.e., power has been cut off from the solenoid now, and the light shows it—but the valve remains open. It is ironic, in a day that will be marked by repeated refusal of the operators and supervisors to believe ominous readings from the reactor-monitoring instruments, that they elect to be misled by this bearer of what they perceived to be good tidings.}}</ref>

The operators had not been trained to understand the ambiguous nature of the PORV indicator and to look for alternative confirmation that the main relief valve was closed. A downstream temperature indicator, the sensor for which was located in the tail pipe between the pilot-operated relief valve and the pressurizer relief tank, could have hinted at a stuck valve had operators noticed its higher-than-normal reading. It was not, however, part of the "safety grade" suite of indicators designed to be used after an incident, and personnel had not been trained to use it. Its location behind the seven-foot-high instrument panel also meant that it was effectively out of sight.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/74 |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California; London, England |page=[https://archive.org/details/threemileislandn00walk/page/74 74] |url-access=registration}}</ref>

===Depressurization of primary reactor cooling system===
Less than a minute after the beginning of the event, the water level in the pressurizer began to rise, even though RCS pressure was falling. With the PORV stuck open, coolant was being lost from the RCS, a [[loss-of-coolant accident]] (LOCA). Expected symptoms for a LOCA were drops in both RCS pressure and pressurizer level. The operators' training and plant procedures did not cover a situation where the two parameters went in opposite directions. The water level in the pressurizer was rising because the steam in the space at the top of the pressurizer was being vented through the stuck-open PORV, lowering the pressure in the pressurizer because of the lost inventory. The lowering of pressure in the pressurizer made water from the coolant loop surge in and created a steam bubble in the reactor pressure vessel head, aided by the decay heat from the fuel.<ref>Kemeny, p. 94.</ref> 

This steam bubble was invisible for the operators, and this mechanism had not been trained. Indications of high water levels in the pressurizer contributed to confusion, as operators were concerned about the primary loop "going solid", (i.e., no steam pocket buffer existing in the pressurizer) which in training they had been instructed to never allow. This confusion was a key contributor to the initial failure to recognize the accident as a LOCA<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |page=16 |doi=10.2172/5395798 |osti=5395798 |quote=A more important factor contributing to the operators' failure to recognize that a LOCA is in progress is the pressurizer water level indicator. Their training on this particular equipment has taught the operators that the only credible check on the amount of coolant in the system is the indicator showing water level in the pressurizer. (In this Babcock & Wilcox reactor, there is no instrument for measuring, as a gas gauge does in an automobile, the amount of fluid in the reactor core portion of the coolant loop—or, stated more simply, the depth of water around the fuel rods.) If the pressurizer level remains high, the operators are not trained to anticipate that coolant water may be leaking out of the primary system. Indeed, the operator training at Met Ed, at B&W, even back in the navy, tells these men that the condition to avoid at all costs is 'going solid'—permitting the pressurizer to fill with water and thus losing the ability to regulate system pressure through the control of the pressurizer steam bubble. The training and the written emergency procedures of the operators never postulated a loss-of-coolant accident through the top of the pressurizer itself, as is happening now. With the relief valve stuck open, the steam bubble vanishes like a jinni out through the valve, and the coolant water right after it. The system pressure continues to be low—a sign of a loss-of-coolant accident. But the pressurizer water level indicator keeps getting higher. Why is this?}}</ref> and led operators to turn off the emergency core cooling pumps, which had automatically started after the PORV stuck and core coolant loss began, due to fears the system was being overfilled.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |pages=16, 17 |doi=10.2172/5395798 |osti=5395798 |quote=Going by the book as it was taught to them, however, the operators continue to read the pressurizer indicator in the old mode: The coolant level is rising; the system is going solid, for heaven's sake. Convinced by this logic that the system, indeed, is overloaded with coolant water, the operators override the emergency system and sharply reduce flow from the HPI pumps. It is a human intervention in the automatic chain of events not inconsistent with the operators' training, but it will have awesome consequences. At Zewe's direction, Operator Ed Frederick shuts down one HPI pump and throttles back the other one from a maximum of 400 gallons per minute (gpm) to about half that flow. Not only does he throttle HPI, Frederick also lifts the plug at the bottom of the reactor coolant system to maximize "letdown" through the normal "makeup and letdown system" that, like a swimming pool filtration system, constantly works to purify primary reactor coolant water. The effect of these two actions is to reduce to a trickle the amount of water being added to the system. This miserly flow rate, perhaps 25 gpm, will continue for the better part of the next 3 hours and is more than offset by the amount of coolant lost every minute through the stuck-open PORV.}}</ref><ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/76/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=76–77 |quote= |access-date=October 24, 2021}}</ref><ref name=upi>{{cite news |url=https://www.upi.com/Top_News/US/2019/03/28/Three-Mile-Island-clings-to-survival-40-years-after-1979-meltdown/1121553642219/?ls=1 |title=TMI clings to survival 40 years after 1979 meltdown |work=[[United Press International]] |date=March 28, 2019 |access-date=March 16, 2022}}</ref>

With the PORV still open, the pressurizer relief tank that collected the discharge from the PORV overfilled, causing the containment building [[Nuclear power plant|sump]] to fill and sound an alarm at 4:11&nbsp;a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing LOCA, but these indications were initially ignored by operators.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |pages=17, 18 |doi=10.2172/5395798 |osti=5395798 |quote=(1) (Operators)... will dismiss two warnings from a temperature instrument showing relief valve discharge temperatures about 100 degrees above normal range. (These are incorrectly reported to Zewe to be about 50 degrees lower than they really are.) He attributes the discrepancy to the fact that the PORV had been leaking anyway, and to residual heat from the early discharge of steam from the PORV when it opened (supposedly) for just a few seconds. (2) At 4:14{{nbs}}a.m., with the accident sequence barely settling in, there are other conspicuous clues that the relief valve is still open. Continued discharge of coolant into the reactor coolant drain tank from the stuck-open relief valve causes the tank's pressure to increase. (3) When the pressure reaches 192 psi, the rupture disc at the top of the tank bursts. Zewe takes note of this around 4:20... With the rupture disc open, coolant from the stuck-open valve running into the reactor coolant drain tank overflows the tank onto the reactor containment building floor. (4) At 4:38{{nbs}}a.m. an auxiliary building operator reports that the automatic containment sump (floor drain) pumps are pumping this water into the next-door auxiliary building... (5) By 5:00{{nbs}}a.m., the temperature inside the containment building is up from 120°F to 170°F, and building pressure has increased from 0 to 2.5 psi—still another sign that the PORV is stuck open.}}</ref><ref name="Kemeny, p. 96">Kemeny, p. 96.</ref> At 4:15&nbsp;a.m., the relief diaphragm of the pressurizer relief tank ruptured, and radioactive coolant began to leak into the general [[containment building]]. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the [[sump pump]]s were stopped at 4:39&nbsp;a.m.<ref name="Kemeny, p. 96"/>

===Partial meltdown and further release of radioactive substances===
At about 5:20{{nbsp}}a.m., after almost 80&nbsp;minutes with a growing steam bubble in the reactor pressure vessel head, the primary loop's four main reactor coolant pumps began to [[cavitation|cavitate]] as a steam bubble/water mixture, rather than water, passed through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, and as the water stopped circulating it was converted to steam in increasing amounts. Soon after 6:00{{nbsp}}a.m., the top of the reactor core was exposed, and the intense heat caused a reaction to occur between the steam forming in the reactor core and the [[Zirconium alloys|zircaloy]] nuclear [[Nuclear fuel|fuel rod]] cladding, yielding [[zirconium dioxide]], [[hydrogen]], and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.<ref>Kemeny, p. 99.</ref>

[[File:Graphic TMI-2 Core End-State Configuration.png|thumb|upright=1.4|A NRC graphic of the TMI-2 core end-state configuration.
{{Ordered list | 2B inlet | 1A inlet | cavity | loose core debris | crust | previously molten material | lower plenum debris | possible region depleted in uranium
| ablated incore instrument guide | hole in baffle plate
| coating of previously molten material on bypass region interior surfaces
| upper grid damage }}  ]]

At 6:00&nbsp;a.m. there was a shift change in the control room. A new arrival noticed that the temperatures in the PORV tail pipe and the holding tanks were excessive, and used a backup—called a block valve—to shut off the coolant venting via the PORV, but around {{convert|32000|USgal|L|abbr=on}} of coolant had already leaked from the primary loop.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |page=19 |doi=10.2172/5395798 |osti=5395798 |quote=... checks over the reactor coolant instruments and quickly concludes there is a steam bubble in the "hot legs"—the pipes leading from the reactor to the steam generators—of the reactor coolant loop. With the coolant system pressure so low, there must be a bubble somewhere else, expanding and forcing water into the pressurizer. "I went to the computer," he will later testify, "and punched out the temperatures on both the [safety] valves and the electromatic (''another term for the PORV'') relief valves." Based on readings showing the relief valve discharge line some 30 degrees hotter than the safety valve discharge lines, Mehler dismisses the pressurizer level reading and moves to a fresh conclusion: The PORV is leaking. Mehler orders the PORV block valve closed... has arrived at exactly the right decision just 20 minutes after coming on the scene fresh from the outside.}}</ref><ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/78/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=78 |quote=Finally..., a shift supervisor who had just arrived at the plant..., concluded from the pressure and temperature readings in the primary loop that the PORV was at least partially open. He was not certain of what was happening in the core, but he reasoned that no harm and perhaps some benefit could be achieved by shutting the offending relief valve. At 6:22{{nbs}}a.m., he ordered that a backup for the PORV, called a block valve, be closed. By that time, about thirty-two thousand gallons of coolant, more than one-third of the volume in the primary system, had flowed out of the stuck-open PORV. None of the staff in the control room took action to determine how long the PORV had been open or to replace the coolant that had escaped. Closing the block valve was a sound decision but insufficient in itself to prevent the severe damage to the core that leaving the PORV open for about two hours and twenty minutes had caused. |access-date=October 24, 2021}}</ref> It was not until 6:45{{nbs}}a.m., 165 minutes after the start of the problem, that radiation alarms activated when the contaminated water reached detectors; by that time, the radiation levels in the primary coolant water were around 300 times expected levels, and the general containment building was seriously contaminated with radiation levels of 800&nbsp;[[rem (unit)|rem]]/[[hour|h]].

==Emergency declaration and immediate aftermath==
At 6:56{{nbs}}a.m. a plant supervisor declared a [[Nuclear safety in the United States#Emergency Classifications|site area emergency]], and less than 30 minutes later station manager Gary Miller announced a general emergency.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/79/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=79 |quote=At 6:56 A.M., one of the plant supervisors—accounts vary on which one—declared a site emergency. The emergency plan for TMI-2 directed that this action be taken if radiation alarms sounded in more than one area. A site emergency meant that there was a possibility of an "uncontrolled release of radioactivity" within the plant's boundaries. It required evacuation of the affected buildings, closure of the gates leading to the plant, and notification of the NRC and the state of Pennsylvania. Less than half an hour later..., the Three Mile Island station manager, declared a general emergency... New and alarmingly high radiation readings in the containment building persuaded him to announce a general emergency, which was defined as having the "potential for serious radiological consequences to the health and safety of the general public." |access-date=October 24, 2021}}</ref> [[FirstEnergy|Metropolitan Edison]] (Met Ed) notified the [[Pennsylvania Emergency Management Agency]], which in turn contacted state and local agencies, Pennsylvania [[Governor of Pennsylvania|Governor]] [[Richard L. Thornburgh]] and [[Lieutenant Governor of Pennsylvania|Lieutenant Governor]] [[William Scranton III]], to whom Thornburgh assigned responsibility for collecting and reporting on information about the accident.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/80/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=80, 81 |quote=Thornburgh assigned responsibility for collecting and reporting information about the accident to Scranton because of his confidence in the ability and integrity of the lieutenant governor. |access-date=October 24, 2021}}</ref> The uncertainty of operators at the plant was reflected in fragmentary, ambiguous, or contradictory statements made by Met Ed to government agencies and to the press, particularly about the possibility and severity of off-site radioactivity releases.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/80/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=80–84 |quote= |access-date=October 24, 2021}}</ref>

Scranton held a press conference in which he was reassuring, yet confused, about this possibility, stating that though there had been a "small release of radiation...no increase in normal radiation levels" had been detected. These were contradicted by another official, and by statements from Met Ed, who both claimed that no radioactivity had been released.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/80/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=80–84 |language=en-us |quote= |access-date=October 24, 2021}}</ref> Readings from instruments at the plant and off-site detectors had detected radioactivity releases, albeit at levels that were unlikely to threaten public health as long as they were temporary, and providing that containment of the then highly contaminated reactor was maintained.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/84/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=84–86 |language=en-us |quote=The levels measured outside TMI-2 on March 28 were, by any standard, very small and unlikely to threaten public health—as long as they did not occur continuously over an extended period. Met Ed had twenty instruments for measuring environmental radioactivity in locations surrounding the site. Although one stack monitor that was calibrated to measure very low levels of radiation went off-scale early in the accident, readings from other instruments provided reasonably reliable information about releases from the plant. The highest reading was 7 millirems per hour; most measurements were in the range of 1 millirem per hour or less. Those values were far below the amount of radiation normally present in the environment. |access-date=October 24, 2021}}</ref>

Angry that Met Ed had not informed them before conducting a steam venting from the plant, and convinced that the company was downplaying the severity of the accident, state officials turned to the NRC.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/87/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=87 |language=en-us |quote=...Scranton held his second news conference of the day... "At this point, we believe there is still no danger to public health." Scranton's statements left no doubt that the state had written off the utility as a partner in responding to the accident. The state hoped that it would realize greater benefits in its efforts to guard public health by collaborating with the NRC. As Critchlow recalled, "I think we just almost instinctively preferred to deal with NRC people." |access-date=October 24, 2021}}</ref> After receiving word of the accident from Met Ed, the NRC had activated its emergency response headquarters in [[Bethesda, Maryland]], and sent staff members to Three Mile Island. NRC chairman [[Joseph Hendrie]] and commissioner Victor Gilinsky<ref>{{cite web |title=Victor Gilinsky |url=https://www.nrc.gov/about-nrc/organization/commission/former-commissioners/gilinsky.html |publisher=U.S. Nuclear Regulatory Commission |access-date=March 6, 2018}}</ref> initially viewed the accident as a "cause for concern but not alarm".<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/89/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=89 |language=en-us |quote=Like [[Joseph Hendrie|Hendrie]], Gilinsky regarded it as cause for concern but not alarm. He tried to keep his previously scheduled appointments to avoid the appearance of a crisis and, in his capacity as acting chairman, he spent much of the day responding to telephone queries from members of Congress and reporters. |access-date=October 24, 2021}}</ref> 

Gilinsky briefed reporters and members of Congress on the situation and informed White House staff, and at 10:00&nbsp;a.m. met with two other commissioners. However, the NRC faced the same problems in obtaining accurate information as the state and was further hampered by being organizationally ill-prepared to deal with emergencies, as it lacked a clear [[command structure]] and did not have the authority either to tell the utility what to do or to order an evacuation of the local area.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/91/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=90, 91 |language=en-us |quote=By tradition, structure, and statutory mandate, the agency was ill prepared and ill equipped to deal with an emergency at a nuclear plant. It was not an operational agency that ran plants or carried out emergency procedures but a deliberative agency that made rules, considered license applications, and conducted inspections. It had no authority to tell the utility what it should do to stabilize the reactor, no capability for operating the plant, and no power to order an evacuation of the surrounding area. Its role was largely limited to collecting information and making recommendations. The NRC lacked a command structure for dealing with a major accident. |access-date=October 24, 2021}}</ref>

In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point".<ref name=Gilinsky2009>{{cite journal |last=Gilinsky |first=Victor |title=Behind the scenes of Three Mile Island |journal=[[Bulletin of the Atomic Scientists]] |date=March 23, 2009 |url=http://thebulletin.org/web-edition/features/behind-the-scenes-of-three-mile-island |access-date=March 31, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090815015736/http://thebulletin.org/web-edition/features/behind-the-scenes-of-three-mile-island |archive-date=August 15, 2009}}</ref> He further wrote: "We didn't learn for years—until the reactor vessel was physically opened—that by the time the plant operator called the NRC at about 8:00&nbsp;a.m., roughly half of the [[Nuclear fuel|uranium fuel]] had already melted."<ref name="Gilinsky2009"/>

It was still not clear to the control room staff that the primary loop water levels were low and that over half of the core was exposed. A group of workers took manual readings from the [[thermocouple]]s and obtained a sample of primary loop water. Seven hours into the emergency, new water was pumped into the primary loop and the backup relief valve was opened to reduce pressure so that the loop could be filled with water. After 16&nbsp;hours, the primary loop pumps were turned on once again, and the core temperature began to fall. A large part of the core had melted, and the system was dangerously radioactive.{{Citation needed|date=March 2018}}

On the day following the accident, March 29, control room operators needed to ensure the integrity of the reactor vessel. In order to do this, someone needed to draw a [[boron]] concentration sample in order to ensure there was enough of it in the primary system to shut down the reactor entirely. Unit 2's chemistry supervisor, Edward "Ed" Houser, volunteered to draw the sample after his co-workers were hesitant. Shift supervisor Richard Dubiel asked Pete Velez, the [[radiation protection]] foreman for Unit 2, to join Houser. Velez would monitor airborne radiation levels and ensure that no [[Radiation exposure|overexposure]] would occur for either of them.{{efn|Attributed to multiple sources: <ref>{{Cite book |last=Gray |first=Mike |title=The warning : accident at Three Mile Island |date=2003 |publisher=Norton & Co |others=Ira Rosen |isbn=0-393-32469-9 |location=New York |oclc=52158956}}</ref><ref>{{Cite news |last=Richman |first=Alan |date=1979-04-04 |title=Radiation Overdose: All in a Day's Work |url=https://www.nytimes.com/1979/04/04/archives/radiation-overdose-all-in-a-days-work-feels-a-little-tired.html |access-date=2023-01-26 |work=The New York Times |language=en-US |issn=0362-4331}}</ref><ref>{{Cite web |last=Yuhas |first=Gregory |date=July 4, 1979 |title=Transcript of TMI-2 investigation with EW Houser |url=https://www.nrc.gov/docs/ML1924/ML19249B027.pdf |website=[[Nuclear Regulatory Commission]]}}</ref><ref>{{Cite web |last=Donaldson |first=Dale |date=July 6, 1979 |title=Transcript of TMI investigation interview of P Velez, Met Ed |url=https://www.nrc.gov/docs/ML1920/ML19208B107.pdf |website=[[Nuclear Regulatory Commission]]}}</ref>}}

Wearing excessive amounts of protective clothing—three pairs of gloves, one pair of rubber boots and a [[respirator]]—the two navigated the reactor auxiliary building to draw the sample. However, Houser had lost his [[Film badge dosimeter|pocket dosimeter]] while taking measurements. Houser had noted the sample he drew looked "like [[Alka-Seltzer]]" and was highly radioactive, with readings as high as 1,000&nbsp;rem/h. The two spent five minutes in the building, then withdrew. Houser had gone past the NRC's quarterly dose limit for radiation exposure (3 rem/qtr in 1979) by one and was only admitted back to work the following quarter.{{efn|Attributed to multiple sources: <ref>{{Cite book |last=Gray |first=Mike |title=The warning : accident at Three Mile Island |date=2003 |publisher=Norton & Co |others=Ira Rosen |isbn=0-393-32469-9 |location=New York |oclc=52158956}}</ref><ref>{{Cite news |last=Richman |first=Alan |date=1979-04-04 |title=Radiation Overdose: All in a Day's Work |url=https://www.nytimes.com/1979/04/04/archives/radiation-overdose-all-in-a-days-work-feels-a-little-tired.html |access-date=2023-01-26 |work=The New York Times |language=en-US |issn=0362-4331}}</ref><ref>{{Cite web |last=Yuhas |first=Gregory |date=July 4, 1979 |title=Transcript of TMI-2 investigation with EW Houser |url=https://www.nrc.gov/docs/ML1924/ML19249B027.pdf |website=[[Nuclear Regulatory Commission]]}}</ref><ref>{{Cite web |last=Donaldson |first=Dale |date=July 6, 1979 |title=Transcript of TMI investigation interview of P Velez, Met Ed |url=https://www.nrc.gov/docs/ML1920/ML19208B107.pdf |website=[[Nuclear Regulatory Commission]]}}</ref>}}

On the third day following the accident, a hydrogen bubble was discovered in the dome{{Clarify|date=April 2022}} of the pressure vessel and became the focus of concern. A hydrogen explosion could breach the pressure vessel and, depending on its magnitude, might compromise the integrity of the containment building leading to a large-scale release of radioactive material. However, it was determined that there was no oxygen present in the pressure vessel, a prerequisite for hydrogen to burn or explode. Immediate steps were taken to reduce the hydrogen bubble, and by the following day it was significantly smaller. Over the next week, steam and hydrogen were removed from the reactor using a [[Catalyst#Heterogeneous catalysts|catalytic]] recombiner and by venting directly into the open air.<ref>{{Citation |last1=Fischer |first1=Robert J. |title=Contingency Planning Emergency Response and Safety |date=2019 |work=Introduction to Security |pages=249–268 |publisher=Elsevier |language=en |doi=10.1016/B978-0-12-805310-2.00011-1 |isbn=978-0-12-805310-2 |pmc=7149346 |last2=Halibozek |first2=Edward P. |last3=Walters |first3=David C.}}</ref>

===Identification of released radioactive material===
The release occurred when the cladding was damaged while the PORV was still stuck open. [[Nuclear fission product|Fission products]] were released into the reactor coolant. Since the PORV was stuck open and the loss of coolant accident was still in progress, primary coolant with fission products and/or fuel was released and ultimately ended up in the auxiliary building. The auxiliary building was outside the containment boundary.

This was evidenced by the radiation alarms that eventually sounded. However, since very little of the fission products released were solids at room temperature, very little [[radiological contamination]] was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. According to the Rogovin report, the vast majority of the radioisotopes released were noble gases xenon and krypton resulting in an average dose of {{convert|1.4|mrem|uSv|abbr=on}} to the two million people near the plant.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |pages=25, 153 |doi=10.2172/5395798 |osti=5395798 |quote=The effects on the population in the vicinity of Three Mile Island from radioactive releases measured during the accident, if any, will certainly be nonmeasurable and nondetectable. During the course of the accident, approximately 2.5 million curies of radioactive noble gases and 15 curies of radioiodines were released. These releases resulted in an average dose of 1.4 mrem to the approximately two million people in the site area. This average dose is less than 1 % of the annual dose from both natural background radiation and medical practice. The 1.4-mrem dose may also be compared to differences in annual doses in background radiation from living in a brick versus a frame house, an additional 14 mrem/yr; or living in the high altitude of Denver rather than in Harrisburg, an additional 80 mrem/yr.}}</ref> In comparison, a patient receives {{convert|3.2|mrem|uSv|abbr=on}} from a chest X-ray—more than twice the average dose of those received near the plant.<ref>{{cite web |url=http://www.physics.isu.edu/radinf/risk.htm |title=Radiation and Risk |website=ISU Health Physics Radinf |access-date=March 6, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20170406055010/http://www.physics.isu.edu/radinf/risk.htm |archive-date=April 6, 2017}}</ref> On average, a U.S. resident receives an annual radiation exposure from natural sources of about {{convert|310|mrem|uSv|abbr=on}}.<ref>{{cite web |title=Backgrounder on Biological Effects of Radiation |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html |website=NRC Library |publisher=U.S. Nuclear Regulatory Commission |access-date=October 26, 2021 |date=March 2017 |quote=On average, a U.S. resident receives an annual radiation exposure from natural sources of about 310 millirem (3.1 millisieverts or mSv). Radon and thoron gases account for two-thirds of this exposure. Cosmic, terrestrial, and internal radiation account for the rest. Man-made sources of radiation from medical, commercial and industrial activities contribute roughly 310 mrem more to our annual exposure. Among the largest of these sources is a computed tomography scan, which accounts for about 150 mrem. Other medical procedures make up another 150 mrem or so each year. Some consumer products such as tobacco, fertilizer, welding rods, exit signs, luminous watch dials and smoke detectors contribute about 10 mrem per year. The pie chart (on the page) shows the sources that make up the average annual U.S. radiation dose of 620 mrem.}}</ref>

Within hours of the accident, the [[United States Environmental Protection Agency]] (EPA) began daily sampling of the environment at the three stations closest to the plant. Continuous monitoring at 11&nbsp;stations was established on April 1 and was expanded to 31 stations on April 3. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area, but measures of [[Beta particle|beta radiation]] were not included because the EPA found no contamination in water, soil, sediment, or plant samples.<ref name="EPA's Role at Three Mile Island">{{cite web |url=http://www.epa.gov/history/topics/tmi/02.htm |title=EPA's Role At Three Mile Island |last=Perham |first=Christine |date=October 1980 |website=EPA.gov |access-date=March 17, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110318094452/http://www.epa.gov/history/topics/tmi/02.htm |archive-date=March 18, 2011}}</ref>

Researchers at nearby [[Dickinson College]]—which had [[radiation monitoring equipment]] sensitive enough to detect Chinese atmospheric atomic weapons-testing—collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely from natural [[radon]] plate-out, not the accident).<ref>{{cite web |url=http://www.threemileisland.org/science/pdfs/what_dickinson_found.pdf |title=What did Dickinson Research Find? |website=ThreeMileIsland.org |access-date=March 17, 2011 |url-status=usurped |archive-url=https://web.archive.org/web/20061003164838/http://www.threemileisland.org/science/pdfs/what_dickinson_found.pdf |archive-date=October 3, 2006}}</ref> Also, the tongues of white-tailed deer harvested over {{convert|50|mi|km|abbr=on}} from the reactor subsequent to the accident were found to have significantly higher levels of cesium-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric nuclear weapons testing.<ref>{{cite journal |last=Field |first=R. W. |date=June 1993 |title=137Cs levels in deer following the Three Mile Island accident |journal=Health Phys |volume=64 |issue=6 |pages=671–674 |doi=10.1097/00004032-199306000-00015 |pmid=8491625}}</ref> Had there been elevated releases of radioactivity, increased levels of iodine-131 and cesium-137 would have been expected to be detected in cattle and goat's milk samples. Elevated levels were not found.<ref>{{cite web |last1=Battist |first1=Lewis |last2=Buchanan |first2=John |last3=Congel |first3=Frank |last4=Nelson |first4=Christopher |last5=Nelson |first5=Mark |last6=Peterson |first6=Harold |last7=Rosenstein |first7=Marvin |url=http://www.threemileisland.org/downloads/210.pdf |title=Population Dose and Health Impact of the Accident at the Three Mile Island Nuclear Station |date=May 10, 1979 |website=ThreeMileIsland.org |access-date=March 6, 2018 |url-status=usurped |archive-url=https://web.archive.org/web/20120401001347/http://www.threemileisland.org/downloads/210.pdf |archive-date=April 1, 2012}}</ref> A later study noted that the official emission figures were consistent with available [[dosimeter]] data,<ref name=hatch97>{{cite journal |last=Hatch |first=M. |display-authors=etal |title=Comments on "A Re-Evaluation of Cancer Incidence Near the Three Mile Island Nuclear Plant" |url=http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=1469856&pageindex=1 |archive-url=https://archive.today/20130801115305/http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=1469856&pageindex=1 |url-status=dead |archive-date=August 1, 2013 |journal=Environmental Health Perspectives |volume=105 |issue=1 |page=12 |year=1997 |pmid=9074862 |pmc=1469856 |doi=10.1289/ehp.9710512|bibcode=1997EnvHP.105...12H }}</ref> though others have noted the incompleteness of this data, particularly for releases early on.<ref>{{cite journal |last1=Wing |first1=S. |last2=Richardson |first2=D. |last3=Armstrong |first3=D. |title=Reply to comments on 'A reevaluation of cancer incidence near the Three Mile Island' |pmc=1469992 |pmid=9171981 |volume=105 |issue=3 |date=March 1997 |journal=Environ. Health Perspect. |pages=266–268 |doi=10.2307/3433255 |jstor=3433255}}</ref>

== Kemeny Commission ==
Several state and federal government agencies mounted investigations into the crisis, the most prominent of which was the ''President's Commission on the Accident at Three Mile Island'', created by U.S. President [[Jimmy Carter]] in April 1979.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/209/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, CA |pages=209–210 |quote=The Three Mile Island crisis was quickly followed by several extensive investigations. The NRC, the state of Pennsylvania, congressional committees, and nuclear industry groups sponsored important studies, but the most prominent of the inquiries were conducted by the President's Commission on the Accident at Three Mile Island. In a speech to the nation about energy policies on April 5, 1979, President Carter announced that he would create an independent commission "to investigate the causes" of Three Mile Island and "to make recommendations on how to improve the safety of nuclear power plants." Six days later he formally established the commission and identified eleven persons he had appointed to it (a twelfth was added a short time later). The projected cost of the study was about $1 million. The White House deliberately avoided placing anyone on the panel who was associated with strong pro- or antinuclear views. |access-date=October 25, 2021}}</ref> The commission consisted of a panel of 12 people, specifically chosen for their lack of strong pro- or anti-nuclear views, and headed by chairman [[John G. Kemeny]], president of [[Dartmouth College]]. It was instructed to produce a final report within six months, and after public hearings, depositions, and document collection, released a completed study on October 31, 1979.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/210/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=210 |language=en-us |quote=The Kemeny Commission operated under stringent time constraints. The president instructed it to submit a final report on Three Mile Island within six months after its first meeting. The commission conducted its investigation by assigning staff members and consultants to task forces that examined technical issues, the roles of the NRC and Met Ed, emergency preparedness, public information, and health effects. After holding a series of public hearings, taking more than 150 depositions, and collecting about three hundred cubic feet of documents, it presented its findings to Carter on October 30, 1979, and released them to the public the following day. The completed study consisted of a 179-page overview and nine volumes of task force reports that totaled more than 2,200 pages. |access-date=October 25, 2021}}</ref>

According to the official figures, as compiled by the 1979 Kemeny Commission from Met Ed and NRC data, a maximum of {{convert|480|PBq|MCi|abbr=on}} of radioactive noble gases, primarily [[xenon]], were released by the event.<ref name="Walker231">{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/231/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=231 |language=en-us |quote=Another significant question that the cleanup of the containment building raised was why more radioactive iodine had not escaped from the plant. Although the accident discharged up to 13 million curies of radioactive noble gases to the environment, it released very little of the much more hazardous iodine-131. A curie is a unit of measurement formerly used to indicate the decay rate (or level of activity) of radioactive substances. Of the estimated 64 million curies of iodine-131 in the core at the time of the accident, less than 20 curies leaked to the atmosphere. This was a far smaller release of iodine-131 than reactor experts had postulated in projecting the consequences of a severe reactor accident before Three Mile Island. Researchers found that most of the iodine-131 in the core had combined with other elements to form compounds that dissolved in water or had attached to metal surfaces in the containment building. Under the conditions in the core and the reactor building, the iodine did not remain in a gaseous state long enough to escape from the plant into the environment. |access-date=October 25, 2021}}</ref> These noble gases were considered relatively harmless, and only {{convert|481|-|629|GBq|Ci|abbr=on}} of [[thyroid cancer]]-causing [[iodine-131]] were released.<ref name="Walker231" /> Total releases according to these figures were a relatively small proportion of the estimated {{convert|370|EBq|GCi|abbr=on}} in the reactor. It was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed,<ref name="FactSheet" /><ref>Kemeny, p. 30.</ref> with {{convert|5|ft|m|1|abbr=on}} of the core gone, and around {{convert|20|ST|t|lk=on|abbr=on}} of uranium flowing to the bottom head of the pressure vessel, forming a mass of [[Corium (nuclear reactor)|corium]].<ref>{{cite journal |last1=McEvily |first1=A. J. Jr. |last2=Le May |first2=I. |year=2002 |title=The Accident at Three Mile Island |url=https://www.jstage.jst.go.jp/article/jsms1963/51/3Appendix/51_3Appendix_1/_pdf |journal=Materials Science Research International |volume=8 |issue=1 |pages=1–8}}</ref> The reactor vessel—the second level of containment after the cladding—maintained integrity and contained the damaged fuel with nearly all of the radioactive isotopes in the core.<ref name="ANS">{{cite web |url=http://www.ans.org/pi/resources/sptopics/tmi/whathappened.html |access-date=November 9, 2008 |title=What Happened and What Didn't in the TMI-2 Accident |website=[[American Nuclear Society]] |url-status=dead |archive-url=https://web.archive.org/web/20110717080337/http://www.touchoilandgas.com/ebooks/A1ioj0/eandpvol7iss2/resources/134.htm |archive-date=July 17, 2011}}{{Verify source|type=archive|date=April 2019}}</ref>

Anti-nuclear political groups disputed the Kemeny Commission's findings, claiming that other independent measurements provided evidence of radiation levels up to seven times higher than normal in locations hundreds of miles downwind from TMI.<ref name="Mangano2004">{{cite journal |last=Mangano |first=Joseph |date=September–October 2004 |title=Three Mile Island: Health Study Meltdown |url=https://journals.sagepub.com/doi/pdf/10.2968/060005010 |journal=[[Bulletin of the Atomic Scientists]] |volume=60 |issue=5 |pages=30–35 |doi=10.2968/060005010 |s2cid=143984619 |issn=0096-3402|url-access=subscription }}</ref> [[Arnold Gundersen|Arnie Gundersen]], a former nuclear industry executive and anti-nuclear advocate,<ref>{{cite web |url=http://fairewinds.com/content/who-we-are |title=Who We Are |website=Fairewinds Associates, Inc |archive-url=https://web.archive.org/web/20100517022024/http://fairewinds.com/content/who-we-are |archive-date=May 17, 2010 |access-date=March 17, 2012 |url-status=dead}}</ref> said "I think the numbers on the NRC's website are off by a factor of 100 to 1,000".<ref name="sturgis09">{{cite web |last=Sturgis |first=Sue |url=https://www.facingsouth.org/2009/04/investigation-revelations-about-three-mile-island-disaster-raise-doubts-over-nuclear-plant-s |title=Investigation: Revelations about Three Mile Island disaster raise doubts over nuclear plant safety |date=April 2, 2009 |website=The Institute for Southern Studies |url-status=dead |archive-url=https://web.archive.org/web/20160814210605/https://www.facingsouth.org/2009/04/investigation-revelations-about-three-mile-island-disaster-raise-doubts-over-nuclear-plant-s |archive-date=August 14, 2016 |access-date=September 4, 2016 |quote=Arnie Gundersen—a nuclear engineer and former nuclear industry executive turned whistle-blower—has done his own analysis, which he shared for the first time at a symposium in Harrisburg last week. "I think the numbers on the NRC's website are off by a factor of 100 to 1,000," he said.}}</ref>{{Verify source|date=April 2019}}<ref>{{cite web |url=http://www.southernstudies.org/images/sitepieces/ThompsonTMIassessment.pdf |title=TMI Assessment (Part 2) – Releases of radiation to the environment |last1=Thompson |first1=Randall |last2=Bear |first2=David |year=1995 |website=The Institute for Southern Studies |archive-url=https://web.archive.org/web/20160416090620/http://www.southernstudies.org/images/sitepieces/ThompsonTMIassessment.pdf |archive-date=April 16, 2016 |url-status=dead |access-date=September 4, 2016}}</ref> Gundersen offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before 2:00&nbsp;p.m. on March 28, 1979, which would have provided the means for a high dose of radiation to occur. Gundersen cites affidavits from four reactor operators according to which the plant manager was aware of a dramatic pressure spike, after which the internal pressure dropped to outside pressure. Gundersen also claimed that the control room shook and doors were blown off hinges. However, official NRC reports refer merely to a "hydrogen burn".<ref name="sturgis09" />{{Verify source|date=April 2019}}

The Kemeny Commission referred to "a burn or an explosion that caused pressure to increase by {{convert|28|psi}} in the containment building",<ref name="PresidentsCommission">{{cite web |last=Kemeny |first=John G. |title=Report of The President's Commission On The Accident at Three Mile Island: The Need For Change: The Legacy Of TMI |url=http://www.threemileisland.org/downloads//188.pdf |display-authors=etal |website=ThreeMileIsland.org |date=October 1979 |access-date=September 30, 2018 |url-status=usurped |archive-url=https://web.archive.org/web/20120401001407/http://www.threemileisland.org/downloads//188.pdf |archive-date=April 1, 2012}}</ref> while ''[[The Washington Post]]'' reported that "At about 2:00&nbsp;pm, with pressure almost down to the point where the huge cooling pumps could be brought into play, a small hydrogen explosion jolted the reactor."<ref>{{cite news |title=The Tough Fight to Confine the Damage |url=https://www.washingtonpost.com/wp-srv/national/longterm/tmi/stories/ch4.htm |newspaper=The Washington Post |year=1979 |access-date=September 30, 2018}}</ref> Work performed for the Department of Energy in the 1980s determined that the hydrogen burn ([[deflagration]]), which went essentially unnoticed for the first few days, occurred 9 hours and 50 minutes after initiation of the accident, had a duration of 12 to 15 seconds and did not involve a [[detonation]].<ref>{{cite report |url=https://digital.library.unt.edu/ark:/67531/metadc1108164/m2/1/high_res_d/6237032.pdf |title=Lessons Learned From Hydrogen Generation and Burning During The TMI-2 Event |last1=Henrie |first1=J. O. |last2=Postma |first2=A. K. |date=May 1987 |publisher=U.S. Department of Energy, Hanford Office |page=2-1 |docket=DOE Contract No. DE-AC06-77RL01030 |quote=The hydrogen burn occurred throughout essentially all of the 2,033,000 ft3 (57,600 m3) containment during a period of approximately 12 s. Less than 5% of the burning took place in the first 6 s, less than 40% during the next 3 s, and more than half of the burning occurred during the last 3 s. There was no detonation. The hottest gas was the gas that burned at approximately 6 s prior to the end of the burn. Even though the gas was losing heat to the unburned gas and surrounding surfaces after it burned, compression heating was dominant and significantly increased its temperature until the pressure peaked. |access-date=October 9, 2021}}</ref><ref>{{cite report |url=https://digital.library.unt.edu/ark:/67531/metadc1204546/m2/1/high_res_d/6312264.pdf |title=Analysis of the Three Mile Island Unit 2 Hydrogen Burn |last1=Henrie |first1=J. O. |last2=Postma |first2=A. K. |date=March 1983 |publisher=U.S. Department of Energy, Hanford Office |volume=4 |page=iii |docket=DE-AV07-76IDO1570 |quote=Prior to the burn (deflagration), the hydrogen was well mixed with the containment air. The average hydrogen concentration was calculated to be 7.9%, wet basis. The hydrogen burn occurred at all three levels in the containment. The burn was initiated somewhere in the lowest level; probably on the west side. Even though the burn time was about 15 sec, nearly all of the burning occurred during a 6-sec period. Over one-half of the burning occurred during the last 3-sec period. |access-date=October 9, 2021}}</ref>

The investigation strongly criticized [[Babcock & Wilcox]], Met Ed, General Public Utilities, and the NRC for lapses in quality assurance and maintenance, inadequate operator training, lack of communication of important safety information, poor management, and complacency, but avoided drawing conclusions about the future of the nuclear industry.<ref>{{cite book |last=Walker |first=J. Samuel |url=https://archive.org/details/threemileislandn00walk/page/210/mode/1up |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |date=2004 |publisher=University of California Press |isbn=0-520-23940-7 |location=Berkeley, California |pages=210 |language=en-us |quote=The commission offered a harsh indictment of Babcock & Wilcox, Met Ed, GPU, and the NRC. It explained that it had no mandate to draw conclusions about the future of the nuclear industry or to make comparisons of nuclear power with other sources of energy. It did not intend to show either that "nuclear power is inherently too dangerous to permit it to continue and expand" or that "the nation should move forward aggressively to develop additional commercial nuclear power." Rather, the commission suggested "that if the country wishes, for larger reasons, to confront the risks that are inherently associated with nuclear power, fundamental changes are necessary if those risks are to be kept within tolerable limits." It maintained that critical improvements in reactor safety required major revisions, if not a revolution, in organization, practices, procedures, and above all, the "mindset" of the nuclear industry and the NRC. |access-date=October 25, 2021}}</ref> The heaviest criticism from the Kemeny Commission said that "... fundamental changes will be necessary in the organization, procedures, and practices—and above all—in the attitudes" of the NRC and the nuclear industry.<ref>{{cite book |last=Kemeny |first=John J |url=https://www.hsdl.org/?view&did=769775 |title=Report of the Commission on The Accident at Three Mile Island—The Need For Change: The Legacy of TMI |date=October 1979 |publisher=U.S. Government Printing Office |isbn=978-1297534478 |location=Washington, D.C. |page=7 |chapter=Overall Conclusion |quote=To prevent nuclear accidents as serious as Three Mile Island, fundamental changes will be necessary in the organization, procedures, and practices—and above all—in the attitudes of the Nuclear Regulatory Commission and, to the extent that the institutions we investigated are typical, of the nuclear industry. |access-date=October 25, 2021}}</ref> Kemeny said that the actions taken by the operators were "inappropriate" but that the workers "were operating under procedures that they were required to follow, and our review and study of those indicates that the procedures were inadequate" and that the control room "was greatly inadequate for managing an accident".<ref>{{cite web |year=1979 |title=Three Mile Island – 1979 Year in Review |url=http://www.upi.com/Audio/Year_in_Review/Events-of-1979/Three-Mile-Island/12311692377023-3/ |access-date=March 6, 2018 |work=United Press International}}</ref>

The Kemeny Commission noted that Babcock & Wilcox's PORV had previously failed on 11&nbsp;occasions, nine of them in the open position, allowing coolant to escape. The initial causal sequence of events at TMI had been duplicated 18&nbsp;months earlier at another Babcock & Wilcox reactor, the [[Davis–Besse Nuclear Power Station]]. The only differences were that the operators at Davis–Besse identified the valve failure after 20&nbsp;minutes, where at TMI it took 80 minutes, and the fact that the Davis–Besse facility was operating at 9% power, against TMI's 97%. Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue.<ref>{{cite journal |last=Hopkins |first=A. |year=2001 |title=Was Three Mile Island a 'normal accident'? |volume=9 |issue=2 |pages=65–72 |doi=10.1111/1468-5973.00155 |newspaper=Journal of Contingencies and Crisis Management}}</ref>

The [[Pennsylvania House of Representatives]] conducted its own investigation, which focused on the need to improve evacuation procedures.<ref>{{Citation |last1=Fischer |first1=Robert J. |title=Contingency Planning Emergency Response and Safety |date=2019 |work=Introduction to Security |pages=249–268 |publisher=Elsevier |language=en |doi=10.1016/B978-0-12-805310-2.00011-1 |isbn=978-0-12-805310-2 |pmc=7149346 |last2=Halibozek |first2=Edward P. |last3=Walters |first3=David C.}}</ref>

In 1985, a television camera was used to see the interior of the damaged reactor. In 1986, [[core sample]]s and samples of debris were obtained from the [[Corium (nuclear reactor)|corium]] layers on the bottom of the reactor vessel and analyzed.<ref>{{cite journal |last1=Akers |first1=D. W. |last2=Jensen |first2=S. M. |last3=Schuetz |first3=B. K. |date=March 1, 1994 |title=Examination of relocated fuel debris adjacent to the lower head of the TMI-2 reactor vessel |url=http://www.osti.gov/bridge/product.biblio.jsp?osti_id=10140801 |doi=10.2172/10140801 |access-date=March 6, 2018 |doi-access=free |website=Department of Energy, Office of Scientific and Technical Information}}</ref>

== Mitigation policies ==
===Voluntary evacuation===
[[File:Three Mile Island accident sign.jpg|thumb|upright=1.3|A sign dedicated in 1999 in [[Middletown, Dauphin County, Pennsylvania|Middletown, Pennsylvania]], near the plant, describing the accident and the evacuation of the area.]]
[[File:Harrisburg Int Airport with Pennsylvania ANG aircraft 1979.jpg|thumb|upright=1.3|right|Three Mile Island in background behind [[Harrisburg International Airport]], a few weeks after the accident]]
On Wednesday, March 28, hours after the accident began, Lieutenant Governor Scranton appeared at a news briefing to say that Met Ed had assured the state that "everything is under control".<ref name=mis>{{cite news |title=A Decade Later, TMI's Legacy Is Mistrust |url=https://www.washingtonpost.com/wp-srv/national/longterm/tmi/stories/decade032889.htm |newspaper=The Washington Post |date=March 28, 1989 |access-date=September 30, 2018}}</ref> Later that day, Scranton changed his statement, saying that the situation was "more complex than the company first led us to believe".<ref name= mis/> There were conflicting statements about radioactivity releases.<ref name=cooke>{{cite book |last=Cooke |first=Stephanie |title=[[In Mortal Hands: A Cautionary History of the Nuclear Age]] |publisher=Black Inc. |date=2009 |page=294}}</ref> Schools were closed, and residents were urged to stay indoors. Farmers were told to keep their animals under cover and on stored feed.<ref name=mis/><ref name=cooke/>

{{blockquote
| text= Based on the advice of the Chairman of the [[Nuclear Regulatory Commission|NRC]] and in the interest of taking every precaution, I am advising those who may be particularly susceptible to the effects of any radiation, that is, pregnant women and pre-school aged children, to leave the area within a five-mile radius of the Three Mile Island facility until further notice. We've also ordered the closings of any schools within this area.
|author=[[Dick Thornburgh]]
}}

Governor Thornburgh, on the advice of NRC chairman Joseph Hendrie, advised the evacuation "of pregnant women and pre-school age children...within a five-mile radius of the Three Mile Island facility". The evacuation zone was extended to a 20-mile radius on March 30.<ref name="Cutter & Barnes">{{cite journal |title=Evacuation behavior and Three Mile Island |last1=Cutter |first1=Susan |author-link1=Susan Cutter |last2=Barnes |first2=Kent |date=June 1982 |journal=Disasters |volume=6 |issue=2 |pages=116–124 |pmid=20958525 |doi=10.1111/j.1467-7717.1982.tb00765.x|bibcode=1982Disas...6..116C }}</ref> Within days, 140,000 people had left the area.<ref name="FactSheet"/><ref name=mis/><ref name="People & Events: Dick Thornburgh">{{cite web |url=https://www.pbs.org/wgbh/americanexperience/features/three-dick-thornburgh/ |title=People & Events: Dick Thornburgh |publisher=[[PBS]] |access-date=March 6, 2018}}</ref> More than half of the {{FORMATNUM:663500}} population within the 20-mile radius remained in that area.<ref name="Cutter & Barnes"/> According to a survey conducted in April 1979, 98% of the evacuees had returned to their homes within three weeks.<ref name="Cutter & Barnes"/>

Post-TMI surveys have shown that less than 50% of the American public were satisfied with the way the accident was handled by Pennsylvania state officials and the NRC, and people surveyed were even less pleased with the utility (General Public Utilities) and the plant designer.<ref name= ota>{{cite web |url=http://www.princeton.edu/~ota/disk3/1984/8421/842111.PDF |title=Public Attitudes Toward Nuclear Power |year=1984 |website=Office of Technology Assessment |page=231 |access-date=September 30, 2018}}</ref>

===Effect on nuclear power industry===
[[File:Nuclear_power_history.svg|right|thumb|upright=1.4|Global history of the use of [[nuclear power]]. The Three Mile Island accident is one of the factors cited for the decline of new reactor construction.]]
According to the IAEA, the Three Mile Island accident was a significant turning point in the global development of nuclear power.<ref>{{cite web |url=http://www.iaea.org/About/Policy/GC/GC48/Documents/gc48inf-4_ftn3.pdf |title=50 Years of Nuclear Energy |website=[[IAEA]] |access-date=December 29, 2008}}</ref> From 1963 to 1979, the number of reactors under construction globally increased every year except in 1971 and 1978. However, following the event, the number of reactors under construction in the U.S. declined from 1980 to 1998, with increasing construction costs and delayed completion dates for some reactors.<ref name=Hultman13>{{cite journal |last1=Hultman |first1=N. |last2=Koomey |first2=J. |title=Three Mile Island: The driver of US nuclear power's decline? |journal=Bulletin of the Atomic Scientists |year=2013 |volume=69 |issue=3 |pages=63–70 |doi=10.1177/0096340213485949 |bibcode=2013BuAtS..69c..63H |s2cid=145756891}}</ref> Many similar Babcock & Wilcox reactors on order were canceled. In total, 52&nbsp;U.S. nuclear reactors were canceled between 1980 and 1984.<ref>{{cite web |url=http://clonemaster.homestead.com/files/cancel.htm |title=Cancelled Nuclear Units Ordered in the United States |date=January 23, 2012 |website=Nuclear Power Plants in the U.S. |access-date=March 6, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20120123212944/http://clonemaster.homestead.com/files/cancel.htm |archive-date=January 23, 2012}}</ref>

The accident did not initiate the demise of the U.S. nuclear power industry, but it did halt its historic growth. Additionally, as a result of the earlier [[1973 oil crisis]] and post-crisis analysis with conclusions of potential overcapacity in [[Base load power plant|base load]], 40 planned nuclear power plants already had been canceled before the accident. At the time of the incident, 129 nuclear power plants had been approved, but of those, only 53 which were not already operating were completed. During the lengthy review process, complicated by the [[Chernobyl disaster]] seven years later, federal requirements to correct safety issues and design deficiencies became more stringent, [[Anti-nuclear movement in the United States|local opposition]] became more stringent, construction times were significantly lengthened and costs skyrocketed.<ref>{{cite news |last=Gertner |first=Jon |title=Atomic Balm? |url=https://www.nytimes.com/2006/07/16/magazine/16nuclear.html |newspaper=[[The New York Times Magazine]] |date=July 16, 2006 |access-date=September 30, 2018}}</ref> Until 2012,<ref>{{cite web |url=http://nuclearstreet.com/nuclear_power_industry_news/b/nuclear_power_news/archive/2012/02/09/nrc-approves-vogtle-reactor-construction-_2d00_-first-new-nuclear-plant-approval-in-34-years-_2800_with-new-plant-photos_2900_-020902.aspx |title=NRC Approves Vogtle Reactor Construction – First New Nuclear Plant Approval in 34 Years |website=Nuclear Street |access-date=September 30, 2018}}</ref> no U.S. nuclear power plant had been authorized to begin construction since the year before, 1978. Globally, the end of the increase in nuclear power plant construction came with the more catastrophic Chernobyl disaster in 1986 (see graph).

===Cleanup===
[[File:TMI cleanup-2.jpg|thumb|A clean-up crew working to remove [[radioactive contamination]] at [[Three Mile Island Nuclear Generating Station|Three Mile Island]]]]
Initially, GPU planned to repair the reactor and return it into service.<ref>{{cite web |last=Holton |first=W. C. |date=September 1990 |title=The Cleanup of Three Mile Island Unit 2 – A Technical History: 1979 to 1990 |url=http://svcf.jp/pdf/Three_Mile_Iland2NP-6931.PDF |access-date=October 24, 2021 |publisher=EPRI (Electric Power Research Institute) |pages=1–4 |quote=For some time after the accident, before the actual damage conditions were well understood, GPU envisioned the unit to return to service. Extensive planning efforts were directed towards this end. |location=Palo Alto, CA}}</ref> However, TMI-2 was too badly damaged and contaminated to resume operations; the reactor was gradually deactivated and permanently closed. TMI-2 had been online for only three&nbsp;months but now had a ruined reactor vessel and a containment building that was unsafe to walk in. Cleanup started in August 1979 and officially ended in December 1993, with a total cleanup cost of about $1&nbsp;billion.<ref name="nytimes1993"/> [[Benjamin K. Sovacool]], in his 2007 preliminary assessment of major energy accidents, estimated that the TMI accident caused a total of $2.4&nbsp;billion in property damages.<ref>{{cite journal |last=Sovacool |first=Benjamin K. |year=2008 |title=The costs of failure: A preliminary assessment of major energy accidents, 1907–2007 |journal=[[Energy Policy (journal)|Energy Policy]] |volume=36 |issue=5 |page=1807 |doi=10.1016/j.enpol.2008.01.040|bibcode=2008EnPol..36.1802S }}</ref>

Efforts focused on the cleanup and decontamination of the site, especially the defueling of the damaged reactor. Starting in 1985, almost {{convert|100|ST|t|abbr=on}} of radioactive fuel were removed from the site. Planning and work was partially hampered by too-optimistic views about the damage.<ref>{{cite web |last=Holton |first=W. C. |date=September 1990 |title=The Cleanup of Three Mile Island Unit 2 – A Technical History: 1979 to 1990 |url=http://svcf.jp/pdf/Three_Mile_Iland2NP-6931.PDF |access-date=October 24, 2021 |publisher=EPRI (Electric Power Research Institute) |page=8-1 |quote=A persistent optimism existed that the damage beyond the known areas was not as bad as some estimates would have it. In fact, it was worse. |location=Palo Alto, California}}</ref>

In 1988, the NRC announced that, although it was possible to further decontaminate the Unit&nbsp;2 site, the remaining radioactivity had been sufficiently contained as to pose no threat to public health and safety. The first major phase of the cleanup was completed in 1990, when workers finished shipping {{convert|150|ST|t|abbr=on}} of radioactive wreckage to Idaho for storage at the Department of Energy's National Engineering Laboratory. However, the contaminated cooling water that leaked into the containment building had seeped into the building's concrete, leaving the radioactive residue too impractical to remove. Accordingly, further cleanup efforts were deferred to allow for decay of the radiation levels and to take advantage of the potential economic benefits of retiring both Unit&nbsp;1 and Unit&nbsp;2 together.<ref name= "nytimes1993"/>

===Health effects and epidemiology===
{{main|Three Mile Island accident health effects}}
In the aftermath of the accident, investigations focused on the amount of radioactivity released. In total, approximately {{convert|2.5|MCi|PBq}} of radioactive gases and approximately {{convert|15|Ci|GBq}} of iodine-131 were released into the environment.<ref>{{cite book |last=Rogovin |first=Mitchell |title=Three Mile Island: a report to the commissioners and to the public. Volume I |date=January 1980 |publisher=U.S. Nuclear Regulatory Commission |location=Washington, D.C. |url=https://www.osti.gov/servlets/purl/5395798 |access-date=October 26, 2021 |page=153 |doi=10.2172/5395798 |osti=5395798 |quote=}}</ref> According to the [[American Nuclear Society]], using the official radioactivity emission figures, "The average radiation dose to people living within 10&nbsp;miles of the plant was eight&nbsp;[[millirem]] (0.08&nbsp;[[Sievert|mSv]]), and no more than 100&nbsp;millirem (1&nbsp;mSv) to any single individual. Eight&nbsp;millirem is about equal to a chest [[X-ray]], and 100&nbsp;millirem is about a third of the average background level of radiation received by US residents in a year."<ref>{{cite news |title=Three-Mile Island cancer rates probed |work=BBC News |date=November 1, 2002 |url=http://news.bbc.co.uk/1/hi/health/2385551.stm |access-date=November 25, 2008}}</ref>

According to health researcher Joseph Mangano, early scientific publications estimated no additional cancer deaths in the {{convert|10|mi|km|abbr=on}} area around TMI, based on these numbers.<ref name=Mangano2004/> Disease rates in areas farther than 10&nbsp;miles from the plant were not examined.<ref name=Mangano2004/> Local activism in the 1980s, based on anecdotal reports of negative health effects, led to scientific studies being commissioned. A variety of epidemiology studies have concluded that the accident had no observable long-term health effects.{{efn|Attributed to multiple sources: <ref name="hatch90"/><ref name="Hatch MC, Wallenstein S, Beyea J, Nieves JW, Susser M 1991 719–724"/><ref>{{cite journal |last=Levin |first=R. J. |year=2008 |title=Incidence of thyroid cancer in residents surrounding the Three-Mile Island nuclear facility |journal=Laryngoscope |volume=118 |issue=4 |pages=618–28 |doi=10.1097/MLG.0b013e3181613ad2 |pmid=18300710 |s2cid=27337295 |quote=These findings, however, do not provide a causal link to the TMI accident.}}</ref><ref>{{cite web |url=https://www.scribd.com/doc/158526327/Settlement-of-Medical-Claims |title=Settlement of Medical Claims |date=February 7, 1985 |via=Scribd |access-date=March 6, 2018}}</ref>}}

A peer-reviewed research article by Dr.{{nbsp}}Steven Wing found a significant increase in cancers between 1979 and 1985 among people who lived within ten miles of TMI.<ref>{{cite journal |last1=Wing |first1=S. |last2=Richardson |first2=D. |last3=Armstrong |first3=D. |last4=Crawford-Brown |first4=D. |title=A reevaluation of cancer incidence near the Three Mile Island nuclear plant: the collision of evidence and assumptions |journal=Environ Health Perspect |pages=52–57 |date=January 1997 |pmid=9074881 |volume=105 |issue=1 |pmc=1469835 |doi=10.1289/ehp.9710552|bibcode=1997EnvHP.105...52W }}</ref> In 2009, Dr. Wing stated that radiation releases during the accident were probably "thousands of times greater" than the NRC's estimates. A retrospective study of the Pennsylvania Cancer Registry found an increased incidence of thyroid cancer in some counties south of TMI (although, notably, not in [[Dauphin County, Pennsylvania|Dauphin County]] where the reactor was located) and in high-risk age groups but did not draw a [[causal]] link between these incidences and the accident.<ref name="ncbi.nlm.nih.gov" /><ref name="Levin RJ; De Simone NF; Slotkin JF; Henson BL. 1238:2064–71" /> The Talbott lab at the University of Pittsburgh reported finding a few, small increased cancer risks within the TMI population.<ref name="Han YY; Youk AO; Sasser H; Talbott EO. 1230–5" /> A more recent study reached "findings consistent with observations from other radiation-exposed populations," raising "the possibility that radiation released from [Three Mile Island] may have altered the molecular profile of [thyroid cancer] in the population surrounding TMI", establishing a potential causal mechanism, although not definitively proving causation.<ref>{{cite journal|title=Altered molecular profile in thyroid cancers from patients affected by the Three Mile Island nuclear accident|first1=David|last1=Goldenberg|first2=Mariano|last2=Russo|first3=Kenneth|last3=Houser|first4=Henry|last4=Crist|first5=Jonathan B.|last5=Derr|first6=Vonn|last6=Walter|first7=Joshua I.|last7=Warrick|first8=Kathryn E.|last8=Sheldon|first9=James|last9=Broach|first10=Darrin V.|last10=Bann|date=July 17, 2017|journal=The Laryngoscope|volume=127|issue=Suppl 3 |pages=S1–S9|doi=10.1002/lary.26687|pmid=28555940|s2cid=40795419}}</ref>

The [[Radiation and Public Health Project]], an organization with little credibility among epidemiologists,<ref>{{cite news |last=Newman |first=Andy |date=November 11, 2003 |title=In Baby Teeth, a Test of Fallout; A Long-Shot Search for Nuclear Peril in Molars and Cuspids |newspaper=The New York Times |url=https://www.nytimes.com/2003/11/11/nyregion/baby-teeth-test-fallout-long-shot-search-for-nuclear-peril-molars-cuspids.html}}</ref> cited calculations by Mangano that showed a spike in infant mortality in downwind communities two years after the accident.<ref name="Mangano2004" /><ref>{{cite news |last=Teather |first=David |date=April 13, 2004 |title=US nuclear industry powers back into life |url=https://www.theguardian.com/business/2004/apr/13/nuclearindustry.usnews |access-date=December 29, 2008 |newspaper=[[The Guardian]] |location=London, England}}</ref> Anecdotal evidence also records effects on the region's wildlife.<ref name="Mangano2004" /> [[John Gofman]] used his own, non-[[peer reviewed]] low-level radiation health model to predict 333 excess cancer or leukemia deaths from the 1979 Three Mile Island accident.<ref name="Gofman John W. xvii" /> The ongoing TMI epidemiological research has been accompanied by a discussion of problems in dose estimates due to a lack of accurate data, as well as illness classifications.<ref>{{cite journal |last1=Wing |first1=S. |last2=Richardson |first2=D. B. |last3=Hoffmann |first3=W. |date=April 2011 |title=Cancer risks near nuclear facilities: the importance of research design and explicit study hypotheses |journal=Environ Health Perspect |volume=119 |issue=4 |pages=417–21 |doi=10.1289/ehp.1002853 |pmc=3080920 |pmid=21147606|bibcode=2011EnvHP.119..417W }}</ref>

===Activism and legal action===
[[File:Singer. Anti-nuke rally in Harrisburg, (Pennsylvania) at the Capitol. - NARA - 540016.tif|right|thumb|upright|Anti-nuclear protest in [[Harrisburg, Pennsylvania]], the state capitol, following the Three Mile Island accident; April 9, 1979]]
The accident enhanced the perceived credibility of anti-nuclear groups and triggered protests around the world.<ref>{{cite journal |last=Carter |first=Luther J. |date=April 13, 1979 |title=Political Fallout from Three Mile Island |journal=[[Science (journal)|Science]] |volume=204 |issue=4389 |pages=154–155 |bibcode=1979Sci...204..154C |doi=10.1126/science.204.4389.154 |pmid=17738077}}</ref><ref>{{cite book |first=Mark |last=Hertsgaard |author-link=Mark Hertsgaard |date=1983 |title=Nuclear Inc. The Men and Money Behind Nuclear Energy |publisher=Pantheon Books |location=New York |pages=95, 97}}</ref> President Carter—who had specialized in nuclear power while in the [[United States Navy]]—told his cabinet after visiting the plant that the accident was minor but reportedly declined to do so in public, in order to avoid offending Democrats who opposed nuclear power.<ref name="evansnovak19790406">{{cite news |last1=Evans |first1=Rowland |last2=Novak |first2=Robert |title=What Carter Found at Three Mile Island |url=https://news.google.com/newspapers?id=sM1RAAAAIBAJ&pg=7107%2C772506 |newspaper=[[Pittsburgh Post-Gazette]] |date=April 6, 1979 |access-date=April 26, 2014 |page=9}}</ref>

Members of the American public, concerned about the release of radioactive gas from the accident, staged numerous [[List of anti-nuclear protests in the United States|anti-nuclear demonstrations]] across the country in the following months. The largest demonstration was held in New York City in September 1979 and involved 200,000&nbsp;people, with speeches given by [[Jane Fonda]] and [[Ralph Nader]].<ref>{{cite book |last1=Hrebenar |first1=Ronald J. |last2=Scott |first2=Ruth K. |title=Interest Group Politics in America |url=https://books.google.com/books?id=YSdz4Cxqnx4C&q=nevada+test+site+protest+demonstration&pg=PA149 |date=1997 |publisher=M.E. Sharpe |isbn=978-1-56324-703-3 |page=149}}</ref><ref name=diablo>{{cite book |last=Giugni |first=Marco |title=Social Protest and Policy Change: Ecology, Antinuclear, and Peace Movements in Comparative Perspective |url=https://books.google.com/books?id=Kn6YhNtyVigC&q=%22three+mile+island%22+anti-nuclear+protests+demonstrations&pg=PA45 |date=2004 |publisher=Rowman & Littlefield |isbn=978-0-7425-1827-8 |page=45}}</ref><ref>{{cite news |url=https://www.nytimes.com/1979/09/24/archives/nearly-200000-rally-to-protest-nuclear-energy-gathering-at-the.html |first=Robin |last=Herman |title=Nearly 200,000 Rally to Protest Nuclear Energy |newspaper=The New York Times |page=B1 |date=September 24, 1979 |access-date=September 30, 2018}}</ref> The New York rally was held in conjunction with a series of nightly "[[No Nukes (album)|No Nukes" concerts]] given at [[Madison Square Garden]] from September 19 to 23 by [[Musicians United for Safe Energy]]. In the previous May, an estimated 65,000&nbsp;people—including California Governor [[Jerry Brown]]—attended a march and rally against nuclear power in Washington, D.C.<ref name=diablo/>

In 1981, citizens' groups succeeded in a class action suit against TMI, winning $25&nbsp;million in an out-of-court settlement. Part of this money was used to found the TMI Public Health Fund.<ref name="Greene1999">{{cite book |last=Greene |first=Gayle |title=The Woman who Knew Too Much: Alice Stewart and the Secrets of Radiation |url=https://books.google.com/books?id=PdkJo5yerGYC&q=%22Alice+Stewart%22+%2B+%22three+mile+island%22&pg=PA178 |date=1999 |publisher=University of Michigan Press |isbn=978-0-472-08783-9 |page=178}}</ref> In 1983, a federal grand jury indicted Metropolitan Edison on criminal charges for the falsification of safety test results prior to the accident.<ref>{{cite news |title=Three Mile Island operator falsified tests: jury |newspaper=[[Ottawa Citizen]] |date=November 8, 1983 |url=https://news.google.com/newspapers?id=Cq8yAAAAIBAJ&dq=three%20mile%20island%20leak%20rates&pg=4415%2C4871603}}</ref> Under a plea-bargaining agreement, Met Ed pleaded guilty to one count of falsifying records and no contest to six other charges, four of which were dropped, and agreed to pay a $45,000&nbsp;fine and set up a $1&nbsp;million account to help with emergency planning in the area surrounding the plant.<ref>{{cite news |title=Three Mile Island plant operator faked leak records |work=Ottawa Citizen |date=February 29, 1984 |url=https://news.google.com/newspapers?id=dtEyAAAAIBAJ&dq=three%20mile%20island%20leak%20rates&pg=1298%2C3906217}}</ref>

According to Eric Epstein, chair of [[List of anti-nuclear groups in the United States#Three Mile Island Alert|Three Mile Island Alert]], the TMI plant operator and its insurers paid at least $82&nbsp;million in publicly documented compensation to residents for "loss of business revenue, evacuation expenses and health claims."<ref>{{cite news |url=http://www.pittsburghlive.com/x/pittsburghtrib/news/multimedia/s_617234.html |archive-url=https://web.archive.org/web/20110319055920/http://www.pittsburghlive.com/x/pittsburghtrib/news/multimedia/s_617234.html |url-status=dead |archive-date=March 19, 2011 |title=Three Mile Island: 30 years of what if... |date=March 22, 2009 |newspaper=Pittsburgh Tribune Review}}</ref> However, a [[class action lawsuit]] alleging that the accident caused detrimental health effects was rejected by [[Harrisburg]] [[United States district court]] Judge Sylvia Rambo, and the final appeal of that decision failed in 2002.<ref>{{Cite news |last= |date=1996-06-08 |title=U.S. Judge Throws Out Claims Against Three Mile Island Plant |url=https://www.nytimes.com/1996/06/08/us/us-judge-throws-out-claims-against-three-mile-island-plant.html |access-date=2025-02-17 |work=The New York Times |language=en-US |issn=0362-4331}}</ref><ref>{{Cite news |date=2002-12-28 |title=Lawsuits over TMI are dead, lawyers say |url=https://www.newspapers.com/article/the-morning-call-lawsuits-over-tmi-are-d/165923199/ |access-date=2025-02-17 |work=The Morning Call |pages=A28 |agency=Associated Press}}</ref>

===Normal accident theory===
The Three Mile Island accident inspired [[Charles Perrow]]'s normal accident theory, which attempts to describe "unanticipated interactions of multiple failures in a complex system". TMI was an example of this type of accident because it was "unexpected, incomprehensible, uncontrollable and unavoidable."<ref>{{cite book |last=Perrow |first=C. |title=Accident at Three Mile Island: The Human Dimensions |date=1982 |publisher=[[Westview Press]] |editor-last1=Sils |editor-first1=D. |location=Boulder, Colorado |pages=173–184 |language=en-us |chapter=The President's Commission and the Normal Accident |editor-last2=Wolf |editor-first2=C. |editor-last3=Shelanski |editor-first3=V.}}</ref>

<blockquote>Perrow concluded that the failure at Three Mile Island was a consequence of the system's immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer what he termed a 'normal accident'. Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely.<ref name=nature477>{{cite journal |last=Pidgeon |first=Nick |title=In retrospect: Normal accidents |date=September 22, 2011 |volume=477 |issue=7365 |pages=404–405 |journal=[[Nature (journal)|Nature]] |bibcode=2011Natur.477..404P |doi=10.1038/477404a |doi-access=free}}</ref></blockquote>

"Normal" accidents, or [[system accident]]s, are so called by Perrow because such accidents are inevitable in extremely complex systems. Given the characteristic of the system involved, multiple failures that interact with each other will occur, despite efforts to avoid them.<ref>{{cite book |last=Perrow |first=Charles |title=Normal Accidents: Living with High-Risk Technologies |location=New York |publisher=Basic Books |date=1984 |page=5}}</ref> Events which appear trivial initially cascade and multiply unpredictably, creating a much larger catastrophic event.<ref name=rev>{{cite web |url=http://esd.mit.edu/WPS/wplit-2003-01.pdf |title='Normal Accidents' by Charles Perrow |first=Daniel E. |last=Whitney |date=July 2003 |work=Massachusetts Institute of Technology |url-status=dead |archive-url=https://web.archive.org/web/20060911075601/http://esd.mit.edu/WPS/wplit-2003-01.pdf |archive-date=September 11, 2006}}</ref>

<blockquote>''Normal Accidents'' contributed key concepts to a set of intellectual developments in the 1980s that revolutionized the conception of safety and risk. It made the case for examining technological failures as the product of highly interacting systems, and highlighted organizational and management factors as the main causes of failures. Technological disasters could no longer be ascribed to isolated equipment malfunction, operator error or acts of God.<ref name=nature477/></blockquote>

===Comparison to U.S. Navy operations===
After the TMI incident, President Carter commissioned a study, ''Report of the President's Commission on the Accident at Three Mile Island'' (1979).<ref name="PresidentsCommission"/>

[[Admiral]] [[Hyman G. Rickover]] was later asked to tell Congress why naval nuclear propulsion (as used in [[Nuclear submarine|submarines]]) had suffered no reactor accidents, defined as the uncontrolled release of fission products to the environment resulting from damage to a reactor core. In his testimony, Rickover said:

<blockquote>Over the years, many people have asked me how I run the [[Naval Reactors]] Program, so that they might find some benefit for their own work. I am always chagrined at the tendency of people to expect that I have a simple, easy gimmick that makes my program function. Any successful program functions as an integrated whole of many factors. Trying to select one aspect as the key one will not work. Each element depends on all the others.<ref>{{cite web |url=http://www.navy.mil/navydata/testimony/safety/bowman031029.txt |title=Statement of Admiral F. L. "Skip" Bowman, U.S. Navy, Director, Naval Nuclear Propulsion Program before the House Committee on Science |date=October 29, 2003 |website=United States Navy Office of Information |access-date=September 30, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20150403135146/https://www.navy.mil/navydata/testimony/safety/bowman031029.txt |archive-date=April 3, 2015}}</ref></blockquote>

== 21st century status ==
[[File:Three Mile Island Nuclear Generating Station.jpg|thumb|upright=1.3|After the accident, Three Mile Island used only one nuclear generating station, TMI-1, which is on the right. TMI-2, to the left, has not been used since the accident.]]
[[File:Three Mile Island Nuclear Generating Station Unit 2.jpg|thumb|upright=1.3|TMI-2 in February 2014. The cooling towers are on the left. The spent fuel pool with containment building of the reactor is on the right.]]
After the incident at TMI-2, the NRC suspended the license to operate TMI-1, which was owned and operated by Met Ed, one of General Public Utilities Corporation's regional utility operating companies. In 1982, the citizens of the three counties surrounding the site voted overwhelmingly in a non-binding resolution to retire Unit 1 permanently. In 1985, a 4–1 vote by the Nuclear Regulatory Commission allowed TMI-1 to resume operations.<ref>{{cite journal |last=Walsh |first=Edward |date=March 1983 |title=Three Mile Island: Meltdown of Democracy? |url=http://thebulletin.org/sites/default/files/TMIWalshMar1983p57to60.pdf |journal=Bulletin of the Atomic Scientists |volume=39 |issue=3 |pages=57–60 |doi=10.1080/00963402.1983.11458968 |bibcode=1983BuAtS..39c..57W}}</ref><ref>{{cite news |url=https://www.washingtonpost.com/archive/politics/1985/05/30/nrc-votes-to-restart-three-mile-island/d7eecd67-eb83-4837-bc9a-cbf6b8a4f97c/ |title=NRC Votes to Restart Three Mile Island |last=O'Toole |first=Thomas |date=May 30, 1985 |newspaper=The Washington Post |access-date=December 15, 2016}}</ref> 

GPU formed General Public Utilities Nuclear Corporation as a subsidiary to own and operate the company's nuclear facilities, including Three Mile Island. In 1996, General Public Utilities shortened its name to GPU Inc. In 1998, GPU sold TMI-1 to AmerGen Energy Corporation, a joint venture between [[Philadelphia Electric Company]] and [[British Energy]]. (GPU was legally obliged to continue to maintain and monitor TMI-2.) In 2001, GPU was acquired by [[FirstEnergy]] Corporation and dissolved, and the maintenance and administration of Unit 2 was contracted to AmerGen.

In 2000, Philadelphia Electric merged with [[Unicom Corp]]oration to form Exelon. In 2003, Exelon bought the remaining shares of AmerGen from British Energy. In 2009, Exelon Nuclear absorbed and dissolved AmerGen. Along with TMI Unit 1, Exelon Nuclear operates [[Clinton Power Station]] and several other nuclear facilities.{{efn|Attributed to multiple sources: <ref>{{cite web |last=Maykuth |first=Andrew |title=Peach Bottom, other U.S. nuclear power plants could be running until 2054. Is it safe? |url=https://www.philly.com/business/energy/aging-nuclear-power-plants-safe-three-mile-island-peach-bottom-us-future-20190228.html |access-date=March 28, 2019 |website=Philadelphia Inquirer |date=February 28, 2019 |language=en-US}}</ref><ref>{{cite web |last=Salamone |first=Jon Harris, Anthony |title=Like Three Mile Island, Talen Energy's nuclear plant under pressure |url=https://www.mcall.com/business/mc-talen-susquehanna-nuclear-plant-future-20170530-story.html |access-date=March 28, 2019 |website=Lehigh Valley Business Cycle |date=May 31, 2017 |language=en-US}}</ref><ref>{{cite web |title=Exelon Generation Formally Integrates AmerGen Assets Into Exelon Nuclear – Exelon |url=http://www.exeloncorp.com/newsroom/Pages/pr_20090108_Generation.aspx |access-date=March 28, 2019 |website=exeloncorp.com}}</ref><ref>{{cite web |url=https://www.chicagotribune.com/news/ct-xpm-2012-09-23-sns-rt-utilities-operationsexelon-threemilel4e8ko0dq-20120923-story.html |title=Exelon's PA Three Mile Island Unit 1 reactor back online |agency=[[Reuters]] |website=Chicago Tribune |date=September 23, 2012 |language=en-US |access-date=March 28, 2019}}</ref><ref>{{cite web |url=https://www.baltimoresun.com/news/bs-xpm-1998-07-18-1998199081-story.html |title=Three Mile Island finds buyer as times change Deal excludes reactor in '79 nuclear accident |last=Murphy |first=John |website=The Baltimore Sun |date=July 18, 1998 |language=en-US |access-date=March 28, 2019}}</ref>}}

Unit 2 continues to be licensed and regulated by the Nuclear Regulatory Commission in a condition known as Post Defueling Monitored Storage.<ref>{{cite web |url=https://www.nrc.gov/info-finder/decommissioning/power-reactor/three-mile-island-unit-2.html |title=Three Mile Island – Unit 2 |publisher=U.S. Nuclear Regulatory Commission |access-date=January 29, 2009}}</ref> The TMI-2 reactor has been permanently shut down with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site, reactor fuel and most core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored. The owner planned to keep the facility in long-term, monitoring storage until the operating license for the TMI-1 plant expired, at which time both plants would be decommissioned.<ref name="FactSheet"/>

In 2009, the NRC granted a license extension which allowed the TMI-1 reactor to operate until April 19, 2034.<ref>{{cite web |title=Three Mile Island 1 – Pressurized Water Reactor |url=https://www.nrc.gov/info-finder/reactor/tmi1.html |publisher=U.S. Nuclear Regulatory Commission |access-date=December 15, 2008}}</ref><ref name="license-extended-reuters">{{cite news |last=DiSavino |first=Scott |title=NRC renews Exelon Pa. Three Mile Isl reactor license |url=https://www.reuters.com/article/companyNewsAndPR/idUSN223175220091022 |publisher=Reuters |date=October 22, 2009 |access-date=October 23, 2009}}</ref> In 2017, it was announced that operations would cease by 2019 due to financial pressure from inexpensive natural gas, unless lawmakers stepped in to keep it open.<ref>{{cite web |url=https://www.cnbc.com/2017/05/30/three-mile-island-power-plant-may-close-in-face-of-cheap-gas.html |title=Three Mile Island Power Plant May Close in Face of Cheap Gas |date=May 30, 2017 |publisher=CNBC News |access-date=May 30, 2017 |url-status=dead |archive-url=https://web.archive.org/web/20170602005925/https://www.cnbc.com/2017/05/30/three-mile-island-power-plant-may-close-in-face-of-cheap-gas.html |archive-date=June 2, 2017}}</ref> When it became clear that the subsidy legislation would not pass, Exelon decided to retire the plant.<ref>{{cite news |url=https://www.npr.org/2019/05/08/721514875/three-mile-island-nuclear-plant-to-close-latest-symbol-of-struggling-industry |title=Three Mile Island Nuclear Plant To Close, Latest Symbol Of Struggling Industry |date=May 8, 2019}}</ref> TMI Unit 1 shut down on September 20, 2019.<ref>{{cite news |url=https://www.kpbs.org/news/2019/sep/20/three-mile-island-nuclear-power-plant-shuts-down/ |title=Three Mile Island Nuclear Power Plant Shuts Down |last=Sholtis |first=Brett |language=en-US |access-date=September 24, 2019 |archive-url=https://web.archive.org/web/20190924195607/https://www.kpbs.org/news/2019/sep/20/three-mile-island-nuclear-power-plant-shuts-down/ |archive-date=September 24, 2019}}</ref> Following the permanent shutdown, Unit 1 is in decommissioning, moving to [[SAFSTOR]] status.<ref>{{cite web |title=Three Mile Island Nuclear Station, Unit 1 |url=https://www.nrc.gov/info-finder/reactors/tmi1.html |website=U. S. NRC |publisher=U. S. Government |access-date=2 January 2023 |quote=NRC will continue to provide licensing oversight and inspections of the plant until the plant is completely decommissioned, including inspections of the Independent Spent Fuel Storage Installation (ISFSI) until the spent fuel is permanently removed from the site and the license is terminated.}}</ref> 

In 2020 the site was purchased by TMI-2 Solutions, a subsidiary of [[EnergySolutions]], with the intent of cleaning up the site for less money than is available in a dedicated fund.<ref name=":0">{{Cite web |last=McDevitt |first=Rachel |date=2023-05-11 |title=Three Mile Island enters new phase of cleanup |url=https://stateimpact.npr.org/pennsylvania/2023/05/11/tmi-enters-new-phase-of-cleanup/ |access-date=2024-01-25}}</ref> On May 8, 2023, TMI-2 Solutions announced that 99% of the nuclear fuel has been cleaned up and that the site has entered the next phase of clean-up that will last until 2029.<ref>{{Cite web |date=2023-05-08 |title=Three Mile Island Unit 2 (TMI-2) Enters Next Phase of Decommissioning Project |url=https://www.tmi2solutions.com/may-8tth-2023 |access-date=2024-01-25}}</ref><ref name=":0"/> TMI-2 Solutions planned to finish clean-up and demolish the plant by 2052.<ref name=":0"/>

In September 2024, Constellation Energy announced plans to restart the Three Mile Island nuclear plant to sell the power to Microsoft, demonstrating the immense power needs of the tech sector as they build data centers to support artificial intelligence.<ref>{{cite web | url=https://edition.cnn.com/2024/09/20/energy/three-mile-island-microsoft-ai | title=Three Mile Island is reopening and selling its power to Microsoft |first=Jordan |last=Valinsky |publisher=[[CNN]] |date=September 20, 2024|access-date=September 20, 2024 }}</ref>  Constellation expects the Unit 1 reactor at Three Mile Island to come back online in 2028, subject to approval by the Nuclear Regulatory Commission. Constellation also plans to apply to extend the plant’s operations to at least 2054.<ref>{{cite news | url=https://www.cnbc.com/2024/09/20/constellation-energy-to-restart-three-mile-island-and-sell-the-power-to-microsoft.html | title=Constellation Energy to restart Three Mile Island nuclear plant, sell the power to Microsoft for AI | first=Spencer | last=Kimball | publisher=[[CNBC]] |date=September 20, 2024|access-date=September 20, 2024 }}</ref>

==Timeline==
{| class="wikitable"
|-
! Date || Event
|-
|{{nowrap|1968–1970}} || Construction
|-
|{{nowrap|April 1974}} || Reactor-1 online
|-
|{{nowrap|February 1978}} || Reactor-2 online
|-
|March 1979||TMI-2 accident occurred. Containment coolant released into environment.
|-
|April 1979||Containment steam vented to the atmosphere in order to stabilize the core.
|-
|July 1980||Approximately 1,591&nbsp;[[Becquerel|TBq]] (43,000&nbsp;[[Curie (unit)|curie]]s) of krypton were vented from the reactor building.
|-
|July 1980||The first manned entry into the reactor building took place.
|-
|November 1980||An Advisory Panel for the Decontamination of TMI-2, composed of citizens, scientists, and state and local officials, held its first meeting in [[Harrisburg]], [[Pennsylvania]].
|-
|December 1980||U.S. 96th Congressional session passed U.S. legislation establishing a five-year [[Nuclear Safety, Research, Demonstration, and Development Act of 1980|nuclear safety, research, demonstration, and development]] program.
|-
|July 1984||The reactor vessel head (top) was removed.
|-
|October 1985||Defueling began.
|-
|July 1986||The off-site shipment of reactor core debris began.
|-
|August 1988||GPU submitted a request for a proposal to amend the TMI-2 license to a "possession-only" license and to allow the facility to enter long-term monitoring storage.
|-
|January 1990||Defueling was completed.
|-
|July 1990||GPU submitted its funding plan for placing $229&nbsp;million in escrow for radiological decommissioning of the plant.
|-
|January 1991||The evaporation of accident-generated water began.
|-
|April 1991||NRC published a notice of opportunity for a hearing on GPU's request for a license amendment.
|-
|February 1992||NRC issued a safety evaluation report and granted the license amendment.
|-
|August 1993||The processing of accident-generated water was completed involving 2.23&nbsp;million gallons.
|-
|September 1993||NRC issued a possession-only license.
|-
|September 1993||The Advisory Panel for Decontamination of TMI-2 held its last meeting.
|-
|December 1993||Post-Defueling Monitoring Storage began.
|-
|October 2009||TMI-1 license extended from April 2014 until 2034.
|-
|May 2019||TMI-1 is announced to be closed in September 2019.
|-
|September 2019 || TMI-1 shutdown at noon on September 20, 2019.
|-
|September 2024 || Constellation signed a twenty year power deal to bring back TMI-1
|}

==Documentaries==
''Meltdown: Three Mile Island'' is a four-part [[docuseries]] released by [[Netflix]] on May 4, 2022.<ref>{{cite web |last=Tinubu |first=Aramide |title='Meltdown: Three Mile Island' Unpacks the Worst Nuclear Incident in America |url=https://www.netflix.com/tudum/articles/meltdown-three-mile-island-release-date-cast-news |website=Netflix.com}}</ref> The documentary recounts the events, controversies, and lingering effects of the accident.<ref name=IMDB>{{cite web |url=https://www.imdb.com/title/tt19757508/ |title=Meltdown: Three Mile Island |publisher=[[IMDb]]}}</ref> Featured in the series are Rick Parks, a TMI nuclear engineer turned whistleblower against [[Bechtel]];<ref>{{cite web|url=https://whistleblower.org/meltdown-three-mile-island/ |title=Meltdown: Three Mile Island|work=[[Government Accountability Project]] |access-date=21 September 2024}}</ref><ref>{{cite web|url=https://www.theguardian.com/tv-and-radio/2022/may/05/meltdown-three-mile-island-netflix-us-nuclear-accident |title=Dodged a Bullet: How Whistleblowers Averted a Second US Nuclear Disaster |last=Horton |first=Adrian |date=5 May 2022 |work=[[The Guardian]] |access-date=21 September 2024}}</ref> Lake Barrett, an independent energy consultant who served as the NRC's on-site director for the TMI-2 cleanup; Eric Epstein, chairman of TMI Alert, a nuclear watchdog organization in central Pennsylvania; [[Michio Kaku]], an American theoretical physicist and critic of nuclear power; and residents of the communities affected by the event.<ref>{{cite web|url=https://variety.com/2022/tv/reviews/meltdown-three-mile-island-netflix-1235256986/ |title='Meltdown: Three Mile Island' Is a Methodical Look at an American Disaster: TV Review |last=D'Addario |first=Daniel |date=May 3, 2022 |publisher=Variety |access-date=January 13, 2023}}</ref><ref>{{cite web|url=https://www.ans.org/news/article-4036/lake-barretts-realitygrounded-perspective-on-netflixs-drama-imeltdown-three-mile-islandi/ |title=Lake Barrett's Reality-Grounded Perspective on Netflix's Drama 'Meltdown: Three Mile Island' |date=June 10, 2022 |website=American Nuclear Society |publisher=Nuclear Newswire |access-date=January 13, 2023}}</ref><ref name=IMDB />

''[[Radioactive: The Women of Three Mile Island]]'' is a 2023 documentary about the accident.<ref>{{Citation |last=Hutner |first=Heidi |title=Radioactive: The Women of Three Mile Island |date=2022-12-06 |type=Documentary |url=https://www.imdb.com/title/tt26654445/ |access-date=2024-03-20 |others=Lynne Bernabei, Linda Braasch, Joyce Corradi |publisher=Three Mile Productions}}</ref>

==In popular culture==
On March 15, 1979, twelve days before the accident, the movie ''[[The China Syndrome]]'' premiered and was initially met with backlash from the nuclear power industry, claiming it to be "sheer fiction" and a "[[character assassination]] of an entire industry".<ref>{{cite news |url=https://www.nytimes.com/1979/03/18/archives/nuclear-experts-debate-the-china-syndrome-but-does-it-satisfy-the.html |title=Nuclear Experts Debate 'The China Syndrome' |last=Burnham |first=David |date=March 18, 1979 |newspaper=The New York Times |access-date=September 30, 2018}}</ref> In the film, television reporter Kimberly Wells ([[Jane Fonda]]) and her cameraman Richard Adams ([[Michael Douglas]]) secretly film a major accident at a nuclear power plant while taping a series on [[nuclear power]]. At one point in the film, an official tells Wells that an explosion at the plant "could render an area the size of the state of Pennsylvania permanently uninhabitable".<ref>{{cite web |last=Southwick |first=Ron |title=Three Mile Island accident was eerily foreshadowed by a Hollywood blockbuster days before |url=https://www.pennlive.com/news/2019/03/the-three-mile-island-accident-followed-a-hollywood-blockbuster.html |website=pennlive.com |date=March 16, 2019 |access-date=March 18, 2019 |language=en-US}}</ref> After the release of the film, Fonda began [[lobbying]] against nuclear power. In an attempt to counter her efforts, [[Edward Teller]] ([[nuclear physicist]], government [[Science Advisory Board|science adviser]], and contributor to the [[Teller–Ulam design|Teller–Ulam thermonuclear bomb design]]), personally lobbied in favor of nuclear power.<ref>{{cite book |last=Benarde |first=Melvin A. |title=Our Precarious Habitat ... It's In Your Hands |url=https://books.google.com/books?id=9O82L8QEZXcC&pg=PA256 |date=October 16, 2007 |publisher=Wiley InterScience |isbn=978-0-470-09969-8 |page=256}}</ref> Teller suffered a heart attack shortly after the incident and joked that he was the only person whose health was affected.<ref>[[q:Edward Teller]]</ref>

Three Mile Island and the associated incident features in the UK Channel 4 show ''[[Utopia_(British_TV_series)|Utopia]]''.

The 2009 superhero film ''[[X-Men Origins: Wolverine]]'' portrays Three Mile Island in the 1970s serving as a mutant prison run by the film's main antagonist William Stryker and his team of scientists, where they experimented on and combined the mutants' powers for the Weapon X and Weapon XI programs. The film portrays the 1979 accident being caused by the destruction from a battle between the characters Wolverine, Sabretooth, and Deadpool.

The Three Mile Island accident plays a pivotal role in ''Heat and Light'', a 2016 novel by [[Jennifer Haigh]].<ref>{{Cite news |last=Maslin |first=Janet |date=2016-04-27 |title=Review: With 'Heat and Light,' Jennifer Haigh Drills Below the Surface |url=https://www.nytimes.com/2016/04/28/books/review-with-heat-and-light-jennifer-haigh-drills-below-the-surface.html |access-date=2024-04-23 |work=The New York Times |language=en-US |issn=0362-4331}}</ref>

[[Midnight Oil]]'s song "Harrisburg" from their 1984 album [[Red Sails in the Sunset (album)|Red Sails in the Sunset]] is about the accident.

==See also==
{{portal|United States|Energy|Nuclear technology}}
* [[Church Rock uranium mill spill]], New Mexico
* [[Forked River Nuclear Power Plant]], New Jersey
* [[Nuclear reactor accidents in the United States]]
* [[Nuclear energy policy of the United States]]
* [[Nuclear safety in the United States]]
* ''[[Three Mile Island: Thirty Minutes to Meltdown]]''
* [[WASH-1400]] – NRC report predicting risks that led to this accident

'''General:'''
* [[List of civilian nuclear accidents]]
* [[Lists of nuclear disasters and radioactive incidents]]
* [[Nuclear and radiation accidents and incidents]]
* [[Nuclear safety and security]]
* [[Process control]]

==Notes==
{{notelist}}

==References==
{{reflist}}

===Sources===
* {{cite book |title=[[Three Mile Island: Thirty Minutes to Meltdown]] |first=Daniel |last=Ford |year=1982 |publisher=Penguin |isbn=978-0-14-006048-5}}
* {{cite book |last=Halden |first=Grace |title=Three Mile Island: The Meltdown Crisis and Nuclear Power in American Popular Culture |date=June 2017 |publisher=Routledge |location=New York |isbn=978-1-138-91764-4}}
* {{cite book |last=Holton |first=W. C. |title=The Cleanup of Three Mile Island Unit 2 – A Technical History: 1979 to 1990 |publisher=EPRI (Electric Power Research Institute) |location=Palo Alto, California |language=en-us |date=September 1990}} ([http://svcf.jp/pdf/Three_Mile_Iland2NP-6931.PDF PDF])
* {{cite book |last=Kemeny |first=John G. |author-link=John G. Kemeny |title=Report of The President's Commission on the Accident at Three Mile Island: The Need for Change: The Legacy of TMI |url=https://archive.org/details/reportofpresiden0000unit_i4h5 |url-access=registration |publisher=The Commission |location=Washington, D.C. |date=October 1979 |isbn=978-0-935758-00-9}}
*{{cite book |last= Nichols |first= Kenneth | authorlink = Kenneth Nichols |title= The Road to Trinity: A Personal Account of how America's Nuclear Policies were Made  |accessdate=  |edition=  |orig-date=  |year= 1987 |publisher= William Morrow and Company |location= New York |isbn= 0-688-06910-X  |oclc=15223648 |pages= 356–364 }}
* {{cite book |last1=Osif |first1=Bonnie A. |last2=Baratta |first2=Anthony |last3=Conkling |first3=Thomas W. |title=[[TMI 25 Years Later: The Three Mile Island Nuclear Power Plant Accident and Its Impact]] |publisher=Conkling |year=2004 |isbn=978-0-271-02383-0}}
* {{cite book |last=Rogovin |first=Mitchell |author-link=Mitchell Rogovin |title=Three Mile Island: A report to the Commissioners and to the Public, Volume I |publisher=[[U.S. Nuclear Regulatory Commission]], Special Inquiry Group |year=1980 |url=http://www.threemileisland.org/downloads/354.pdf |url-status=usurped |archive-url=https://web.archive.org/web/20101130124554/http://threemileisland.org/downloads//354.pdf |archive-date=November 30, 2010}}
* {{cite book |last=Thornburgh |first=Dick |author-link=Dick Thornburgh |title=Where the Evidence Leads |publisher=[[University of Pittsburgh Press]] |year=2010 |isbn=978-0-8229-6112-3}}
* {{cite book |last=Vilanova |first=Santiago |title=El síndrome nuclear. El accidente de Harrisburg y el riesgo nuclear en España |date=1980 |publisher=Bruguera |isbn=978-84-02-07390-7}}
* {{cite book |last=Walker |first=J. Samuel |title=Three Mile Island: A Nuclear Crisis in Historical Perspective |publisher=[[University of California Press]] |location=Berkeley, California |language=en-us |year=2004 |isbn=978-0-520-23940-1}} ([https://books.google.com/books?id=0EDwbQF5k24C&q=%22three+mile+island%22+%2B+books Google Books])

==External links==
{{Commons category|Three Mile Island accident}}
* [http://www.eia.gov/nuclear/state/pennsylvania/xls/threemileisland.xls TMI web page from the US Department of Energy's Energy Information Administration]
* [http://tmi.dickinson.edu/ "Three Mile Island 1979 Emergency"], a website about the accident, with many reports and other relevant documents created by nearby [[Dickinson College]]
* [http://www.tmia.com Three Mile Island Alert], a nuclear watchdog group founded two years before the TMI accident
* [http://www.tmia.com/node/164 What's wrong with the "fact sheet"] purports to correct errors in the NRC report
* [http://www.efmr.org/ EFMR] citizens radiation monitoring group for the Three Mile Island and Peach Bottom nuclear plants
* [http://alsos.wlu.edu/qsearch.aspx?browse=places/Three+Mile+Island,+Pennsylvania Annotated bibliography for Three Mile Island from the Alsos Digital Library for Nuclear Issues] {{Webarchive|url=https://web.archive.org/web/20140407082527/http://alsos.wlu.edu/qsearch.aspx?browse=places%2FThree+Mile+Island%2C+Pennsylvania |date=2014-04-07 }}
* [http://www.library.pitt.edu/thornburgh/collection/series19.html#tmi Video] {{Webarchive|url=https://web.archive.org/web/20110612134524/http://www.library.pitt.edu/thornburgh/collection/series19.html#tmi |date=2011-06-12 }} and [http://www.library.pitt.edu/thornburgh/collection/series20.html#tmi audio] {{Webarchive|url=https://web.archive.org/web/20110612133057/http://www.library.pitt.edu/thornburgh/collection/series20.html#tmi |date=2011-06-12 }} relating to the Three Mile Island accident, from the [http://www.library.pitt.edu/thornburgh/index.html Dick Thornburgh Papers] {{Webarchive|url=https://web.archive.org/web/20110612132203/http://www.library.pitt.edu/thornburgh/index.html |date=2011-06-12 }} at University of Pittsburgh
* [http://www.ratical.org/radiation/KillingOurOwn/KOO.pdf Killing Our Own] a review of subsequent casualties by Harvey Wasserman and Norman Solomon with Robert Alveraez and Eleanore Walters
* [https://www.sciencedaily.com/releases/2007/01/070129152941.htm Three Mile Island – Failure Of Science Or Spin?, Science Daily]
* [https://www.washingtonpost.com/wp-srv/national/longterm/tmi/whathappened.htm Crisis at Three Mile Island] by ''The Washington Post''
* [https://guides.libraries.psu.edu/tmi Three Mile Island Research and Document Guide] at Penn State University Libraries
* {{cite web |url=https://www.gao.gov/assets/emd-80-109.pdf |title=Three Mile Island: The Most Studied Nuclear Accident in History |date=September 9, 1980 |website=www.gaonet.gov |publisher=[[Government Accountability Office|U.S. Government Accountability Office]] |oclc=7975712}}
* [http://tapewrecks.blogspot.com/2014/03/radioactive-releases-music-of-three.html Radioactive Releases... The Music of Three Mile Island] – A growing collection of over 300 songs inspired by the TMI accident
* [http://large.stanford.edu/courses/2012/ph241/tran1/docs/188.pdf Report Of The President's Commission On The Accident at Three Mile Island.pdf] – Report Of The President's Commission On The Accident at Three Mile Island
* [https://librivox.org/report-of-the-presidents-commission-on-the-accident-at-three-mile-island/ Report of the President's Commission on the Accident at Three Mile Island] – [[LibriVox]] Public domain audiobook of the final report
* [https://onedrive.live.com/redir?resid=5F13EC2E699BFA49%21169060&authkey=%21AAWj0ULBwOQlSvk&page What Really Happened at Three Mile Island?], a PowerPoint presentation on the TMI-2 cleanup by Lake Barrett, who directed the project for the NRC

{{Coord|40.15329|-76.72534|region:US_type:event|display=title}}
{{U.S. nuclear plants}}
{{Nuclear and radiation accidents and incidents}}
{{Presidency of Jimmy Carter}}
{{Authority control}}

[[Category:Three Mile Island accident| ]]
[[Category:1979 disasters in the United States]]
[[Category:1979 in the environment]]
[[Category:1979 in Pennsylvania]]
[[Category:Radiation accidents and incidents]]
[[Category:1979 industrial disasters]]
[[Category:March 1979 in the United States]]
[[Category:Civilian nuclear power accidents]]
[[Category:Disasters in Pennsylvania]]
[[Category:Environment of Pennsylvania]]
[[Category:INES Level 5 accidents]]
[[Category:History of Dauphin County, Pennsylvania]]
[[Category:Nuclear accidents and incidents in the United States]]
[[Category:Nuclear history of the United States]]
[[Category:Presidency of Jimmy Carter]]
[[Category:Susquehanna River]]