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{{Short description|Retired research nuclear reactor}}
{{for multi|the "NRX" series of [[nuclear thermal rocket]] designs|NERVA|the Neoreactionary movement (NRx)|Dark Enlightenment|the racing series|Nitro Rallycross}}
'''NRX''' ('''National Research Experimental''') was a [[heavy-water-moderated]], light-water-cooled, [[nuclear research reactor]] at the Canadian [[Chalk River Laboratories]], which came into operation in 1947 at a design power rating of 10 MW (thermal), increasing to 42 MW by 1954. It was Canada's most expensive science facility and the world's most powerful nuclear research reactor at its construction.<ref>{{Cite web |url=http://www.nrc-cnrc.gc.ca/aboutUs/nrc90/achievements/ww2effort_e.html |title=Scientific Research Helps World War II War Effort - National Research Council Canada<!-- Bot generated title --> |access-date=2007-09-06 |archive-url=https://web.archive.org/web/20080225145407/http://www.nrc-cnrc.gc.ca/aboutUs/nrc90/achievements/ww2effort_e.html |archive-date=2008-02-25 |url-status=dead }}</ref>
NRX was remarkable for its heat output and the number of [[free neutron]]s it generated. In the late 1940s, NRX was the most intense neutron source in the world.{{cn|date=January 2025}}

NRX experienced the world's first major reactor accident outside of Russia on 12 December 1952. The reactor began operation on 22 July 1947 under the [[National Research Council of Canada]] and was taken over by [[Atomic Energy of Canada Limited]] (AECL) shortly before the 1952 accident. The accident was cleaned up, and the reactor was restarted within two years. NRX operated for 45 years, then shut down permanently on 30 March 1993.<ref>{{cite web
 |url=https://nucleus.iaea.org/RRDB/RR/HeaderInfo.aspx?RId=26
 |title=IAEA Research Reactor Database
 |access-date=2017-10-19
}}</ref> Decommissioning is underway at the Chalk River Laboratories site.

NRX was the successor to Canada's first reactor, [[ZEEP]]. Because the operating life of a research reactor was not expected to be very long, in 1948, planning started for the construction of a successor facility, the [[National Research Universal reactor]], which started self-sustained operation (or "went critical") in 1957.

== Design ==
{{Unreferenced section|date=May 2009}}

Two main processes govern a heavy water-moderated reactor. First, the water slows down ([[neutron moderator|moderates]]) the [[neutron]]s which are produced by nuclear fission, increasing the chances of the high energy neutrons causing further fission reactions. Second, [[control rod]]s absorb [[neutron]]s and adjust the power level or shut down the reactor in the course of regular operation. Inserting the control rods or removing the heavy water moderator can stop the reaction.

The NRX reactor incorporated a calandria, a sealed vertical [[aluminium]] cylindrical vessel with a diameter of {{convert|8|m}} and height of {{convert|3|m}}. The core vessel held about 175 {{convert|6|cm||spell=in|adj=mid|-diameter}} vertical tubes in a hexagonal lattice, {{convert|14,000|l}} of heavy water and [[helium]] gas to displace air and prevent [[corrosion]]. The water level in the reactor could be adjusted to help set the power level. Fuel elements or experimental items were sitting in the vertical tubes and surrounded by air. This design was a forerunner of the [[CANDU]] reactors.

The fuel elements contained fuel rods {{convert|3.1|m}} long, {{convert|31|mm}} in diameter and weighing {{convert|55|kg}}, containing [[uranium]] fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium [[coolant]] tube with up to {{convert|250|l/s}} of cooling water from the [[Ottawa River]] flowing through it. Between the coolant sheath and the calandria, an airflow of {{convert|8|kg/s}} was maintained.

Twelve of the vertical tubes contained [[control rod]]s made of [[boron carbide]] powder inside steel tubes. These could be raised and lowered to control the reaction, with any seven inserted being enough to absorb sufficient neutrons that no chain reaction could happen. The rods were held up by [[electromagnet]]s so that a power failure would cause them to fall into the tubes and terminate the reaction. A [[pneumatic]] system could use air pressure from above to quickly force them into the reactor core or from below to slowly raise them from it. Four were called the ''safeguard bank'' while the other eight were controlled in an automatic sequence. Two buttons on the main panel in the control room activated magnets to seal the rods to the pneumatic system, and the button to cause the pneumatic insertion of the rods into the core was located a few feet away.

== History ==

[[File:NRX_Pile_Building_and_ZEEP_Building-_Cooling_Tanks_1945.jpg|thumb|right|NRX and Zeep buildings 1945]] NRX was for a time the world's most powerful [[research reactor]], vaulting Canada into the forefront of physics research. Emerging from a [[World War II]] cooperative effort between [[UK|Britain]], the [[United States]], and [[Canada]], NRX was a multipurpose research reactor used to develop new isotopes, test materials and fuels, and produce [[neutron radiation]] beams, that became an indispensable tool in the blossoming field of [[condensed matter physics]].

The nuclear physics design of NRX emerged from the [[Montreal Laboratory]] of Canada's [[National Research Council of Canada|National Research Council]], which was established at the [[University of Montreal]] during WWII to engage a team of Canadian, British, and other European scientists in top-secret heavy-water reactor research. When the decision was made to build the NRX at what is now known as [[Chalk River Laboratories]], the detailed engineering design was contracted to Canada's [[Defence Industries Limited]] (DIL), who subcontracted construction to Fraser Brace Ltd.

In the early days of cancer [[radiation therapy]], the NRX reactor was the world's only source of the isotope [[cobalt-60]], first used to bombard tumours in 1951.<ref>{{cite web |last1=Semeniuk |first1=Ivan |title=Jimmy Carter's atomic bond with Canada |url=https://www.theglobeandmail.com/canada/article-jimmy-carter-nuclear-reactor-chalk-river/ |website=The Globe and Mail |access-date=17 November 2023 |date=19 March 2023 |quote=In the early days of cancer radiation therapy, it also provided the world's only ready source of the radioactive isotope Cobalt-60, which Canadian researchers first used to bombard tumours in 1951.}}</ref>

In 1994, Dr. [[Bertram Brockhouse]] shared the [[Nobel Prize in Physics]] for his work in the 1950s at NRX, which advanced the detection and analysis techniques used in the field of neutron scattering for condensed matter research.

Based on this design, the [[CIRUS reactor]] was built in India. It was ultimately used to produce [[plutonium]] for India's [[Operation Smiling Buddha]] [[nuclear test]].<ref>{{cite book |last = Richelson |first = Jefferey T. |author-link = Jeffrey T. Richelson |title = Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea |date = March 1999 |publisher = WW Norton |isbn = 978-0-393-05383-8 |url-access = registration |url = https://archive.org/details/spyingonbombamer00rich }}</ref>

== Accident ==
On 12 December 1952, the NRX reactor suffered a partial [[nuclear meltdown|meltdown]] due to operator error and mechanical problems in the shut-off systems. For test purposes, some fuel channels had been disconnected from high-pressure water cooling and were connected by hoses to a temporary cooling system; one low-power channel was cooled only by airflow.<ref name=":0">{{Cite web |last=Guillemette |first=Mélissa |date=2022-07-13 |title=Chalk River: The Forgotten Nuclear Accidents {{!}} The Walrus |url=https://thewalrus.ca/nuclear-accidents/ |access-date=2023-03-04 |website=[[The Walrus]] |language=en-US}}</ref>

During tests at low power, with low coolant flux through the core, the supervisor noticed several [[control rod]]s being pulled from the core; an operator in the basement had incorrectly opened pneumatic valves. The wrongly-opened valves were immediately closed, but some control rods did not reenter the core and were stuck in almost withdrawn positions, but still low enough for their status lights to indicate them as lowered. Due to a miscommunication between the supervisor and the control room operator, the wrong buttons were pressed when the supervisor asked to lower the control rods into the core. Instead of sealing the withdrawn control rods to the pneumatic system, the safeguard bank of four control rods was accidentally withdrawn from the core. The operator noticed that the power level was exponentially increasing, doubling every 2 seconds, and tripped the reactor. However, three safeguard control rods were not inserted into the core, and the fourth took an abnormally long time, about 90 seconds, to insert while the power kept rising. After just 10 seconds, the core reached 17 MW(thermal). The cooling water boiled in some tubes connected to the temporary cooling system, and some of them ruptured; the positive [[void coefficient]] of the reactor led to a further power increase. About 14 seconds later, valves were manually opened to drain the heavy water moderator from the calandria. As this took some time to become effective, power increased for five more seconds, peaked at an estimated 100 MW, then went down as the moderator level decreased and was at zero 25 seconds later. The accident, from low to high to zero power, took about 108 seconds. Meanwhile, some [[fuel element failure|fuel elements melted]] and were pierced in several places; the helium cover gas leaked, and the air was aspirated inside. Hydrogen and other gases evolved by radiation-induced cooling water dissociation; 3–4 minutes later, [[oxyhydrogen]] exploded in the calandria. During the incident, some gaseous fission products were vented to the atmosphere, and heavy water in the calandria was contaminated with the cooling water and the fission products.<ref name=":0" />

To remove the fuel [[decay heat]], the water cooling system was kept operating, leaking contaminated coolant to the reactor basement. About {{convert|10|kCi|TBq|lk=on|sigfig=1}} of radioactive materials, contained in about {{convert|4500|m3|gal}} of water,<ref name=":0" /> collected in the basement of the reactor building during the next few days.<ref>[https://books.google.com/books?id=2KYVftKE9NUC&dq=%22nrx+incident%22&pg=PA135 Introduction to Nuclear Power - Collier, Geoffrey Hewitt - Google Books]. Books.google.cz. Retrieved on 2013-12-06.</ref>

Clean-up of the reactor building required several months of work, assisted by 150 US Navy personnel after [[Hyman G. Rickover|Admiral Rickover]] requested permission to send US nuclear submarine personnel to learn about cleaning up nuclear contamination. The US contingent included future US president [[Jimmy Carter]], at the time a lieutenant in the US nuclear submarine program who was in charge of 12 men.<ref>{{cite web|url = https://www.pbs.org/wgbh/americanexperience/features/three-president-carter/ |title = The American Experience: Meltdown at Three Mile Island |publisher = PBS |access-date = 2007-06-29 }}</ref> The NRX [[reactor core]] and calandria, damaged beyond repair, were removed and buried, and an improved replacement was installed; the refurbished reactor started operating 14 months and 5 days after the accident.<ref name=Jedicke>{{cite web |last1=Jedicke |first1=Peter |title=The NRX Incident |url=http://www.cns-snc.ca/media/history/nrx.html |website=Canadian Nuclear Society |publisher=Fanshawe College |access-date=4 June 2019 |location=London, Ontario |date=1989 |archive-date=14 June 2019 |archive-url=https://web.archive.org/web/20190614032616/http://www.cns-snc.ca/media/history/nrx.html |url-status=dead }}</ref> The clean up was primarily performed by 850 Atomic Energy of Canada staff, assisted by about 170 Canadian and 150 US military personnel, and 20 contractors.<ref name=":0" /><ref>{{Cite web |title=Jimmy Carter and the NRX Accident – How Legends Grow – The Society for the Preservation of Canada's Nuclear Heritage |date=23 December 2021 |url=https://nuclearheritage.com/jimmy-carter-and-the-nrx-accident-how-legends-grow/ |access-date=2024-03-18 |language=en-US}}</ref>

The lessons learned in the 1952 accident advanced the field of reactor safety significantly,<ref name=winfield>{{Citation| last1=Winfield | first1 =D.J. | last2=Alsop | first2=C.A. | contribution = Optimization of Safety System Test Frequencies | year = 2003| title = Research Reactor Utilization, Safety, Decommissioning, Fuel and Waste Management| publisher = International Atomic Energy Agency|page=39| citeseerx =10.1.1.127.961 }}</ref> and the concepts it highlighted (diversity and independence of safety systems, guaranteed shutdown capability,<ref name=winfield /> efficiency of [[man-machine interface]]) became fundamentals of reactor design.{{Citation needed|date=April 2019}} The incident was the world's first severe nuclear reactor accident.<ref name=":0" />

==See also==
* [[Chicago Pile-1]]
* [[Nuclear power]]
* [[Nuclear fission]]
* [[Nuclear power plant]]
* [[Nuclear waste]]

==References==
{{Reflist}}

==External links==
* {{cite web|url=http://www.cns-snc.ca/media/history/nrx.html|title=The NRX incident|first=Peter|last=Jedicke|date=1989|publisher=[[Fanshawe College]]|location=[[London, Ontario|London]]|access-date=2010-02-22|archive-date=2016-01-08|archive-url=https://web.archive.org/web/20160108180651/https://cns-snc.ca/media/history/nrx.html|url-status=dead}}
* {{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucacc.html|title=Reactor Accidents|publisher = [[Georgia State University]]}}
* {{cite web|url=http://www.nuclearfaq.ca|title=Canadian Nuclear FAQ|first=Jeremy|last=Whitlock|author-link=Jeremy Whitlock}}
* {{cite web|url=http://www.nuclearheritage.com|title=Society for the Preservation of Canada's Nuclear Heritage, Inc.}}

{{Nuclear power in Canada}}

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[[Category:1947 establishments in Ontario]]
[[Category:1993 disestablishments in Ontario]]
[[Category:Atomic Energy of Canada Limited]]
[[Category:Energy infrastructure completed in 1947]]
[[Category:Government buildings completed in 1947]]
[[Category:Nuclear reactors]]
[[Category:Nuclear accidents and incidents]]
[[Category:Nuclear research reactors]]