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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. At the time of its construction it was Canada's most expensive science facility and the world's most powerful nuclear research reactor. NRX was remarkable both in terms of its heat output and the number of neutrons it generated. When a nuclear reactor is operating its nuclear chain reaction generates many billions of free neutrons, and in the late 1940s NRX was the most intense neutron source in the world.

NRX experienced one of the world's first major reactor accidents 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 restarted within two years. NRX operated for 45 years, being shut down permanently on 8 April 1993. It is currently undergoing decommissioning 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 construction of a successor facility, NRU, which went critical in 1957.