NRX

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. NRX was remarkable for its heat output and the number of free neutrons it generated. In the late 1940s, NRX was the most intense neutron source in the world.

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. 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.