Chalk River: One of the Most Powerful Nuclear Reactors of its Era

The NRX in 1955. © Library and Archives Canada
A sample being removed from the self-serve unit of the NRX reactor at the Chalk River Laboratories in 1955. © Library and Archives Canada

For the week of Monday, July 19, 2021.

On July 22, 1947, the National Research Experimental (NRX) reactor at Chalk River Laboratories in Chalk River, Ontario, became operational.

Canadian scientific research into nuclear physics dates back to Ernest Rutherford’s work, between 1898 and 1907, on atomic theory at McGill University in Montréal, Quebec, where he discovered the existence of alpha and beta radiation. Academic understanding had advanced considerably by the time Canada’s first nuclear reactor, the Zero Energy Experimental Pile (ZEEP), went online in September 1945. ZEEP, the first nuclear reactor outside the United States, continued operating until 1970, initially at 1 watt (thermal) and later at 250 watts. In 1947, the much more powerful NRX reactor began operating at 10 megawatts, designed for a maximum output of 20 megawatts. The NRX was upgraded several times, eventually reaching 42 megawatts.

In 1946, physicist Wilfrid Bennett Lewis was made director of the Chalk River Laboratories, which was home to the NRX, the world’s most powerful research reactor for several years. As an international centre for atomic energy, the Chalk River Laboratories helped position Canada at the forefront of global physics research. It produced radionuclides and neutrons for scientific and medical research, conducted gamma ray experimentation, and assisted the United States with their program of nuclear-powered submarines. The NRX operated under the National Research Council of Canada until 1952, when the Division of Atomic Energy became the newly-formed Crown Corporation named Atomic Energy Canada Limited (AECL).

In December 1952, an accident occurred with the operation of the NRX. Misinformation from the control room, in conjunction with defective shut-off rods, caused the reactor to overheat. It released a large amount of radioactive material from the fuel, some of which escaped into the reactor building and the surrounding environment. Canadian Army and United States Navy personnel worked with facility staff to assist in the cleanup, and the reactor was rebuilt and returned to operation within two years.

Based partially on the success of the NRX over its years of operation, AECL proposed a new design for a nuclear reactor generating power for commercial use. This led AECL, Canadian General Electric, and Ontario Hydro to collaborate on building a prototype nuclear power plant that first operated in 1962. The Nuclear Power Demonstration, as it was known, was the first of the Canada Deuterium Uranium (CANDU) prototypes, a reactor now used in seven countries.

The NRX shut down in January 1992. As Canada’s first large-scale endeavour into nuclear reactor research, it paved the way for future innovations, including the larger National Research Universal research reactor and the CANDU power reactors.

Ernest Rutherford is a designated national historic person. The Historic Sites and Monuments Board of Canada (HSMBC) advises the Government of Canada on the commemoration of National Historic Persons—individuals who have made unique and enduring contributions to the history of Canada.

The National Program of Historical Commemoration relies on the participation of Canadians in the identification of places, events and persons of national historic significance. Any member of the public can nominate a topic for consideration by the Historic Sites and Monuments Board of Canada. Information on how to participate in this process is available here:

Learn more about Parks Canada’s approach to public history by checking out the Framework for History and Commemoration (2019) on our website.