NRX Reactor

Following World War II, Canada found itself with the world’s second-largest nuclear infrastructure (after the U.S.), and in a position to lead the world in sub-atomic research. It continued its support of the British and United States weapons programs, while continuing efforts to develop a powerful nuclear research reactor. In 1947, the 20 megawatt National Research Experimental (NRX) came online at the Chalk River facility. NRX was always the focus of the National Research Council’s Atomic Project. It was designed to be the workhorse of science and innovation. NRX was for a time the world’s most powerful research reactor, by the mid-1950s producing over 40 million watts of heat and vaulting Canada into the forefront of physics research.

Although its original purpose was to demonstrate pilot scale, production of weapons-grade plutonium for the U.S., which it did until the early 1950s, NRX had mechanisms for producing isotopes and channels for conducting experiments with beams of neutrons. It had a great many beam ports around its perimeter to allow neutrons and gamma rays out, as well as locations where experiments could be inserted into the core. At the time, nuclear science and technology was in its infancy. National Research Council (NRC) scientists made sure that NRX was a versatile and a flexible machine.

NRX Reactor
NRX Reactor Hall – the core is inside the central cylindrical structure. Experiments are crowded around beam holes.

One early innovation that arose was the production of isotopes. NRX, like ZEEP, used natural uranium as its fuel, and heavy water as its moderator. Once a nuclear chain reaction was started, it would provide an intense and constant source of neutrons that offer a far more efficient way to manufacture isotopes than any method previously available.

Using the vast numbers of free neutrons in the core of NRX, atoms of an element could capture neutrons and become heavier isotopes of the same element. Initially, small quantities of isotopes were produced — mainly for use in universities as chemical tracers.

But a revolutionary innovation was on the horizon: the field of modern nuclear medicine. In three years, Canada had built a world-class nuclear laboratory campus at Chalk River and at its centre, the world’s most powerful nuclear reactor: an intense source of neutrons.

Nuclear Medicine

Up to that point, radium had been used in certain medical treatments. However, scientists recognized that cobalt-60 could provide an intense source of gamma radiation that could be used to kill cancer cells.

Global nuclear medicine largely started in Canada in 1951, when the first two cancer-treatment machines using cobalt-60 (radioisotopes) were built. The world’s first colbalt-60 cancer-therapy machine was built by Eldorado Mining and Refining Ltd. (later to become part of Atomic Energy of Canada Limited), and operated at Victoria Hospital in London, Ontario, on October 27, 1951. Dr. Harold Johns, a Canadian physicist responsible for radiation therapy for the Saskatchewan Cancer Commission in Saskatoon, also designed a cobolt-60 beam therapy unit that first treated a patient in Saskatoon on November 8, 1951, just 12 days after the Eldorado unit was first used in London, Ontario. For many years, Canada was the only country that could produce cobalt-60, and today it supplies over 80% of the world’s demand.

A major disruption of NRX’s isotope supply began on December 12, 1952, when the reactor suffered a partial meltdown due to operator error and mechanical problems in the shut-off systems. Some fuel cladding burst, and as a result there was a release of radioactive material, mostly contained within the NRX building. Clean-up of the site required several months with the reactor core being removed and buried, and a new core put in its place. The refurbished reactor was operating again, this time at a higher power level (ultimately 42 megawatts), within two years. The lessons learned in the 1952 accident advanced the field of reactor safety significantly, and the concepts it highlighted (diversity and independence of safety systems, guaranteed shutdown capability, efficiency of man-machine interface) became fundamentals of reactor design.

While NRX was initially designed during wartime with a military role in mind, its designers had many other scientific and industrial uses in mind as well. While scientists flocked from around the world to Chalk River to conduct research with this new and powerful tool, one of the first applications explored was the production of medical isotopes such as iodine-131, phosphorus-32, carbon-14, and cobalt-60.

Growing from that first innovation, Canada is now home to the world’s largest medical isotope industry. After 45 years of esteemed service, NRX was taken offline in 1992.