The origins of NRX trace back to a clandestine laboratory at the University of Montreal, where during World War II, teams of Canadian, British, and European scientists worked on heavy-water reactor research under conditions of strict secrecy. When the decision came to build a full-scale research reactor, the chosen site was a remote stretch of forest along the Ottawa River, roughly 180 kilometers northwest of the capital. Construction was contracted to Defence Industries Limited and subcontracted to Fraser Brace Ltd. On July 22, 1947, the NRX -- National Research Experimental -- achieved its first sustained nuclear reaction. At 10 megawatts thermal, it was Canada's most expensive science facility, and within a few years its power output climbed to 42 megawatts. The reactor's heavy-water moderator gave it a neutron flux 10 to 20 times greater than comparable graphite reactors, making it an unmatched tool for physics research and isotope production.

NRX quickly proved itself far more than an engineering curiosity. In the early 1950s, the reactor became the world's only source of cobalt-60, a radioactive isotope that physicians first used to bombard cancerous tumours in 1951. That breakthrough helped launch the modern era of radiation therapy, saving countless lives over the decades that followed. The reactor's intense neutron beams also attracted physicists who would push the boundaries of condensed matter research. Dr. Bertram Brockhouse conducted his pioneering neutron scattering experiments at NRX during the 1950s, work so fundamental that it earned him a share of the 1994 Nobel Prize in Physics. The reactor's design influence extended further still -- the NRX blueprint was adapted to build India's CIRUS reactor, which later produced the plutonium for India's first nuclear weapon test, Operation Smiling Buddha, in 1974. A single Canadian reactor, built for peaceful research, left fingerprints on both life-saving medicine and geopolitical upheaval.

The accident began with a test and a misunderstanding. On that December morning in 1952, operators were running the reactor at low power when a supervisor noticed control rods being pulled from the core -- an operator in the basement had incorrectly opened pneumatic valves. The valves were closed, but some rods stuck in nearly withdrawn positions while their status lights falsely showed them as lowered. Then came a fatal miscommunication: when the supervisor asked the control room to lower the rods, the operator pressed the wrong buttons, accidentally withdrawing the safeguard bank of four additional rods. Power began doubling every two seconds. The operator tripped the reactor, but three of the safeguard rods refused to drop. Within ten seconds, power surged to 17 megawatts. Cooling water boiled and ruptured its temporary hoses. Fourteen seconds later, operators manually opened valves to drain the heavy water moderator. Power spiked to an estimated 100 megawatts before finally collapsing as the moderator drained away. Fuel elements melted, hydrogen gas accumulated, and an oxyhydrogen explosion rocked the calandria. Radioactive fission products vented into the atmosphere.

The damaged reactor core was so contaminated that it had to be removed and buried. Admiral Hyman Rickover, father of the US nuclear submarine program, requested permission to send 150 Navy personnel to Chalk River -- not out of charity, but to gain firsthand experience with nuclear contamination cleanup. Among those Americans was a young lieutenant named Jimmy Carter, who led a team of twelve men into the irradiated facility. The cleanup was primarily carried out by 850 Atomic Energy of Canada staff, supplemented by 170 Canadian military personnel, the US contingent, and 20 contractors. The damaged calandria was extracted, a new and improved core was designed and installed, and just 14 months and 5 days after the accident, NRX was back online. The reactor would continue operating for another four decades, finally shutting down permanently on March 30, 1993, after 45 years of service.

The 1952 NRX incident was the world's first severe nuclear reactor accident, predating Three Mile Island by nearly three decades. The lessons extracted from those 108 seconds of chaos became foundational principles of reactor design: diversity and independence of safety systems, guaranteed shutdown capability, and careful attention to the human-machine interface. NRX's own design -- the calandria, the heavy-water moderator, the vertical fuel channels -- became the architectural ancestor of Canada's CANDU reactor fleet, which today generates power around the world. The Chalk River site where NRX once operated is now undergoing decommissioning, a slow and painstaking process of dismantling a facility that shaped nuclear science, medicine, and safety for half a century.