Canada's nuclear credentials stretched back decades before the laboratory opened. Ernest Rutherford had conducted pioneering research at McGill as early as 1899. In 1940, George Laurence of the National Research Council began packing bags of uranium dioxide from the Eldorado Mine at Port Radium into a wooden bin lined with paraffin wax, trying to demonstrate that a nuclear reactor was feasible. His experiments failed -- impurities in the petroleum coke moderator absorbed too many neutrons -- but they put Canada on the atomic map. Meanwhile, Cominco's smelting plant in Trail, British Columbia, was quietly producing heavy water, a substance that had been selling for up to $1,130 per pound. When the French refugee scientists needed somewhere to continue their reactor work, Canada offered both the raw materials and the political neutrality the project demanded.

The Montreal Laboratory began in borrowed quarters at McGill before moving to permanent space at the Universite de Montreal in March 1943. The team was deliberately international: Goldschmidt and Pierre Auger from France, Placzek from Czechoslovakia, Paneth and von Halban from Austria, R. E. Newell and F. R. Jackson from Britain, joined by Canadians George Volkoff, Bernice Weldon Sargent, George Laurence, and gifted young researchers like J. Carson Mark and Leo Yaffe. But brilliance alone could not build a reactor. Although Canada possessed vast uranium ore deposits and heavy water production capacity, the Americans controlled access to both. Anglo-American cooperation broke down, leaving the Montreal scientists stranded without the materials they needed. It took the Quebec Agreement of 1943, merging Britain's Tube Alloys program with the Manhattan Project, to break the impasse and get American help flowing again.

John Cockcroft arrived as the new director in May 1944, and the program's center of gravity shifted 200 kilometers northwest to the Chalk River Laboratories in Ontario. The Montreal team designed two reactors there. The small ZEEP went critical on September 5, 1945 -- just weeks after the bombing of Hiroshima -- and the larger NRX followed on July 22, 1947. Originally designed at 10 megawatts, the NRX was later upgraded to 42 MW through engineering improvements, replacing stainless-steel-clad uranium rods cooled by heavy water with aluminum-clad rods cooled by light water. For a time, NRX stood as the most powerful research reactor in the world, providing Britain, the United States, and Canada with fissile plutonium, uranium-233, and medical isotopes like phosphorus-32. By the end of 1946, the entire Montreal Laboratory program had cost an estimated US$22.2 million, excluding heavy water -- a bargain for a project that advanced reactor science by years.

On September 5, 1945, the same day ZEEP went critical, a Soviet cipher clerk named Igor Gouzenko walked out of the USSR's Ottawa embassy carrying copies of cables that exposed a GRU espionage network operating inside Canada's nuclear program. Among the agents was Alan Nunn May, a British physicist who had secretly handed tiny samples of uranium-233 and uranium-235 to a Soviet handler. Fred Rose, a sitting member of Parliament, was implicated. Bruno Pontecorvo, who defected to the Soviet Union in 1950, remains the prime suspect for leaking blueprints of the NRX reactor. When the spy ring became public in February 1946, American trust in sharing nuclear secrets with its allies evaporated almost overnight.

The Montreal Laboratory had been a triumph of international cooperation, but the peace corroded everything. In November 1945, the British announced Cockcroft's appointment to head a new atomic research establishment in England -- without consulting Canada, and while the NRX was still under construction. The Canadians regarded it as a slap in the face. Then, at a Combined Policy Committee meeting in February 1946, the British announced plans to build their own reactor in the UK, again without telling Ottawa first. An outraged C. D. Howe instructed ambassador Lester B. Pearson to declare nuclear cooperation between Britain and Canada finished. Anglo-American collaboration collapsed weeks later when Truman announced the United States would not help Britain build a plutonium production reactor. The Montreal Laboratory closed in July 1946, its wartime alliances dissolved into mutual suspicion, but its reactors at Chalk River kept running -- monuments to what three nations built together before politics pulled them apart.