Given the focus that Soviet intelligence had on nuclear spying in the 1940s, and the extent to which they penetrated the American outposts of the Manhattan Project, did they attempt, or achieve, any theft of information from Canada's Chalk River Nuclear Laboratories? If they did, is it possible that the neutrino physicist Bruno Pontecorvo, who worked there and later defected to the Soviet Union, was involved?
Yes, Chalk River was a target for Soviet intelligence during the era of the atom spies. The Soviets were aware of and had penetrated the British atomic programme, 'Tube Alloys', some time before it was merged into the Manhattan Project in mid-1943. The role of the Montreal and Chalk River Laboratories in that project, coupled with a relatively lax counterintelligence environment made Canada an obvious target.
Canada played host to one of the first and most significant defections of the Cold War. In September 1945, Igor Gouzenko, a cipher clerk working for Soviet military intelligence (GRU) at the embassy in Ottawa defected. During debriefing by the RCMP, MI5 and the FBI, Gouzenko handed Western counterintelligence a treasure trove of evidence pointing to far-reaching and ongoing Soviet intelligence operations in Canada.
One of the dozens of people identified as Soviet agents on the basis of Gouzenko's information was the British physicist Alan Nunn May, who had joined the Tube Alloys project while working at the University of Cambridge and had subsequently been assigned to the Montreal Laboratory, and thence to Chalk River. He was recruited by the GRU in Canada and eventually arrested in 1946, after his return to Britain. It's often difficult to appraise exactly how much or what information a spy handed over; at the very least, May is believed to have provided samples of radioactive isotopes and at least some technical data.
On Pontecorvo, the verdict is mixed. No-one has ever produced decisive proof that he was a spy, but the rule about absence of evidence not being evidence of absence applies. Certainly Pontecorvo was a communist; certainly he defected. It's the question of 'why' that lingers. Some sources (most notably, KGB defector Oleg Gordievsky) insist he was a Soviet agent, but both MI5 and the FBI have kept much of their files on him classified, and nothing has emerged from the KGB archives to conclusively indict him. In any case, the matter is far from settled. The physicist Frank Close published a book on Pontecorvo earlier this year; I haven't read it, but it's received some very good reviews.
For further reading: Christopher Andrew's The Sword and the Shield and Jonathan Haslam's new book Near and Distant Neighbours are good general-purpose histories of Soviet intelligence. Allen Weinstein's The Haunted Wood covers Stalin-era Soviet espionage in the United States, and so prominently features the atom spies.