Spot on ColinB
Farm Hall indeed settles the matter about why the remaining aryan physicists' pooch was walking tentatively around Germany in mid-1945.
Some tunnels under Mauthausen with Uranium in them (anyone think to check the geology?) does not a combination of the Oak Ridge, Argonne, Los Alamos and Hanford sites make.
The USSR was already deeply engaged in nuclear weapons development, despite wartime stringency; but then, just the knowledge that a simple reactor-based plutonium programme was underway makes the road to executing one rather easier in the face of a demanding and despotic leader.