Actually I am confident that the full story on how the Soviets got atomic knowledge remains highly classified. For an alternative point of view on that subject read Robert Wilcox's "Japan's Secret War". A very strong argument can be made that the Japanese were far farther in the development of atomic bombs then the Germans and the Soviets got much of their knowlegde from Japanese scientists in what is now North Korea.
What has to always be remembered about the early days of all the nuclear programs is that almost no one knew the dangers of radiation, or that you could screw around with units of plutonium and inadvertently set off uncontrolled fision. We know of one such accident occuring in the Soviet Arctic ~ that was the famous 50 Megaton blast. Another undoubtedly occured in Manchuria and eliminated almost all of the Japanese nuclear scientists.
That's the reason I don't think the Russians got much help from the Japanese.