[jude24]

many Americans are still not aware that in 1943, under the Lend-Lease program, tons of nuclear materials were shipped to the USSR.

Source? One the one hand I could easily believe that, but a claim like that needs a bit more documentation.

[jamaksin]

"Dark Sun - The Making of the Hydrogen Bomb" By Richard Rhodes, Simon & Schuster, New York, 1995. ISBN 0-684-80400-X

Chapter 5 "Super Lend-Lease"
Pages 99-100 lists nuclear materials sent.
FYI - text is available in paperback edition.