Richenbacker was a WW1 ace. He wrongly assumed that any radioactive materials were uranium. Plenty of other radioactive materials had military uses in 1940s.
On August 2nd, 1939, Albert Einstein wrote a, now famous letter, to President Roosevelt. In the letter, he related his concern regarding the fact that the Germans were working on a new bomb to be fueled by nuclear fission, and because of that, they had halted their exports of URANIUM.
Given this information, I question your response to my entry. Why, with that knowledge, would we be sending uranium to the USSR? And, surely FDR was aware of activities carried out by American Lend-Lease planes.