All this talk about U-235 or things labeled 'U-235' is bonkers. U235 is 0.7% (0.007 by weight or 7 parts per thousand) of natural U, and because it has identical chemistry to U238 (99.3% or 993 parts per thousand of natural U), it is horrendously tough to separate. (Only U-235 is fissile.) The Y-12 plant in Oak Ridge, which produced the U235 for the Little Boy Hiroshima bomb, was enormous. The plant used 15,000 tons of silver from the US silver reserves, due to a copper shortage, in the calutrons (cyclotrons) at the plant. There were 1,152 of these huge things at the Y-12 complex which took 22,000 to build and employed over 5,000 people to run. An entire city was built around it and the K25 gaseous diffusion plant, Oak Ridge, TN. It would have taken at least this much effort for the Japanese to even have hope of building a uranium fission weapon. Even to this day, uranium enrichment is a tremendously difficult process. A few bombed cyclotrons in Japan stopping them from building a bomb is an idea that is too stupid for words.