理研の「ウラニウム爆弾」構想 : 第二次世界大戦期の日本の核兵器研究

抄録

Soon after the dropping of the Hiroshima bomb, Yoshio Nishina, an experimental physicist who was in charge of the Army's development of nuclear weapons at Riken, the Institute of Physical and Chemical Research, could understand that it was an atomic bomb because its energy release given in Truman's statement coincided with the one that his colleague Hidehiko Tamaki estimated a few years ago. This suggests that they knew of the magnitude of nuclear explosions. Uraniumu bakudan (uranium bomb), Japanese physicists' bomb at the time, is, however, known to be a kind of nuclear reactor out of control. The "bomb" of this kind is not very powerful because it is based on a slow-neutron reaction. This paper challenges to reproduce Japanese physicists' calculations at the time, and shows that they thought that they could explode their uraniumu bakudan, a slow- reactor bomb, with a quite high efficiency. This led them to expect that the energy release from their bomb would be of 20 K ton TNT equivalence that accidentally coincided with the energy release of the Hiroshima bomb.