How far Japan's wartime atomic research progressed

Japan's wartime atomic research remained far from a usable bomb despite Yoshio Nishina's expertise. Material shortages are distinguished from uncertain accounts of calculation errors and scientists' intentions.

Question

How close was Japan to developing an atomic bomb during the Second World War?

Short answer

Japan had capable physicists but remained at an early stage and lacked the materials needed to produce a wartime bomb.

Notes

Japan's atomic programme is assessed as less advanced than Germany's wartime effort. One panelist, recalling a single book read some time earlier, attributes part of the problem to badly mistaken critical-mass calculations and explicitly cautions against relying too heavily on that recollection. Yoshio Nishina's position at Riken and access to cyclotrons gave him substantial influence over Japanese nuclear research. Material shortages made a wartime weapon impractical, while projects could still support his scientific work. Clarke separately speculates that some senior scientists may have agreed to calculations that discouraged military development. That suggestion is an interpretation of their possible intentions, whose role in the programme's failure remains uncertain.