In the fall of 1940, the Japanese army concluded that constructing an atomic bomb was indeed feasible. The Institute of Physical and Chemical Research, or Rikken, was assigned the project under the direction of Yoshio Nishina. The Japanese Navy was also diligently working to create its own “superbomb” under a project was dubbed F-Go, headed by Bunsaku Arakatsu at the end of World War II. The F-Go program began at Kyoto in 1942. However, the military commitment wasn’t backed with adequate resources, and the Japanese effort to an atomic bomb had made little progress by the end of the war.
Japan’s nuclear efforts were disrupted in April 1945 when a B-29 raid damaged Nishina’s thermal diffusion separation apparatus. Some reports claim the Japanese subsequently moved their atomic operations to Konan [Hungnam, now part of North Korea]. The Japanese may have used this facility for making small quantities of heavy water. The Japanese plant was captured by Soviet troops at war’s end, and some reports claim that the output of the Hungnam plant was collected every other month by Soviet submarines.
There are indications that Japan had a more sizable program than is commonly understood, and that there was close cooperation among the Axis powers, including a secretive exchange of war materiel. The German submarine U-234, which surrendered to US forces in May 1945, was found to be carrying 560 kilograms of Uranium oxide destined for Japan’s own atomic program. The oxide contained about 3.5 kilograms of the isotope U-235, which would have been about a fifth of the total U-235 needed to make one bomb. After Japan surrendered on 15 August 1945, the occupying US Army found five Japanese cyclotrons, which could be used to separate fissionable material from ordinary uranium. The Americans smashed the cyclotrons and dumped them into Tokyo Harbor.