There are several evidences that Japan had a nuclear program more developed than is usually imagined, and also that there was a close cooperation between the Axis powers, including trade of secret war materials.
The fact that the leading physicists in Japan were collected from mid-1940 to research the military applications of nuclear fission is not new.
The German submarine U-234, which surrendered to U.S. forces in May 1945 after the surrender of Germany, was carrying, among other things, 560 kg of uranium oxide for the nuclear program that was devised to produce the atomic bomb
The uranium oxide contained about 3.5 kg of U-235 isotope, about one fifth of the total of U-235 required to produce a bomb.
Throughout the 1930s, Japan was equal to the developments in experimental physics of the greatest powers. The renowned Japanese scientist Yoshio Nishina close associate of Niels Bohr and a contemporary of Albert Einstein, inclusive, had worked several years in Copenhagen Niels Bohr laboratories.
The Japanese were also at an advanced stage in construction techniques of cyclotrons, with a small device installed in Tokyo.
After the Japanese surrender on August 15, 1945, the occupying U.S. Army found five Japanese cyclotrons that could be used to separate fissile uranium common. The Americans destroyed the cyclotron and threw what was left of them in Tokyo Bay.
In the autumn of 1940, Japanese Army studies concluded that the construction of an atomic bomb was feasible. The years 1940 and 1941 were a period of intense military interest in the possibilities of atomic weapons.
In 1941, the then Prime Minister and War Minister Hideki Tojo
determined that the project (with code name "Ni-Go") of bomb sponsored by the Japanese Army was would be performed in the Institute for Physical and Chemical Research (RIKEN), with more than 100 researchers under the direction of Yoshio Nishina.
The Japanese Navy also joined forces to create their own "superbomb" under the project named "F-Go" (F from fission), led by physicist Bunsaku Arakatsu.
In 1943, in Kyoto, the Navy project started with the main objective of replacing oil by nuclear energy. Oil became a precious commodity increasingly scarce, undermining the Japanese war effort.
The Institute for Physical and Chemical Research and a research facility in Hungnam, in the occupied Korea, also contributed to these efforts.
However, the scientific enterprise and military was not provided with adequate resources, and effort of the Japanese Navy to manufacture the atomic bomb had little progress until the end of the war.
The nuclear efforts in Japan were interrupted in April 1945, when a raid of B-29 bombers damaged the equipment of thermal diffusion separation of Yoshio Nishina.
Some accounts claim that Japanese moved their atomic research operations to Konan (now Hungnam in North Korea) which, even according to the German historian Rainer Karlsch Japanese scientists were planning to carry out a test of an atomic bomb on 12 August 1945.
The Japanese may have used this facility to produce small amounts of heavy water. However, this "factory" Japanese was captured by Soviet troops at the end of the war, and there are indications that the production of Hungnam plant was monthly collected by Soviet submarines.