During World War II, Japan had several programs exploring the use of nuclear fission for military technology, including nuclear reactors and nuclear weapons. Like the similar wartime programs in Nazi Germany, it was relatively small, suffered from an array of problems brought on by lack of resources and wartime disarray, and was ultimately unable to progress beyond the laboratory stage during the war.
The Institute of Physical and Chemical Research building in Taisho period
The third director of the RIKEN Institute Masatoshi Okochi submitted a report on "Possibility of Uranium Bomb Manufacturing" in May 1941.
Dr. Yoshio Nishina completed this "small" cyclotron in 1937, the first cyclotron constructed outside the United States (and the second in the world).
The second RIKEN cyclotron, completed in 1943
Yoshio Nishina
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Yoshio Nishina was a Japanese physicist who was called "the founding father of modern physics research in Japan". He led the efforts of Japan to develop an atomic bomb during World War II.
Yoshio Nishina, Llewellyn Thomas and Friedrich Hund, in Copenhagen in 1926