Japanese nuclear weapons program

During World War II, the Empire of 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, they were comparatively small, suffered from Allied air raids, shortages, disarray, and did not progress beyond the laboratory stage.

The Imperial Japanese Army initiated the "Ni-Go Project" for nuclear weapons at the RIKEN institute, led by physicist Yoshio Nishina. Work was limited to cyclotron research, production of small quantities of uranium hexafluoride, and an unsuccessful attempt to enrich it via thermal diffusion in a Clusius tube.

The Imperial Japanese Navy also supported the "F-Go Project", at Kyoto Imperial University, led by physicist Bunsaku Arakatsu and involving Hideki Yukawa. This group focused on designing an ultracentrifuge to enrich uranium hexafluoride, but did not construct any before the end of the war.

Japan has not since had a nuclear weapons program. Its postwar constitution is interpreted to forbid its possession of weapons of mass destruction. It is party to the Nuclear Non-Proliferation Treaty, and has enacted laws affirming its Three Non-Nuclear Principles: to not possess, manufacture, or permit the introduction of nuclear weapons. However, the United States stationed nuclear weapons in Japan and in the Ryukyu Islands from the 1950s until 1972.

Japan is today considered an exemplar latent or threshold nuclear state, capable of developing weapons in a short timespan. It is unique among non-nuclear weapons states in that it possesses a full nuclear fuel cycle and plutonium stockpile, as part of its civilian nuclear energy industry, and advanced developments in the industries necessary to make nuclear weapons.