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.
Background
In 1934, Tohoku University professor Hikosaka Tadayoshi's "atomic physics theory" was released. Hikosaka pointed out the huge energy contained by nuclei and the possibility that both nuclear power generation and weapons could be created. In December 1938, the German chemists Otto Hahn and Fritz Strassmann sent a manuscript to Naturwissenschaften reporting that they had detected the element barium after bombarding uranium with neutrons; simultaneously, they communicated these results to Lise Meitner. Meitner, and her nephew Otto Robert Frisch, correctly interpreted these results as being nuclear fission and Frisch confirmed this experimentally on 13 January 1939. Physicists around the world immediately realized that chain reactions could be produced and notified their governments of the possibility of developing nuclear weapons.World War II
The leading figure in the Japanese atomic program was Yoshio Nishina, a close associate of Niels Bohr and a contemporary of Albert Einstein. Nishina had co-authored the Klein–Nishina formula. Nishina had established his own Nuclear Research Laboratory to study high-energy physics in 1931 at RIKEN Institute, which had been established in 1917 in Tokyo to promote basic research. Nishina had built his first cyclotron in 1936, and another, 220-ton cyclotron in 1937. In 1938 Japan also purchased a cyclotron from the University of California, Berkeley.File:RIKENFirstCyclotron.jpg|thumb|Dr. Yoshio Nishina completed this "small" cyclotron in 1937, the first cyclotron constructed in Japan, and one of the few outside the United States.
Due to the German-Japanese alliance resulting from Germany's Four Year Plan, Japan and its military had already been pursuing nuclear science to catch up to the West in nuclear technology. This allowed for Nishina to introduce quantum mechanics to Japan.
In 1939, Nishina recognized the military potential of nuclear fission, and was worried that the Americans were working on a nuclear weapon which might be used against Japan.
In August 1939, Hungarian-born physicists Leo Szilard and Eugene Wigner drafted the Einstein–Szilard letter, which warned of the potential development of "extremely powerful bombs of a new type".
The United States started the investigations into fission weapons in the United States, which eventually evolved into the massive Manhattan Project, and the laboratory from which Japan purchased a cyclotron became one of the major sites for weapons research.
In the early summer of 1940, Nishina met Lieutenant-General Takeo Yasuda on a train. Yasuda was at the time director of the Army Aeronautical Department's Technical Research Institute. Nishina told Yasuda about the possibility of building nuclear weapons. However, the Japanese fission project did not formally begin until April 1941 when Yasuda acted on Army Minister Hideki Tōjō's order to investigate the possibilities of nuclear weapons. Yasuda passed the order down the chain of command to Viscount Masatoshi Ōkōchi, director of the RIKEN Institute, who in turn passed it to Nishina, whose Nuclear Research Laboratory by 1941 had over 100 researchers.
B-Research
Meanwhile, the Imperial Japanese Navy's Technology Research Institute had been pursuing its own separate investigations, and had engaged professors from the Imperial University, Tokyo, for advice on nuclear weapons. Before the Attack on Pearl Harbor in 1941, Captain Yoji Ito of the Naval Technical Research Institution of Japan initiated a study that would allow for the Japanese Navy to use nuclear fission. After consulting with Professor Sagane at Tokyo Imperial University, his research showed that nuclear fission would be a potential power source for the Navy.This resulted in the formation of the Committee on Research in the Application of Nuclear Physics, chaired by Nishina, that met ten times between July 1942 and March 1943. After the Japanese Navy lost at Midway, Captain Ito proposed a new type of nuclear weapons development designated as "B-Research" by the end of June 1942. By December, deep in the project, it became evident that while an atomic bomb was feasible in principle, "Japanese scientists believed that it would be difficult for even the United States to realize the application of atomic energy in time to influence the outcome of the war." This caused the Navy to lose interest and to concentrate instead on research into radar.
Ni-Go Project
In 1942 the Army was not discouraged, and soon after the Committee issued its report it set up an experimental project at RIKEN, the Ni-Go Project. Its aim was to separate uranium-235 by thermal diffusion, ignoring alternative methods such as electromagnetic separation, gaseous diffusion, and centrifugal separation.By spring 1944, the Nishina Project barely made any progress due to insufficient uranium hexafluoride for its Clusius tube. The previously provided uranium within the copper tube had corroded and the project was unable to separate U-235 isotopes.
By February 1945, a small group of scientists had succeeded in producing a small amount of material in a rudimentary separator in the RIKEN complex—material which RIKEN's cyclotron indicated was not uranium-235. The separator project came to an end in March 1945, when the building housing it was destroyed by a fire caused by the USAAF's Operation Meetinghouse raid on Tokyo. No attempt was made to build a uranium pile; heavy water was unavailable, but Takeuchi Masa, who was in charge of Nishina's separator, calculated that light water would suffice if the uranium could be enriched to 5–10% uranium-235.
While these experiments were in progress, the Army and Navy searched for uranium ore, in locations ranging from Fukushima Prefecture to Korea, China, and Burma. The Japanese also requested materials from their German allies and of unprocessed uranium oxide was dispatched to Japan in April 1945 aboard the submarine, which however surrendered to US forces in the Atlantic following Germany's surrender. The uranium oxide was reportedly labeled as "U-235", which may have been a mislabeling of the submarine's name and its exact characteristics remain unknown; some sources believe that it was not weapons-grade material and was intended for use as a catalyst in the production of synthetic methanol to be used for aviation fuel.
The attack also effectively destroyed the Clusius tube and any chances of the Japanese producing an atomic bomb in time to influence the war in their favor and rival the West in nuclear weaponry.
According to the historian Williams, "The same lack of sufficient high quality uranium that had impeded the German atomic project had also, as it turned out, obstructed Japanese attempts to make a bomb." This was the conclusion of the Manhattan Project Intelligence Group, who also reported Japan's nuclear physicists were just as good as those from other nations.
F-Go Project
In 1943, a different Japanese naval command began a nuclear research program, the F-Go Project, under Bunsaku Arakatsu at the Imperial University, Kyoto. Arakatsu had spent some years studying abroad including at the Cavendish Laboratory at Cambridge under Ernest Rutherford and at Berlin University under Albert Einstein. Next to Nishina, Arakatsu was the most notable nuclear physicist in Japan. His team included Hideki Yukawa, who would become in 1949 the first Japanese physicist to receive a Nobel Prize.Early on in the war Commander Kitagawa, head of the Navy Research Institute's Chemical Section, had requested Arakatsu to carry out work on the separation of Uranium-235. The work went slowly, but shortly before the end of the war he had designed an ultracentrifuge which he was hopeful would achieve the required results. Only the design of the machinery was completed before the Japanese surrender.
File:拆除中的荒勝研究室加速器.png|thumb|left|Bunsaku Arakatsu's accelerator demolition by GHQ, 24 November 1945.
After Arakatsu and Nishina's meeting, in spring 1944, the Army-Navy Technology Enforcement Committee formed due to lack of progress in the development of Japanese nuclear weapons. This led to the only meeting of the leaders of the F-Go Project scientists, on 21 July 1945. After the meeting, nuclear weaponry research ended as a result of the destruction of the facility that housed isotope separation research, known as Building 49.
Shortly after the surrender of Japan, the Manhattan Project's Atomic Bomb Mission, which had deployed to Japan in September, reported that the F-Go Project had obtained 20 grams a month of heavy water from electrolytic ammonia plants in Korea and Kyushu. In fact, the industrialist Jun Noguchi had launched a heavy water production program some years previously. In 1926, Noguchi founded the Korean Hydro Electric Company at Konan in north-eastern Korea: this became the site of an industrial complex producing ammonia for fertilizer production. However, despite the availability of a heavy-water production facility whose output could potentially have rivalled that of Norsk Hydro at Vemork in Norway, it appears that the Japanese did not carry out neutron-multiplication studies using heavy water as a moderator at Kyoto.