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The Japanese program to develop nuclear weapons was conducted during World War II. Like the German nuclear weapons program, it suffered from an array of problems, and was ultimately unable to progress beyond the laboratory stage before the atomic bombings of Hiroshima and Nagasaki and the Japanese surrender in August 1945.
Today, Japan's nuclear energy infrastructure makes it eminently capable of constructing nuclear weapons at will. The de-militarization of Japan and the protection of the United States' nuclear umbrella have led to a strong policy of non-weaponization of nuclear technology, but in the face of nuclear weapons testing by North Korea, some politicians and former military officials in Japan are calling for a reversal of this policy.
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.
The leading figure in the Japanese atomic program was Dr. 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 (the Institute for Physical and Chemical Research), which had been established in 1917 in Tokyo to promote basic research. Nishina had built his first 26-inch (660 mm) cyclotron in 1936, and another 60-inch (1,500 mm), 220-ton cyclotron in 1937. In 1938 Japan also purchased a cyclotron from the University of California, Berkeley.
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. Indeed, in 1939, President Franklin D. Roosevelt started the first investigations into fission weapons in the United States, which eventually evolved into the massive Manhattan Project, and the laboratory from which Japan purchased its own cyclotron would become 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 Tojo's order to investigate the possibilities of nuclear weapons. Yasuda passed the order down the chain of command to Okochi Masatoshi, director of the Riken Institute, who in turn passed it to Nishina, whose Nuclear Research Laboratory by 1941 had over 100 researchers.
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. 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. It concluded in a report that while an atomic bomb was, in principle, feasible, "it would probably be difficult even for the United States to realize the application of atomic power during the war". This caused the Navy to lose interest and to concentrate instead on research into radar.
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 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 two months later when the building housing it was destroyed in 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 carried out searches for uranium ore, in locations ranging from Fukushima Prefecture to Korea, China, and Burma. The Japanese also requested materials from their German allies and 560 kg (1,200 lb) of unprocessed uranium oxide was dispatched to Japan in April 1945 aboard the submarine U-234, 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.
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 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 (to spin at 60,000 rpm) which he was hopeful would achieve the required results. Only the design of the machinery was completed before the Japanese surrender.
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 (now known as Hungnam) 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.
Historian Rainer Karlsch has alleged that shortly before the end of the war US intelligence acquired information to the effect that Japanese scientists had planned to conduct a test of a nuclear weapon near Hungnam on 12 August 1945. However, this could not be verified as the Red Army occupied Konan a few days later, before US occupation authorities could investigate fully.
On 16 October 1945 Nishina sought permission from the American occupation forces to use the two cyclotrons at the Riken Institute for biological and medical research, which was soon granted; however, on 10 November instructions were received from the US Secretary of War in Washington to destroy the cyclotrons at the Riken, Kyoto University, and Osaka University. This was done on 24 November; the Riken's cyclotrons were taken apart and thrown into Tokyo Bay.
In a letter of protest against this destruction Nishina wrote that the cyclotrons at the Riken had had nothing to do with the production of nuclear weapons, however the large cyclotron had officially been a part of the Ni-Go Project. Nishina had placed it within the Project by suggesting that the cyclotron could serve basic research for the use of nuclear power, simply so that he could continue working on the device; the military nature of the Project gave him access to funding and kept his researchers from being drafted into the armed forces. He felt no qualms about this because he saw no possibility of producing nuclear weapons in Japan before the end of the war.
In 1946 the Atlanta Constitution published a story by reporter David Snell, who had been an investigator with the 24th Criminal Investigation Detachment in Korea after the war, which alleged that the Japanese had successfully tested a nuclear weapon near Konan before being captured by the Russians. He claimed that he had received his information at Seoul in September 1945 from a Japanese officer to whom he gave the pseudonym of Captain Wakabayashi, who had been in charge of counter-intelligence at Konan. Although SCAP officials, who were responsible for strict censorship of all information about information of Japan's wartime interest in nuclear physics, were dismissive of Snell's report.
Under the 1947-48 investigation, comments were sought from Japanese scientists who would or should have known about such a project. Further doubt is cast on Snell's story by the lack of evidence of large numbers of Japanese scientists leaving Japan for Korea and never returning. Snell's claims were repeated by Robert K. Wilcox in his 1985 book Japan's Secret War: Japan's Race Against Time to Build Its Own Atomic Bomb. The book also included what Wilcox stated was new evidence from intelligence material which indicated the Japanese might have had an atomic program at Konan. These specific claims were dismissed in a review of the book by Department of Energy employee Roger M. Anders which was published in the journal Military Affairs, an article written by two historians of science in the journal Isis and another article in the journal Intelligence and National Security.
In 1946 talking about his wartime efforts Arakatsu made the claim he was making "tremendous strides" towards making an atomic bomb and that Russia probably already had one.
Since the bombing of Hiroshima and Nagasaki, Japan has been a staunch upholder of antinuclear sentiments. Its postwar Constitution forbids the establishment of offensive military forces, and in 1967 it adopted the Three Non-Nuclear Principles, ruling out the production, possession, or introduction of nuclear weapons. Despite this, the idea that Japan might become a nuclear power has persisted. After China's first nuclear test in 1964, Japanese Prime Minister Eisaku Satō said to President Lyndon Johnson when they met in January 1965, that if the Chinese Communists had nuclear weapons, the Japanese should also have them. This shocked Johnson's administration, especially when Sato added that "Japanese public opinion will not permit this at present, but I believe that the public, especially the younger generation, can be 'educated'."
Throughout Sato's administration Japan continued to discuss the nuclear option. It was suggested that tactical nuclear weapons, as opposed to larger strategic weapons, could be defined as defensive, and therefore be allowed by the Japanese Constitution. A White Paper commissioned by future Prime Minister Yasuhiro Nakasone opined that it would be possible that possessing small-yield, purely defensive nuclear weapons would not violate the Constitution, but that in view of the danger of adverse foreign reaction and possible war, a policy would be followed of not acquiring nuclear weapons "at present".
The Johnson administration became anxious about Sato's intentions and made securing Japan's signature to the Nuclear Non-Proliferation Treaty (NPT) one of its top priorities. In December 1967, to reassure the Japanese public, Sato announced the adoption of the Three Non-Nuclear Principles. These were that Japan would not manufacture, possess, or permit nuclear weapons on Japanese soil. The principles, which were adopted by the Diet, but are not law, have remained the basis of Japan's nuclear policy ever since.
According to Kei Wakaizumi, one of Sato's policy advisers, Sato realized soon after making the declaration that it might be too constraining. He therefore clarified the principles in a February 1968 address to the Diet by declaring the "Four Nuclear Policies" ("Four-Pillars Nuclear Policy"):
It followed that if American assurance was ever removed or seemed unreliable, Japan might have no choice but to go nuclear. In other words, it kept the nuclear option available.
In 1969 a policy planning study for Japan's Foreign Ministry concluded that Japan should, even if it signed the NPT, maintain the economic and technical ability to develop and produce nuclear weapons in case it should ever become necessary, for example due to the international situation.
Japan finally signed the NPT in 1970 and ratified it in 1976, but only after West Germany became a signatory and the US promised "not to interfere with Tokyo's pursuit of independent reprocessing capabilities in its civilian nuclear power program".
In 1995 the Clinton administration pushed the Japanese government to endorse the indefinite extension of the NPT, but it opted for an ambiguous position on the issue. A former Japanese government official recalled, "We thought it was better for us not to declare that we will give up our nuclear option forever and ever". However, eventually pressure from Washington and other nations led to Japan's supporting the indefinite extension.
In 1998 two events strengthened the hand of those in Japan advocating that the nation should at least reconsider if not reverse its non-nuclear policy. Advocates of such policies included conservative academics, some government officials, a few industrialists, and nationalist groups.
The first of these events was India and Pakistan both conducting nuclear tests; the Japanese were troubled by a perceived reluctance on the part of the international community to condemn the two countries' actions, since one of the reasons Japan had opted to join the NPT was that it had anticipated severe penalties for those states who defied the international consensus against further nuclear proliferation. Also, Japan and other nations feared that an Indian nuclear arsenal could cause a localized nuclear arms race with China.
The second event was the August 1998 launch of a North Korean Taepodong-1 missile over Japan which caused a public outcry and led some to call for remilitarization or the development of nuclear weapons. Fukushiro Nukaga, head of the Japan Defense Agency, said that his government would be justified in mounting pre-emptive strikes against North Korean missile bases. Prime Minister Keizo Obuchi reiterated Japan's non-nuclear weapon principles and said that Japan would not possess a nuclear arsenal, and that the matter was not even worthy of discussion. However, it is thought that Koizumi implied he agreed that Japan had the right to possess nuclear weapons when he added, "it is significant that although we could have them, we don't".
Earlier, Shinzo Abe had said that Japan's constitution did not necessarily ban possession of nuclear weapons, so long as they were kept at a minimum and were tactical weapons, and Chief Cabinet Secretary Yasuo Fukuda had expressed a similar view.
While there are currently no known plans in Japan to produce nuclear weapons, it has been argued Japan has the technology, raw materials, and the capital to produce nuclear weapons within one year if necessary, and some analysts consider it a de facto nuclear state for this reason. For this reason Japan is often said to be a "screwdriver's turn" away from possessing nuclear weapons.
Significant amounts of reactor-grade plutonium are created as a by-product of the nuclear energy industry, and Japan was reported in December 1995 to have 4.7 tons of plutonium, enough for around 700 nuclear warheads. Japan also possesses an indigenous uranium enrichment plant which could hypothetically be used to make highly enriched uranium suitable for weapon use. Japan has also developed the M-V three-stage solid fuel rocket, similar in design to the U.S. LGM-118A Peacekeeper ICBM, which could serve as a delivery vehicle, and has experience in re-entry vehicle technology (OREX, HOPE-X). Toshiyuki Shikata, a government adviser and former lieutenant general, indicated that part of the rationale for the fifth M-V Hayabusa mission, from 2003 to 2010, was that the reentry and landing of its return capsule demonstrated "that Japan's ballistic missile capability is credible."
It has been pointed out that as long as Japan enjoys the benefits of a "nuclear-ready" status held through surrounding countries, it will see no reason to actually produce nuclear arms, since by remaining below the threshold, albeit with the capability to cross it at short notice, Japan can expect the support of the US while posing as an equal to China and Russia.