World War II: Japanese Nuclear Weapons/Genshi Bakudan Program

World War II nuclear war

Figure 1.--

There are many unanswered questions about the German atomic bomb program. Even less is known about the Japanese program. The Japanese were also interested in nuclear weapons. Japan had nuclear scientists who learned about the German expeiments and were aware of the weapons potential. Both the Japanese Army and Navy has small atomic weapons program. There is considerable controversy concening the progress made. The Japanese began mining uranium at Konan, North Korea. The mine an research facility was seized by the Soviets after they declared war on Japan (August 1945). Konan is now the source of the uranium for North Korea's atomic bombs. Unlike the Anglo-American relationship, there sees to have been relatively limited scientific cooperation between Germsany and Japan. What did occur was primarily German technology delivered to Japan. The Germans at first wanted to be paid. And they were concerned about the future relationship between the two countries. As the War went against Germany, the Germans were more willing to transfer technology. The problem was how to transfer it. Mine laying submarines were converted to carry cargos. We know that shipments took place, because of German accounts and the fact that German submarine crews were found in Japan after the War. And more importantly, one of the submarines, the U-234, surrended to the U.S. Navy after the NAZIs surrenderd (May 1945). It contained enriched uranium as part of the cargo. Just what cargos got through to Japan earlier is not known with any surity. The U-234 incident clearly shows that uranium was a priority concern of the Japanese by the end of the War.

Japanese Nuclear Physics

Japan in the 20th century emerged as an important industrial nation with a competent scientific establishment. It was the leading industrial and scientific country in Asia, but the country's scientific and industrial capacity was small in comparison to that of the the United States. This was especially the case because unlike the Axis countries, Britain and America cooperated closely on scientific research. And the Anglo-American effort was further nutured by refugee European scientisrs, many of them Jewish. One of Japan's leading nuclear pysicist was Dr. Hikosaka Tadayoshi, a professor at Tohoku University. Dr. Hikosaka published his atomic physics theory (1934). Hikosaka theorized that atomic nuclei held a huge energy potential that could both generate electrical power as well as be used for weapons. Another important Japanese physicist was Dr. Yoshio Nishina. Dr. Yoshio was friendly with both Niels Bohr and Albert Einstein. Hec worked for several years at Bohr/s institure in Denmark. Dr. Nishina was a highly competent physicist. He had co-authored the Klein-Nishina Formula which theorized the interactions gamma rays and matter.. Dr Nishina had set up his own Nuclear Research Laboratory to study high-energy physics (1931) at the Riken Institute for Physical and Chemical Research. The Riken Institute was opened in Tokyo to pursue basic scientific resarch (1917). Dr. Nishina had built his first basic 26 inch cyclotron (1936). He then built a much larger 60 inch 220 ton cyclotron (1937). Japan also purchased another cyclotron from the University of California at Berkeley (1938). There were other competent Japanese pysocists, including Ryokichi Sagane who ad attened the University of California at Berkeley, studyingbunder Ernest Lawrence. Both Hikosaka and Yoshio were highly competent phyicists. Japan thus had some first rate theoretical physicists. The county did not, however, have a large number of trained pysicists like the American British, and refugee team assembled for the Manhattan Project. Nor did the country have the industrial infrastructure that could carry outa massive effort like the Manhattan Project. The country did not have the electrical generating capcity needed or large numbers of scientists and engineer who could be assigned to the multitude of tasks that needed to be solved as part of building a bomb. And there were notable weaknesses in some scientific areas. One author writes, Japan "lacked the knowledhe in certain aspects of science and technologyrequired forc a successful weapons program. It is reported that at one meeting some physicist suggested that uranium being a dense material, woukd naturally concentrate itself in 'wrinkles' of the Earth's crust. This ignorance of geological facts and uranium mining ws taken seriously by scientists on the project and led them astray." [Ragheb] There were many other areas in which the Jaoanese were weak. Masa Takeuchi oversaw the creation of thermal gaseous uranium enrichment equipment. It took him 18 months. Similar equipment was devised by the americans in a few weeks. [Shapley]

Government Commitment

Japanese scientists in the best of conditions would have had trouble building a bomb. Fortunately for the allies, they were not operating in the best of conditions. And at the onset of the War did not even have Government support for the effort. Japan's military leaders who launched the War experienced dizzying success with conventioal weaponry. There was little interest in fabtastic new weapons which must have sounded like star wars to them. Their original concept was a short, sharp war to seize the Southern Resource Zone (SRZ). They believed that the United States occupied with the NAZIs in Europe would not have the capability or the will to wage a costly war with Japan. Only when the war began to go against Japan did the militarists turn to their scientists in increasing desperation for possible futurist weapons that could stop the American juggernaught that they had set in motion. The Governnment failked to use the country's scientific resources infficently. The military men running Japan for the most part had very narriow backgrounds and had never lived outside of their country. Many were suspicious of the urbane scientists, many who had travelked abd studied abroad. [Ragheb] The Government formed the Japan Society for the Promotion of Science. Government resources were used to etablish several new labortories. And priorituies were eablished for scientific research. The Caninet approved the General Plan for te establishment of a New Scientific and Technological Structure. The Government set up a Science Mobilization Council which organized the war time sciece effort into 32 priority areas. [Low] While some halting steps were taken toward establishing an atomic program, militry officials did not take a possible atomic weapon very seriously. And by the time they did, the American submarine campaign (1943) and then the strategic bombing campaign (1944) began to disrupt the Japanese war economy. American air attacks destroyed some of the Japanese thermal diffusion plants, discouraging the Japanese scientists. [Ragheb] The Americans were unaware that the Japanese had atomic facilities, but the incendiary bombing destroyed large areas of Japanese cities.

German Work

German chemists Otto Hahn and Fritz Strassmann submitted a paper to Naturwissenschaften reporting that they had detected the element barium after bombarding uranium with neutrons (December 1938). Lise Meitner has been a member of the research team, but her name was omoted because she was Jewish. Friends had spirited her out of Germany. Hahn wrote to Meitner and she with her nephew Otto Robert Frisch concluded that what had occurred was nuclear fission on a small scale. The small amount of uramium involved and the the use of mostly U-238 (U-235 is a very small component of naturally occuring uranium) meant that there was no chain reaction. Frisch confirmed the results with his own experiments (January 13, 1939).

Scientists Learn of German Work

Physicists around the world immediately realized that thedoretically chain reactions could be produced which would be a destructive weapon of unimaginable proprtions. At this time it was clear that NAZI Germany was about to launch a war. The fact that German scientists were leading nuclear research sent shuders through the scientific comminity. As a result, physicists around the world began informing their governments of the possibility of nuclear weapons anbd the German lead. The Japanese were also interested in nuclear weapons. Japan had nuclear scientists who learned about the German expeiments and were aware of the weapons potential. Dr. Nishina upon learning of the German work immediately recognized the weapons potential of nuclear fission. By this time, war was breaking out in Europe and relations with the United States were beginning to break down because of the Japanese invasion of China. He was concerned that the United States might launch an atomic bomb program. Unknown to Dr. Nishina, his associate Albert Einstein had contacted Pressident Roosevelt who took the first tentative steps toward just such a project. The University of California at Berkeley from which Japan purchased a cycloton was just one of the American research institutes that would be involved in the American project.

Weapons Research

One of the poorly studied subjects of World War II is the Japanese nuclear weapons program, largely because the Japanese government and researchers do not want to raise the topic. It goes against the Japanese effort to pose their country as a victim of World War II largely because of the American use of nuclear weapons. Both the Japanese Army and Navy has atomic weapons programs. Of the two, the Army program was the more important. There is considerable controversy concening the progress made. The initial Japanese research was conducted in Japan. Once the American strategic bombing campaign began, the Japanese moved to their safer Konan facilities in northern Korea.

Army program: Ni-Go

Nishina encountered Lieutenant-General Takeo Yasuda on a train (early summer 1940). Yasuda was working as the director of the Army Aeronautical Department's Technical Research Institute. Nishina told Yasuda about both the possibility of building nuclear weapons and the potential destructive power. The Army actually initiated a nuclear program with the goal of building a uranium fission device (April 1941). Yasuda was resonding to orders from Prime Minister Hideki Tojo whio ordered him to investigate the potential for nuclear weapons. Yasuda gave resonsibility for this research to Okochi Masatoshi, director of the Riken Institute. And at the Riken Institute is was Nishina and his Nuclear Research Laboratory who to do the work. By this time Nishina had more than 100 scientists on his staff. [Dahl, pp. 279–285.] The Army and Navy Navy established a joint Committee on Research in the Application of Nuclear Physics. The Committee was chaired by Dr. Nishina.. The joint Committee met 10 times (July 1942 - March 1943). The members concluded that a fusion weapon was feasible, "it would probably be difficult even for the United States to realize the application of atomic power during the war". There does not seem to have been any consideration of a dirty bomb. The Army had already been ordered to established an experimental project at Riken, called the Ni-Go Project. the findings of the joint Committee does not see\m to have deterred the Army team. The stated objective was produce enriched uranium (U-235) as the first step in profucing a fusion weapon. The scientists decided to enrich uranium by employing the thermal diffusion method, one of several possible methods. As far as we call tell, there was at this time no consulations with the Germans.

Navy program: F-Go

Armies and navies around the world often engage in devisive rivalries. This was especially true in Japan. While the Army was the senior service, the Navy also commanded substantial prestige. Ahnd the Imperial Navy also assessed the weapons potential of nuclear energy. We are not sure but it appears to have been the result of the Army's investigation. Commander Kitagawa, head of the Navy Research Institute's Chemical Section, requested Bunsaku Arakatsu at the Imperial University in Kyoto to begin enriching uranium. The subject was also persued by the Navy's Technology Research Institute. They contacted scientists at the Imperial University in Tokyo, for technical experise. As a result of prelininary consultations, the Army and Navy Navy established a joint Committee on Research in the Application of Nuclear Physics (1942-43). The fact that a nuclearv weapon was not one which could be easily achieved appears to have discouraged Navy planners. Rather the Navy secided to focus its resources on radar research. [Dahl, pp. 279–285.] The American lead in radar was by 1943 causing increasing problems for the Imperial Navy as well as the air wings of the Army and Navy. And the the American lead was in large meaure the result of techhnology provided by the British. Such sharing was much less as part of the Axis alliance. Germany at the time had very advanced radar technology. And there appears to have been no sharing of nuclear techonolgy until later in the War. Despite the projected time frame, Army planners were not discouraged about the enterprise. New Japanese naval commabndewrs has another look at the nuclear weapns issue (1943). They ordered a nuclear research program be initiated--the F-Go Project. The Navy's uranium program was put in the hands of Bunsaku Arakatsu at the Imperial University in Kyoto. Arakatsu had an impressive record, inclusing time spent at the finest European research institutes. He worked in the Cavendish Laboratory at Cambridge under Ernest Rutherford. He also worked at Berlin University under Albert Einstein. Nishina and Arakatsu were Japan's two most ilustrious nuclear physicists. Arakatsu assembled a research team which included Hideki Yukawa who after the War would become the first Japanese physicist to be awearded a Nobel Prize (1949). Arakatsuc as part of hos efforts to enrich uranium designed an ultracentrifuge (to spin at 60,000 rpm) which he hoped could finally permit the produc\tion of substantial amounts of enriched uranium. Arakatsuc was unable, however to actually build the aparatus before Japan was forced to surrender (August 1945). [Dees, p. 96.]

Obtaining Uranium

Another huge problem the Japanese faced is that they did not have uranium ore that the needed to produce fissionanle materials. Japanese military conquests, unlike the Germans, did not give them access to already mined uranium ore. As a result, both the Imperial Army and Navy attempted to find uranium resources which could be mined. They considered areas in Japan (Fukushima Prefecture), Korea, China, and Burma. The Japanese also asked for their German Axis allies to provide enriched uranium. A japanese submarine was dispatched to transport it. And when this failed, the Germans tried to ship the uranium by out-going U-boat. Just how much the Germans shipted is a matter of some contoversy. Al we know for sure is that the U-234 had enriched uranium aboard when it surenderdc to the Americans (May 1945). There are no confirmed shipments that are known to have reached Japan. By the last months of thewar when these shipment were attempted, Allied ASW ecapabilities were making it increasinly dew The Japanese began mining uranium at Konan, North Korea. The mine and research facility was seized by theSoviets at the end of the War (August 1945).

Col. Touransouke Kawashima

Primeminister Tojo ordered Colonel Kawashima (he may have been a general) to innitiate a Japanese atomic bomb project (fall 1942). Kawashima reports that, "The prime minister commented vthat the war might be decided by atomic bomb. I don't think prime minister Tojo had any idea what vthey were." [Chen] The greatest problem for the Japanese as it was fior the Americans was obtaining the adequate quantities of weapons grade uranium. needed for a Japanese atomic bomb. We know from NARA Magic decrypts that Kawashima attempted to obrain Uranium Pitchblende from the Germans. [Magic decrypt--Signal sent to the Japanese Embassy in Berlin, July 7, 1943.] The Germans wanted to know the purpose. Kawashima attempted to hide the purpose. He said it was for a catalyst to produce jet fuel from coal. The Germans either did not believe him or wanted details on the catalyst for their synthetic fuel program. [August 1943] This forced Kawashima's hand. He signaled the Japabnese Embassybin Berlin explaining it was for a nuclear weapons project [November 1943] The Germans agreed to the shipments. The Germans had Uranium ore from Jac-y-mov (Joachimsthal) in western Czechoslovakia. It was partially refined at the Degaussa plant in Oranienberg. The initial plan was to ship the uranium oxide (yellow cake) on Japanese I-class submarines. We do not know if any such shipments were made. We do know that few Japanese submarines in 1944 made it to France, let along the roundctrip back to Japan. Of course after D-Day, the French Atlantic ports rapidly fell into Allied hands. There are reports that some such shipments did make it through. This is impossible to verify, in part because Japanese sources are very reluctant to release information on their World War II atomic program. The Japanese attitude is to portray themselves, using the American atomic bomb attacks, as a victim of the War. Information showing that Jpan had an active atomic progrm goes against the conventional Japanese view of the War. We do know that the I-52 failed to make it to France (July 1944). By this time if the war, Allied ASw operations was making it increasonly dangerous for Axis submarines to operare in the Atlaytic.
After D-Day closed the French ports, the Germans agreed to send the Japanese uranium oxide on out-bound U-boat (August 1944). It was difficult enough for the Japanbese to reach French ports, making it to German ports was considered virtuallt impossible.

Axis Cooperation

Unlike the Anglo-American relationship, there sees to have been relatively limited scientific cooperation between Germsany and Japan. What did occur was primarily German technology delivered to Japan. The Germans wanted to be paid. And they were concerned about the future relationship between the two countries. As the War went against Germany, the Germans were more willing to transfer technology. The problem was how to transfer it. The Germans converted mine laying submarines to carry cargos. We know that shipments took place, because one of the submarines, the U-234, surrended to the U.S. Navy after the NAZIs surrenderd and it contained 560 kg enriched uranium oxide. U-234 was a XB type mine layer, but converted as a Japan-transporter. It departed Kiel headed for Kristiansand, Norway (March 25, 1945). The ship departed Norway for Japan (April 16, 1945). The cargo included drawings, a Me-262 jet fighter in crates, and 560kg of uranium oxide. There were also several high ranking German experts on various weapons technologies as well as two Japanese officers. Cpt. Fehler received cease-fire orders (May 4). He decided to surrender to the Americans. The two Japanese officers onboard either commited suiside or were killed by the Germans. The Captain claimed to know nothing bout the uranium. Just what cargos and scientific research papers got through to Japan is not known. Nor do we know if the Japanese just asked for the enriched U-235 on their own or if there were discussions about nuclear weapons. There could not have been technical discussions because as far as we know, Japanese phyicists never traveled to Germany or German pysicists to Japan. We only know that uranium must have been very high on the list of Japanese priorities to have been included on the last U-boat out of Germany to Japan. Some authors believe there was uranium on earlier outgoing U-boats. Col. Kawashima claims a U-boat with 2 tons of uranium oxide was sunk. [Kawashima] Kawashima did not specify the U-boat or where it was sunk, but he was probably referring to the U-864 sunk off Norway. The remains of the U-864 are known, but there is no confirmation that uranium was aboard. Researchers addressing this question are interested in several individual submarines (U234, 871, 874, and 876) as well as three Italian submarines. [Bybee]

Dirty Bomb

There is no real evidence that the Japanese ever built a thermonuclear device. There is considerable evidence suggesting that the Japanese were contemplating attacks on the United States using dirty bombs ascearly as 1943. The Japanese had submarines which could launch planes. Given the small number of planes involved, using them for a priority target (Panama Canal) or a particularly deadly weapon (dirty bomb). To make a dirt bomb the Japanese needed enriched uranium. And we know that from 1943 thatvthe Japanese were trying to get enriched uranium from the Germans who had achieved some sucess in isolating the U-235 isotope. By this time of the War, the Allies had stopped exchanges through surface merchant ships. So shipment had to be done by submarine. Both Germany and Japan, of course had large sunmarine fleets. Magic decrypts indicate that the Japanese placed a high priority to enriched uranium in these exchanges. Just what they wanted it for is an open question, but the only realistic alternative open to them was a dirty bomb. The Japanese attempted to use their submarines to bring German enriched uranium back to Japan. Magic decrypts alerted the Allies and the I-52 was sunk in the mid-Atlantic along with gold the Germans demanded in payment. [Billings] After the Japanese submarines were sunk. The Japanese still tried to get the enriched uranium through U-boat deliveries. Whuch we know about because the U-234 had uranium aboard when it surrendered at the end of the War (May 1945). Not known with any certainty is the number of other Japanese and German submarines sunk with enriched uranium aboard or if any made it to Japan. We know some German submarines made it to Japan, but we do not know if they made it with enriched uranium. The historical record is somewhat clouded by the reluctance of Japanese sources to discuss the issue.


A good deal of sensational claims have been made about the Japanese aromic program. Some even claim that the Japanese test fired a nuclear device. This is highly unlikely for a variety of reasons, most prominately producing weapons grade uranium required a huge industrial effort and massive amounts of electricity. Japan simply did not have that capacity. The generally accepted academic assessment is that Japans had only a few qualified physicists. Even so their theoretical concepts were better than the Germans. The Japanese calculations, for example, were more accurate than those of Heisnberg. Japan did not, however, have the industrial capacity to build a bomb in 1945. In this regard they had even less capability than the Germans. The major difficulty was refining the Uranium ore to produce enriched uranium (uranium with a high percentage of the U-235 isotope). This required not only a substahtial industrial plant, but massive quantities of electricity. The American Manhattan Project used an huge amount iof tghe country's electrical production. Neither Japan or Germany had the electrical generating capacity needed nor spare capcity thst coulf bec used. The Germans and Japanese were able to isolate small quantities for laboratory use, but were far away from the quantities needed for a nuclear device. [Roades]

Soviet Seizure

The Soviets seized the Japanese facilities and uranium mind at Konan after they declared war on Japan (August 1945). Details on what they found have never been released. They were closed during the Soviet era anhd as far as well know were not rekease after the disolutiin of the Siviet Union. Konan is now the source of the uranium for North Korea's atomic bombs.

'Hog Wild' Incident (August 1945)

A few weeks after the end of the War, but even before the signing ceremony in Tokyo Bay, four Soviet Yak-9D fighters forced down the 20th Air Foirce B-29 Superfortress 'Hog Wild' (August 29, 1945). [Bybee] This occurred over the city of Konan, the location of the Japanese uranium mine in North Korea. Konan is now known as Hungnam. The American plane was on a mission to drop supplies to Allied POWs at Konan. The Soviets believed it to be a spy mission. The Soviets were well aware of the American Manhattan Project because of the extensive NKVD espionage network. Thus Stalin was not surprised when President Truman informed him of the American atomic bomb at Potsdam (July 1945). He hd already ordered a crash program to build a Soviet bomb.

Hitorical Research

The Japanese loss of the Pacific war and the terrible destrucgtion of the strategic bombing campaign was taraumatic event. Japan in the post-War period embraced pacifism with the sane intensity that they had embraced miitarism before the war. Rather than confronting forthrightly what Japan had done, as was the case with the Germans, the Japanese set out to assume the guise of a victim rather than as one of the chief aggrssir nation culpable of horrendous crimes and attroicities. The American atmic bombing was seized upon by the myth makers. The Japanese peopkle were the only people to hsve suffered an atomic attack. The Government thus did not want their own atomic program publicized. One author explains thst the various atomic laboratories were dismantled. The only atomic research permitted in post-War Japan were medical and biological studies of the survivors of the two attacks. [Ragheb]


Billings, Richard N. Battleground Atlantic: How the Sinking of a Single Japanese Submarine Assured the Outcome of World War II, 311p.

Bybee, John D. E-mail nessage, August 3, 2011.

Chen, P.Y. "Allied sinking of German U-boat hindered Japanese A-bomb project,".

Dahl, Per F. Heavy Water and the Wartime Race for Nuclear Energy (CRC Press, 1999).

Dees, Bowen C. The Allied Occupation and Japan's Economic Miracle: Building the Foundations of Japanese Science and Technology 1945-52 (Routledge, 1997).

Kawashima, Touransouke. Video taped statement broadcasted on Japan Broadcasting Corporation (NHK) (1982). NHK has since denied requests from researchers for a transcript or copy of the video tape. So we do not know precisely what was said.

Low, Morris Fraser. "Japan's secret war?'Instant' scientific manpower and Japan's World war II bomb poroject," nnals of Science Vol. 47 (1990), pp. 347-360.

NARA Magic decrypts.

Ragheb, M. Chapter 3: "Japnese nuclear weapons progrm" (January 21, 2012).

Roades, Richard. The Making of the Atomic Bomb.

Shapley, Deborah. "Nuclear weaponshistory: Japan's war time bomb projdcts revealed," Science Vol. 199, No. 4325, pp. 152-57.


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Created: 12:18 AM 4/4/2011
Last updated: 1:12 AM 2/4/2012