Nuclear Bombs

Photograph of Nagasaki mushroom cloud

National Archives

The Pacific War saw the first and, one hopes, the last use of nuclear weapons in warfare.  The city of Hiroshima was destroyed on 6 August 1945 using a "Little Boy" gun bomb, and Nagasaki was destroyed three days later using a "Fat Man" implosion bomb.  Casualties in both attacks were extremely heavy:  An estimated 130,000 persons perished at Hiroshima, of which two-thirds were civilians, while casualties at Nagasaki were around 70,000.  These figures for a single strategic bombing operation were exceeded only by the casualties from the fire bombing of Tokyo.

Nuclear weapons derive their explosive energy from the fission of the unstable nuclei of very heavy elements.  Each such fission splits a nucleus into two smaller nuclei and a number of neutrons, releasing a vast amount of energy in the process -- about sixteen million times the explosive power per unit weight of a conventional chemical explosive.  The neutrons released by each fission are capable of triggering the fission of other nuclei.  If conditions are contrived so that these neutrons are unable to escape from the mass of fissile material,  a chain reaction takes place that releases the available energy extremely rapidly.  

The potential of nuclear explosives was recognized before the United States entered the war.  The famed physicist Albert Einstein wrote a letter to Roosevelt advising him of the danger that Germany would develop a nuclear weapon, and on 6 December 1941, the day before the attack on Pearl Harbor, Roosevelt authorized the U.S. Army, in collaboration with the British government, to set up the Manhattan Project to produce fissile materials and to research the design of a workable weapon.  Further details of the U.S./British collaboration were worked out between Churchill and Roosevelt on 18 June 1942, including an agreement that most of the work would be done in the United States, where there was no immediate danger of air attack.

Work on the first nuclear reactor began in November 1942 at the University of Chicago, under the leadership of Enrico Fermi, who had fled Fascist Italy with his Jewish wife.  Experiments with this reactor confirmed that a chain reaction was possible, and that it could be controlled via the phenomenon of delayed fission neutrons.  Most of the neutrons from a fission (prompt neutrons) are released at once, but a small fraction, around 1-2%, are released more slowly (delayed neutrons).  So long as a reactor is not prompt critical (critical from the release of prompt neutrons alone) the slower release of delayed neutrons gives the time margin necessary to control the reaction.  This is what distinguishes a reactor from a bomb, and it makes the peaceful use of nuclear energy possible.

On 28 December 1942, Roosevelt ordered that cooperation with the British be reduced to a minimum, due to his concerns that the British meant to exploit American research commercially after the war.   However, on 20 July 1943 an agreement was reached with the British that increased cooperation between the two powers, who pledged to share their knowledge, to refrain from using nuclear weapons without the other's consent, and to keep atomic information away from third parties.

Two weapon designs were developed during the war.  The gun bomb was based on uranium-235, a rare isotope of natural uranium whose separation from the more abundant but non-fissile uranium-238 took place at a plant built at Oak Ridge, Tennessee.  Ground was broken for the plant on 1 February 1943 and it would eventually employ 82,000 scientists and engineers. The gun bomb (Little Boy) fired a slug of uranium-235 into a uranium-235 target, producing an aggregate mass large enough to trap neutrons (a "critical mass")  and produce an explosion.  This design was considered sufficiently foolproof, and uranium-235 was in sufficiently short supply, that Little Boy was not tested prior to its first use over Hiroshima.

The implosion bomb was based on plutonium-239, a synthetic element produced by bombarding uranium-238 with neutrons in nuclear reactors built at Hanford, Washington.  In this weapon, a mass of plutonium-239 was surrounded by a large chemical explosive charge, which compressed the plutonium-239 to the point where neutrons could no longer escape and a chain reaction took place.  This design was theoretically capable of producing a more powerful explosion than a gun bomb, but it was also less robust, so it was tested in the desert near Alamagordo, New Mexico, on 16 July 1945.

Truman had decided by 24  July 1945 that the bomb should be used by 10 August, confiding in his diary that he wished

to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless, and fanatic, we as the leader of the world for the common welfare cannot drop this terrible bomb on the old capital [Kyoto] or the new [Tokyo].

(Gilbert 1989) However, by this point in the war, there were few worthwhile military targets in Japan that were not located in sizable cities, and a list of four cities containing significant military targets that had not already been heavily attacked was made up. These were Hiroshima, Kokura, Nagasaki, and Yokohama; Kyoto was originally on the list but was replaced with Nagasaki. There seems to have been little sense at the time that the employment of nuclear weapons represented the crossing of any significant moral divide.

Both nuclear attacks were delivered by unescorted B-29s modified to have the necessary range and the speed to clear the target after releasing the weapons.  The base of operations was the island of Tinian, where special facilities had been prepared for loading the aircraft.  The bombs themselves were not armed until the aircraft were safely in flight.  The attacks were delivered in daylight, and the attacking aircraft were few enough in number that the Japanese did not scramble interceptors, thinking the raids were weather observation flights. 

The first raid, led by Colonel Paul Tibbetts in the Enola Gay, was almost perfectly executed, and the bomb exploded very close to its aiming point, a bridge at the center of Hiroshima.  By contrast, the second raid, led by Major Charles Sweeney in Bock's Car, was poorly executed.  Weather over the targets was very poor that day, and Sweeney's crew bombed by radar, disobeying strict orders to abort the mission if it proved impossible to bomb visually.  The Nagasaki bomb missed its aiming point by well over a mile, and Bock's Car returned to Okinawa rather than Tinian because it was running low on fuel.

The Americans felt that the use of two nuclear weapons was required to convince the Japanese that more were available in their arsenal. In fact, Manhattan Project leaders estimated that seven more bombs would be ready by 31 October 1945. Tokyo was originally to be the next target, but intelligence reports of a massive Japanese buildup on Kyushu led Marshall to ask that the remaining bombs be reserved for tactical use in case an invasion of Japan was ordered.

The Japanese quickly realized the nature of the weapon used against Hiroshima, and concluded that the only conceivable defense was to destroy the nuclear bomber force. However, an attempt to concentrate 200 bombers and 2000 suicide troops on northern Honshu for a suicide raid on the Marianas was preempted by an American carrier strike on 9 August, the day of the Nagasaki bombing. The Americans assembled extensive intelligence to brief their pilots, who attacked at treetop level and claimed 251 aircraft destroyed and 141 damaged in spite of Japanese efforts to camouflage their aircraft at dispersed revetments. A single kamikaze retaliated with a hit on destroyer Borie.

Just how much influence the nuclear attacks had on the Japanese decision to surrender has been heavily debated by historians in the decades since.  The attacks coincided with the Russian invasion of Manchuria, and it has been claimed that the Russian intervention had more of an effect on the Japanese government and the Emperor than the nuclear attacks.  It has also been claimed that the collapse of the Japanese economy would have been sufficient to force a surrender. Japanese historian Hando Kazutoshi has stated that "For Japan's civilian politicians, the dropping of the atomic bombs was the last straw. For the Japanese Army, it was the Russian invasion of Manchuria." Marquis Kido Koichi, the Lord Keeper of the Privy Seal and close advisor to the Emperor, later said that "the feeling that the emperor and I had about the atomic bombing was that the psychological moment we had long waited for had finally arrived to resolutely carry out the termination of the war.... We felt that if we took the occasion and utilized the psychological shock of the bomb to follow through, we might perhaps succeed in ending the war" (Wolk 2010). However, Kido may have been telling his American interrogators what he thought they wanted to hear. It seems likely that it was the combination of all three factors that finally induced the Emperor to order his government to accept the Allied terms.

The nuclear attacks produced something close to panic in the civilian population. The exodus from the cities, already underway due to the fire bombing attacks, was accelerated, and the subsequent appearance of even a single Allied aircraft caused the population to scramble for shelter.

Frank (1999) has made a strong case that Japan was far from ready to accept an unconditional surrender prior to the nuclear attacks. Had the bombs not been used, the United States might have forced a surrender by destroying the rail transport system of Honshu, but the resulting famine might have starved up to ten million Japanese.

The nuclear attacks have often been depicted as uniquely horrible.  Horrible they certainly were, but but they were hardly unique.  The fire bombings of Tokyo and the rape of Nanking likely produced more casualties, and all three pale in comparison with the Holocaust in Europe.  The Second World War was one of the greatest outpourings of brutality the world has ever witnessed, and the nuclear attacks seem less remarkable viewed against this backdrop.  However, they were harbingers of more deadly nuclear weapons to come, which threatened to destroy civilization if the subsequent Cold War had ever turned hot.  Indeed, the threat of nuclear annihilation may explain why there has not yet been a Third World War:  The threat of Mutual Assured Destruction works.

In mid-September 1945, Foreign Minister Shigemitsu Mamoru telegraphed Japanese diplomats in Europe (Drea 2009):

Since the Americans have recently been raising an uproar about the question of our mistreatment of prisoners, I think we should make every effort to exploit the atomic bomb question in our propaganda.

This has been the consensus Japanese attitude to questions of war guilt ever since.

Japanese Nuclear Research. The basic physical principles on which nuclear weapons are based were widely known when war broke out in the Pacific, and Japan had capable physicists, including Yukawa Hideki, who predicted the existence of the meson and would be awarded the Nobel Prize in Physics in 1949. Arakatsu Bunsaku and Hagiwara Tokutaro were performing cyclotron experiments as early as 1939 in which they determined that each uranium fission produced an average of 2.6 neutrons and that a chain reaction was possible. By spring of 1940, Yasuda Takeo, an Army lieutenant general with a background in electrical engineering who led the Army Aeronautical Technology Research Institute, ordered Lieutenant Colonel Suzuki Tatsusaburo to "explore the possibility of an atomic bomb" (Grunden 2005).

The Navy organized a Physics Discussion Group in July 1942 that included Nishina Yoshio, Yukawa's mentor, who is sometimes described as the father of Japanese physics. Nishina had by then already explored the feasibility of an atomic weapon for the Army, which organized its own group. However, Japanese efforts to develop nuclear energy were hampered by a lack of depth in the Japanese technical community and a paucity of resources. There was little sense of urgency even after the Japanese heard (false) rumors in 1939 that the Americans had succeeded in powering a small turbine with nuclear energy. Interest did ramp up somewhat after the debacle at Midway. However, as so often happened, the Navy program was independent of the Army program, to the point where even Nishina could not share findings across the two groups. This led to considerable duplication of effort.

Japanese naval leaders, such as Vice Admiral (Engineering) Shibuya Ryutaro, saw some potential for nuclear power as a substitute for scarce oil for powering warships, a reasonable objective given Japan's resources and the pressing need for an alternative to oil as an energy source. However, Arakatsu, who led the Navy's F-Go atomic research program, later claimed that his only concern was keeping Japanese physicists out of the military draft, and the Navy program lagged far behind that of the Army.

The Army's NI-go team was led by Nishina, who was given 700,000¥ (about $175,000) to construct a new research building at the Institute for Physical and Chemical Research (Riken). However, the effort to separate U-235 from natural uranium using thermal diffusion of uranium hexafluoride in Clausius tubes made little progress. The Japanese experienced considerable difficulty with their vacuum technology and with the production of uranium hexafluoride. By 17 November 1944 the Japanese had concluded that no weapon could be completed in time to affect the outcome of the present war. An air raid on 13 April 1945 destroyed most of what remained of the Army program.

Efforts to build a nuclear reactor were abandoned in the spring of 1943. Natural uranium, which contains very little U-235, cannot sustain a chain reaction without a neutron moderator, such as graphite or heavy water. Both were in short supply.  Heavy water could be produced at the Japan Nitrogenous Fertilizer Corporation (Nichitsu) plant at Hungnam, which drew upon the abundant electrical power of the Fusenko and other hydroelectric projects. However, production never exceeded 50 mililiters (2.7 tablespoons) per month, a pitifully inadequate amount even if highest priority had been given to heavy water production. The electrolysis plant was destroyed in an air raid on 29 June 1945.

Even obtaining uranium was difficult. The most promising mine was at Kikune, near Seoul, but it was found that even at maximum production the mine could produce just 10 kg of refined uranium oxide per month. Some 4500 tons of tin mining tailings from Malaya were shipped to Japan but proved to contain little uranium. Another 100 pounds (45 kg) of uranium was obtained from Japanese ceramics shops, while 200 pounds (90 kg) were allegedly obtained on the Shanghai black market. Finally, the Japanese Ambassador to Germany arranged for two tons of pitchblende and 1200 pounds (560 kg) of uranium oxide to be sent to Japan by submarine, but the first submarine was sunk en route and the second submarine surrendered to the U.S. Navy upon the German capitulation.

A report on the Japanese Army's nuclear research program was prepared by David Snell, an intelligence officer who had been a reporter for the Atlanta Constitution prior to the war. Snell claimed that Tojo had made plans to use any nuclear weapons Japan developed to force an end to the war in China, and that the Japanese researchers had very nearly completed a nuclear bomb at a research facility in Korea before being captured by the Russians. A close examination of the available evidence fails to support Snell's assertion that the Japanese program had made such progress, but it has nonetheless become the basis for conspiracy theories of a cover up of the Japanese program.

Photo Gallery

First nuclear reactor


Calutron operators at Oak Ridge

U.S. Army

Plutonium plant at Hanford

U.S. Army

Los Alamos main gate




Trinity test at 9 seconds


"Little Boy"


"Fat Man"


Hiroshima mushroom cloud


Hiroshima following the nuclear attack


Nagasaki mushroom cloud



Drea (2009)

Frank (1999)
Gilbert (1989)

Grunden (2005)

Hastings (2007)

Maga (2001)

Marston (2005)

Morison (1959)

Nakagawa (1997)

Prados (1995)

Rhodes (1995)

Wolk (2010)

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