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 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 nonfissile uranium-238 took place at a plant built at Oak Ridge, Tennessee. 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.
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 there is reason to believe that Sweeney's crew bombed by radar, disobeying strict orders to abort the mission if it proved impossible to bomb visually, and then falsified their mission report. 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.
Just how much influence the nuclear attacks had on the Japanese decision to surrender has been heavily debated 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. However, 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 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 produced more casualties, and all three pale in comparison with the Holocaust. 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 doctrine of Mutual Assured Destruction works.
References
The Pacific War Online Encyclopedia
(c) 2007 by Kent G.
Budge. Index