Although the German nuclear program had significant potential because of a strong prewar scientific community, it failed to produce a weapon. Historians debate the reason for the German failure, with arguments citing a lack of resources and high-level support, basic technological and scientific errors or failures, and possible intentional slowdowns by German scientists. The Japanese program, with some assistance from Germany, made limited progress in basic research but did not come close to producing a viable weapon. Soviet nuclear research activities were disrupted by the German invasion in June 1941 and the occupation of the Soviet industrial heartland. However, the Soviet research effort was reborn in late 1942, motivated by appeals from Russian physicists and reports of nuclear advances in Germany as well as Britain and the United States. The Soviet nuclear weapons project continued throughout the war and afterward. Assisted by information provided by an active espionage network in the United States and Great Britain and the contributions of captured German scientists, the Soviets exploded a nuclear device on 29 August 1949.
Only the Americans, assisted by British, Canadian, and émigré scientists, were successful in developing an atomic bomb during the war. The perception of a significant German scientific capability, reinforced by intelligence reports of progress in nuclear matters, was a strong motivation for the British and American research efforts. In the United States, émigré scientists including Leo Szilard and Enrico Fermi were key participants in and advocates of nuclear research. In August 1939, Albert Einstein wrote a letter to President Franklin D. Roosevelt that drew attention to the potential of the concept and gained increased government support for the American scientific research effort.
The U.S. government accelerated its nuclear research activities in August 1942 by creating a highly secret top-priority program that was given the cover designation of "Manhattan Engineer District," normally shortened to the manhattan Project. Brigadier General (later Major General) Leslie Groves, a U.S. Army engineer, directed the program. The central research facility was established at Los Alamos, New Mexico, with support provided by a nationwide network of facilities including university laboratories, industrial sites, and major new nuclear production sites at Hanford, Washington, and Oak Ridge, Tennessee. The manhattan Project was a massive undertaking that involved solving the theoretical foundations for nuclear fission and fusion designs and then the practical engineering challenge of creating a functional weapon.
The manhattan Project produced two successful atomic bomb designs. Both designs brought sufficient fissile material together quickly to create a supercritical mass, which was triggered into a rapid fission chain reaction by an initiator that injected neutrons into the mass. The result was the explosive release of the binding nuclear energy. The simplest design used a gun assembly to fire one subcritical mass, termed a bullet, into a second subcritical mass, the target, to create a supercritical mass. The gun-assembly weapon used uranium-235 as fuel and was the design of the "Little Boy" bomb dropped on Hiroshima on 6 August 1945. The second approach used an implosion technique and plutonium-239 as the fuel. The implosion design used shaped conventional high-explosive charges to compress a subcritical mass into a denser mass that became supercritical. The implosion design was used for the first atomic test explosion on 16 July 1945, when a device named "Gadget" was detonated at the Trinity Site on the Alamagordo Bombing Range in New Mexico. The "Fat Man" bomb dropped on Nagasaki on 9 August 1945 was also an implosion design.
The two bomb designs included significant emphasis on safety. In addition to designed safety and arming procedures, key parts of the bombs were not inserted until after the B-29s of the 509th Composite Group took off from their base on Tinian Island. This procedure was intended to prevent a full nuclear explosion if an accident occurred, although the Little Boy design still had significant inherent safety problems. Both bombs used radar altimeters for fusing and had a barometric altimeter as a backup.
The effects of nuclear weapons are generally described in terms of a comparable amount of thousands of tons of conventional explosives, kilotons, of yield. The "Little Boy" bomb was estimated to have the explosive power of 15 kilotons and the "Fat Man" bomb 21 kilotons. This description does not fully describe the effects of these weapons, as in a normal bomb design approximately half of the yield was produced in blast energy, approximately one third of the energy was thermal radiation, and the remainder was released in the form of radiation.
Although the atomic bombs created by the manhattan Project used nuclear fission, the fusion process had been considered in the theoretical discussions and was recognized as having even greater destructive potential. The fusion weapon design, often referred to as the "Super," was not produced until after the war. Fusion weapons, known as thermonuclear weapons or hydrogen bombs, used a fission detonation to trigger the fusion reaction, which generated significantly higher destructive power. The United States detonated the first fusion device, with a yield of 10 megatons (million tons), in October 1952 in a South Pacific test. The development and use of atomic bombs in World War II and the subsequent development of thermonuclear weapons had a significant effect on postwar military forces and national security policies, especially in the form of deterrence doctrines that dominated the Cold War period. Jerome V. Martin
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Jerome V. Martin