When a neutron strikes the nucleus of an atom of the isotopes uranium 235 or plutonium-239, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In the process of splitting, a great amount of thermal energy, as well as gamma rays and two or more neutrons, is released. Under certain conditions, the escapingneutrons strike and thus fission more of the surrounding uranium nuclei, which then emit more neutrons that split still more nuclei. This series of rapidly multiplying fissions culminates in a chain reaction in whichnearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb.
Fission releases an enormous amount of energy relative to the material involved. When completely fissioned, 1 kg (2.2 pounds) of uranium-235 releases the energy equivalently produced by 17,000 tons, or 17 kilotons, of TNT. The detonation of an atomic bomb releases enormous amounts of thermal energy, or heat, achieving temperatures of several million degrees in the exploding bomb itself. This thermal energy creates a large fireball, the heat of which can ignite ground fires that can incinerate an entire small city. Convection currents created by the explosion suck dust and other ground materials up into the fireball, creating the characteristic mushroom-shaped cloud of an atomic explosion. The detonation also immediately produces a strong shock wave that propagates outward from the blast to distances of several miles, gradually losing its force along the way. Such a blast wave can destroy buildings for several miles from the location of the burst. Large quantities of neutrons and gamma rays are also emitted; this lethal radiation decreases rapidly over 1.5 to 3 km (1 to 2 miles) from the burst. Materials vaporized in the fireball condense to fine particles, and this radioactive debris, referred to as fallout, is carried by the winds in the troposphere or stratosphere. Since the radioactive contaminants include such long-lived radioisotopes as strontium-90 and plutonium-239, they can have lethal effects for weeks after the explosion.
President Franklin D. Roosevelt, influenced by a letter signed by Albert Einstein (1879-1955), authorized a feasibility study in October 1939, and in February 1940 a $6,000 research grant was approved. By 1945 over $2 billion would be spent on the project. On December 2, 1942 the first self-generated chain reaction of a uranium-graphite pile was put into operation by a research team led by Enrico Fermi (U.S., born Italy) at the University of Chicago. Following this test, the Manhattan Project was authorized. A factory for the production of uranium 235 was built at Oak Ridge, Tenn., and a secret plant for plutonium preparation was constructed at Hanford. Wash. A laboratory with manufacturing capabilities was established at Los Alamos, NM., with J. Robert Oppenheimer (U.S.) appointed director in March 1943.
The first atomic bombs were built in the United States during World War II under a program called the Manhattan Project. One bomb, using plutonium, was successfully tested on July 16, 1945, at a site 193 km (120 miles) south of Albuquerque, New Mexico The first atomic bomb to be used in warfare used uranium. It was dropped by the United States on Hiroshima, Japan, on August 6, 1945. The explosion, which had the force of more than 15,000 tons of TNT, instantly and completely devastated 10 square km (4 square miles) of the heart of this city of 343,000 inhabitants. Of this number, 66,000 were killed immediately and 69,000 were injured; more than 67 percent of the city's structures were destroyed or damaged. The next atomic bomb to be exploded was of the plutonium type; it was dropped on Nagasaki on August 9, 1945, producing a blast equal to 21,000 tons of TNT. The terrain and smaller size of Nagasaki reduced destruction of life and property, but 39,000 persons were killed and 25,000 injured; about 40 percent of the city's structures were destroyed or seriously damaged. The Japanese initiated surrender negotiations the next day. (Adapted from the ‘Encyclopedia of Britannica’ and ‘Inventions and Discoveries’)