Atomic Bomb See Also: Hahn, Otto, Hydrogen Bomb Weapon deriving its great ExplosiveForce from the sudden release of Nuclear Energy through the fission, or splitting, of heavy Atomic nuclei. The first atomic bomb was successfully tested by the U.S. near Alamogordo, New Mexico, on July 16, 1945. In the final stages of World War II the U.S. dropped atomic bombs on Hiroshima on Aug. 6, 1945, and on Nagasaki three days later to force Japan to surrender. Atomic bombs were subsequently developed by the USSR (1949), Great Britain (1952), France (1960), China (1964), and India (1974), and a number of other nations, particularly Pakistan and Israel, are believed to have atomic bombs or the capability to produce them readily. The USSR's nuclear arsenal has been divided since 1991 among Russia, Ukraine, Kazakhstan, and Belarus. Practical fissionable nuclei for atomic bombs are the Isotopes Uranium-235 and Plutonium-239, which are capable of undergoing chain reaction. If the Mass of the fissionable material exceeds the critical mass (the smallest mass of a fissionable material that will sustain a nuclear chain reaction at a constant level), the chain reaction multiplies rapidly into an uncontrollable release of Energy. Basically, a chain reaction occurs during fission when the release of neutrons from the splitting of one atom leads to the splitting of others. An atomic bomb is detonated by bringing together very rapidly (e.g., by means of a chemical explosion) two subcritical masses of fissionable material. The ensuing explosion produces great amounts of Heat, a Shock Wave, and intense Neutron and gamma Radiation. The region of the explosion becomes radioactively contaminated, and Wind-borne radioactive products may be deposited elsewhere as fallout.