由某些重元素，如钸或铀的原子核裂变突然释放能量而产生巨大爆炸威力的武器（参阅nuclear fission）。一个原子弹包含了2磅（1公斤）铀－235能释放出相当於17,000吨的黄色炸药（TNT）；原子弹爆炸时释放出巨大的热能形成一个巨大的火球，同时立即产生强大的冲击波，和被称为落尘（fallout）的放射性的尘埃。第二次世界大战期间，美国通过实施「曼哈顿计画」（Manhattan Project）制造出第一批原子弹。1945年7月16日在新墨西哥的沙漠试爆成功。在实战中使用的第一枚原子弹是铀弹，1945年8月6日，美国将该弹投於日本广岛；第二枚，3天後投於日本长崎。战後，美国在太平洋和内华达州进行了数十次原子弹爆炸试验。1949年苏联首次试爆原子弹成功，随後，英国（1952）、法国（1960）、中国（1964）和最近的印度（1974）、巴基斯坦（1998）相继试验了各自的核子武器。以色列和南非被怀疑曾试爆核子武器。亦请参阅Non-proliferation of Nuclear Weapons, Treaty on the。
atomic bomb
Weapon whose great explosive power results from the sudden release of energy upon the splitting, or fission, of the nuclei of such heavy elements as plutonium or uranium (see nuclear fission). An atomic bomb containing 2 lbs (1 kg) of uranium-235 will generate a 17-kiloton explosion that creates a huge fireball, a large shock wave, and lethal radioactive fallout. The first atomic bomb, developed by the Manhattan Project during World War II, was set off on July 16, 1945, in the New Mexico desert. The first bomb used in warfare was dropped by the U.S. on Hiroshima on August 6, 1945; a second bomb was dropped on Nagasaki three days later. After the war, the U.S. conducted dozens of atomic tests in the Pacific and in Nevada. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952), France (1960), and China (1964), and subsequently India (1974) and Pakistan (1998). Israel and South Africa are also suspected of having tested atomic weapons. See also hydrogen bomb, Non-Proliferation Treaty.
