由某些重元素，如鈽或鈾的原子核裂變突然釋放能量而產生巨大爆炸威力的武器（參閲nuclear fission）。一個原子彈包含了2磅（1公斤）鈾－235能釋放出相當於17,000噸的黃色炸藥（TNT）；原子彈爆炸時釋放出巨大的熱能形成一個巨大的火球，同時立即產生強大的衝擊波，和被稱為落塵（fallout）的放射性的塵埃。第二次世界大戰期間，美國通過實施「曼哈頓計畫」（Manhattan Project）製造出第一批原子彈。1945年7月16日在新墨西哥的沙漠試爆成功。在實戰中使用的第一枚原子彈是鈾彈，1945年8月6日，美國將該彈投於日本廣島；第二枚，3天後投於日本長崎。戰後，美國在太平洋和內華達州進行了數十次原子彈爆炸試驗。1949年蘇聯首次試爆原子彈成功，隨後，英國（1952）、法國（1960）、中國（1964）和最近的印度（1974）、巴基斯坦（1998）相繼試驗了各自的核子武器。以色列和南非被懷疑曾試爆核子武器。亦請參閲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.
