原爆 Nuclear weapon
原子爆弾
原子爆弾（げんしばくだん、英: atomic bomb）、あるいは原爆は、ウランやプルトニウムなどの元素の原子核が起こす核分裂反応を使用した核爆弾であり、初めて戦争において攻撃用に実使用された核兵器である。原子爆弾は、核爆発装置に含まれる。水素爆弾を含めて「原水爆」とも呼ばれる。 核兵器は通常兵器と比較して威力が極めて大きいため、大量破壊兵器に位置付けられ、核不拡散条約や部分的核実験禁止条約などで、実戦での使用が禁止されている。ビキニでの実験で原水爆防止運動が日本で起こった。
A nuclear weapon[a] is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission (fission or atomic bomb) or a combination of fission and nuclear fusion reactions (thermonuclear weapon[b]), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter. Nuclear warfare is widely considered a global catastrophic risk. Nine sovereign states are believed to possess nuclear weapons as of 2026: the United States, Russia, the United Kingdom, France, China, India, Pakistan, North Korea and Israel.
The majority of nuclear weapons have energy yields between 100 and 1,000 kilotons of TNT. Yields in the low kilotons can destroy cities. The effects of nuclear weapons include extreme blast damage, heat, ionizing radiation, and firestorms, followed by radioactive nuclear fallout, an electromagnetic pulse, radar blackout, and acute or chronic radiation syndrome in humans and animals. Hundreds of nuclear explosions would likely cause nuclear winter and nuclear famine.
The first nuclear weapons were developed by the United States in collaboration with the United Kingdom and Canada during World War II in the Manhattan Project. Production requires a large scientific-industrial complex, primarily for fissile material, either nuclear reactors and reprocessing plants producing plutonium, or uranium enrichment facilities. Nuclear weapons have been used twice in war, in the 1945 atomic bombings of Hiroshima and Nagasaki that killed between 150,000 and 246,000 people. Nuclear deterrence, including mutually assured destruction, aims to prevent nuclear warfare via the threat of unacceptable damage and the danger of escalation to nuclear holocaust. A nuclear arms race for weapons and their delivery systems was a defining component of the Cold War. Nuclear weapons have had yields between 50 megatons for the Tsar Bomba, and 10 tons for the W54.
