Nuclear weapon

A nuclear weapon is a weapon which derives its destructive force from nuclear reactions of either nuclear fission or the more powerful fusion. As a result, even a nuclear weapon with a relatively small yield is significantly more powerful than the largest conventional explosives, and a single weapon is capable of destroying or seriously disabling an entire city.

In the history of warfare, nuclear weapons have been used only twice, both during the closing days of World War II. The first event occurred on the morning of August 6, 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of these weapons, which resulted in the immediate deaths of around 100,000 to 200,000 individuals (mostly civilians) and even more over time, was and remains controversial &mdash; critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war (see Atomic bombings of Hiroshima and Nagasaki for a full discussion).

Since that time, nuclear weapons have been detonated on over two thousand occasions for testing and demonstration purposes. The only known countries to have detonated such weapons are the United States, Soviet Union, United Kingdom, France, People's Republic of China, India, and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse).

Various other countries may hold nuclear weapons but have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has made several changing statements about the abandonment of its nuclear weapons programs, often dependent on the political climate at the time) but has never conducted a confirmed test and its weapons status remains unclear; and Iran currently stands accused by a number of governments of attempting to develop nuclear capabilities, though its government claims that its acknowledged nuclear activities, such as uranium enrichment, are for peaceful purposes. (For more information see List of countries with nuclear weapons.)