Nuclear Weapons, explosive weapons that gain their explosive power from nuclear reactions. Nuclear weapons generate much more energy than weapons using chemical explosives such as TNT. Nuclear weapons can be in the form of aerial bombs, artillery shells, torpedoes, mines, or missile warheads. The nuclear weapons are much more dangerous than any conventional weapon. ( section “Military Missiles.”) Nuclear devices can be exploded in the atmosphere or on or under a land or water surface.

Nuclear weapons are of two types, the fission weapons, also known as atomic bombs or atomic weapons, and the thermonuclear weapons, also called as hydrogen bombs, hydrogen weapons, or fusion weapons. When the nuclei of certain kinds of uranium or plutonium atoms get split, its matter converts into energy in fission weapons. In thermonuclear weapons matter converts into energy when pairs of certain kinds of hydrogen nuclei combine to form single nuclei. Most nuclear weapons are thermonuclear, and they are much more powerful than fission weapons.

A nuclear explosion immediately creates a luminous fireball consisting of ionized matter. The explosion also creates a powerful blast wave. In an atmospheric explosion, the fireball rapidly rises and forms a mushroom-shaped cloud. The blast wave moves away from the fireball at supersonic speed and can demolish buildings over large areas. Heat emitted by the fireball can cause serious skin burns and even start fires from a great distance. The explosion also produces highly penetrating nuclear radiation that can cause serious illness or death. Radioactive matter created during the explosion can leave a region virtually uninhabitable for some time.

The devastating power of nuclear weapons has twice been used in warfare. On August 6, 1945, during World War II, a United States B-29 bomber, Enola Gay, dropped a 9,000-pound (4,082-kg) nuclear device on Hiroshima, Japan. The explosion of this atomic bomb (as the weapon was called) resulted in a huge number of deaths—the exact figures are unknown, but estimates range from 68,000 to 200,000 persons. The city was largely destroyed. On August 9, another B-29, Great Artist, released a 10,000-pound (4,536-kg) nuclear device over the Japanese city of Nagasaki, with similar results. Five days later Japan surrendered to the United States and its Allies, bringing World War II to an end.

Many of the nuclear weapons that have been tested since World War II have hundreds of times the explosive power of the atomic bombs dropped on Japan. After the United States' monopoly on nuclear weapons ended in the summer of 1949, when the Soviet Union exploded its first nuclear device, the world was faced during the Cold War with the awesome possibility of a nuclear exchange between the United States and the Soviet Union that conceivably could have destroyed civilization owing to their extreme enmity. It is believed that the fear of nuclear war helped in maintaining peace during this period. Many nations have since sought ways to control such weapons and reduce the risk of nuclear war.

The Cold War ended in 1991, with the collapse of the Soviet Union, but during the previous decades both the United States and the Soviet Union had built up their nuclear armaments to such an extent that each had enough capacity to destroy all the major cities of the other. Each had also maintained a second-strike capacity—that is, no nuclear attack could destroy enough of its weapons to prevent an effective retaliatory strike. This situation was referred to as mutual assured destruction, or MAD, because a nuclear attack by one country on the other would likely lead to the devastation of both countries. Small, potent arsenals or supplies of nuclear weapons were developed by United Kingdom and France during the Cold War.

During the late 20th century, the United States worked on a defensive missile system called the Strategic Defense Initiative (SDI), popularly called “Star Wars.” The system was conceived as a means to destroy all incoming ballistic missiles, and thereby end reliance on mutual assured destruction. The end of the Cold War and the growing difficulties in developing such a system virtually put an end to the program.

The United States and Russia have the greatest number of nuclear weapons Other countries that possess nuclear weapons are the United Kingdom, France, China, India, and Pakistan. In addition, Ukraine, Belarus, and Kazakhstan, which together with Russian were part of the Soviet Union, possess nuclear weapons. Several other countries, including Israel and North Korea, may also have nuclear weapons.

Although the end of the Cold War lessened concern over a major nuclear exchange, a new concern of the world community is the possibility of proliferation of nuclear weapons among small nations, such as North Korea, that have aggressive foreign policies. In 2006, North Korea tested a small nuclear device, and it believed that Iran is also planning to build nuclear weapons. South Africa also had produced nuclear weapons, which they voluntarily destroyed later during the late 20th century.

The increasing number of countries with nuclear arms has increased the danger of destruction by nuclear weapons over regional conflicts. In 2002, India and Pakistan faced similar tensions over the Kashmir issue when both the counties equipped with nuclear weapons exchanged artillery fire, as the nations were on the verge of a war.