atom bomb and nucleus reaction with image

The mushroom cloud of the atomic bombing of the Japanese city of Nagasaki on August 9, 1945 rose some 11 mi (18 km) above the bomb'shypocenter.

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or from a combination of fission and fusionreactions (thermonuclear bomb). Both bomb types release large quantities of energy from relatively small amounts of matter. The first test of a fission ("atomic") bomb released an amount of energy approximately equal to 20,000 tons of TNT (84 TJ). The firstthermonuclear ("hydrogen") bomb testreleased energy approximately equal to 10 million tons of TNT (42 PJ).A thermonuclear weapon weighing little more than 2,400 pounds (1,100 kg) can release energy equal to more than 1.2 million tons of TNT (5.0 PJ). A nuclear device no larger than traditional bombs can devastate an entire city by blast, fire, and radiation. Since they are weapons of mass destruction, theproliferation of nuclear weapons is a focus ofinternational relations policy.

Nuclear weapons have been used twice inwar, both times by the United States againstJapan near the end of World War II. On August 6, 1945, the U.S. Army Air Forcesdetonated a uranium gun-type fission bombnicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the U.S. Army Air Forces detonated aplutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki. These bombings resulted in the deaths of approximately 200,000 civilians andmilitary personnel from injuries sustained from the explosions. The ethics of these bombings and their role in Japan's surrenderare subjects of debate.

Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over two thousand times fortesting and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are (chronologically by date of first test) the United States, the Soviet Union(succeeded as a nuclear power by Russia), the United Kingdom, France, China, India,Pakistan, and North Korea. Israel is believed to possess nuclear weapons, though, in apolicy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy,Turkey, Belgium and the Netherlands arenuclear weapons sharing states. South Africa is the only country to haveindependently developed and then renounced and dismantled its nuclear weapons.

The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but its effectiveness has been questioned, and political tensions remained high in the 1970s and 1980s. Modernisation of weapons continues to this day.