<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 fusion reactions (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 first thermonuclear ("hydrogen") bomb test released 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, the proliferation of nuclear weapons is a focus of international relations policy . </P> <P> Nuclear weapons have been used twice in war, both times by the United States against Japan near the end of World War II . On August 6, 1945, the U.S. Army Air Forces detonated a uranium gun - type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the U.S. Army Air Forces detonated a plutonium implosion - type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki . These bombings resulted in the deaths of approximately 200,000 civilians and military personnel from injuries sustained from the explosions . The ethics of these bombings and their role in Japan's surrender are subjects of debate . </P>

Where does a nuclear bomb get its energy from