A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions. Both bomb types release large quantities of energy from relatively small amounts of matter. A nuclear weapon (illegally called an atomic bomb, atomic bomb, atomic bomb, nuclear warhead, A bomb or nuclear bomb) fission and fusion reactions (thermonuclear bomb). Both types of bombs release large amounts of energy from a relatively small amount of matter. The first test of a fission bomb ("atomic") released an amount of energy equivalent to 20,000 tonnes of TNT (84 TJ). The first test of a thermonuclear bomb ("hydrogen") released an energy of about 10 more tonnes of TNT (42 PJ). A thermonuclear weapon weighing just over 2,400 pounds (1,100 kg) can release energy equivalent to more than 1.2 million tonnes of TNT (5.0 PJ). A nuclear device no larger than conventional bombs can devastate an entire city by explosion, fire and radiation. Since these are weapons of mass destruction. Nuclear weapons were used twice during the war, both by the United States against Japan in the Second World War. On August 6, 1945, the US military air force detained a uranium-type fission bomb nicknamed "little boy" over Hiroshima, a Japanese city; Three days later, on August 9, the US Air Force detained a plutonium implosion-type fission bomb known as a "fat man" over the Japanese city of Nagasaki. These bombings left nearly 200,000 civilians and the military dead, wounded and dead. These attacks and their role in The Surrender of Japan are subject to debate.