A weapon of mass destruction (also called an atom bomb , nuke, atomic bomb, atomic warhead , A-bomb, or nuclear bomb) is an device that derives its destructive force from nuclear reactions, either fission (fission bomb) or from a mixture of fission and fusion reactions (thermonuclear bomb). Both bomb types release large quantities of energy from relatively small amounts of matter. the primary test of a fission ("atomic") bomb released an amount of energy approximately adequate to 20,000 plenty of TNT (84 TJ). the primary thermonuclear ("hydrogen") bomb test released energy approximately adequate to 10 million plenty of TNT (42 PJ). A thermonuclear weapon weighing little quite 2,400 pounds (1,100 kg) can release energy adequate to quite 1.2 million plenty of TNT (5.0 PJ). A nuclear device no larger than traditional bombs can devastate a whole city by blast, fire, and radiation. Since they're weapons of mass destruction, the proliferation of nuclear weapons may be a focus of diplomacy policy. Nuclear weapons are used twice in war, both times by the us against Japan near the top of war II. On Transfiguration , 1945, the U.S. Army Air Forces detonated a uranium gun-type atom 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 atom bomb nicknamed "Fat Man" over the japanese city of Nagasaki. These bombings caused injuries that resulted within the deaths of roughly 200,000 civilians and military personnel. The ethics of those bombings and their role in Japan's surrender are subjects of debate.