ATOMIC BOMBS, powerful explosive nuclear weapon fueled by the splitting, or fission, of the nuclei of specific isotopes of uranium or plutonium in a chain reaction. The strength of the explosion created by an atomic bomb is on the order of the strength of the explosion that would be created by thousands of tons of TNT (see Trinitrotoluene). An atomic bomb must provide enough mass of plutonium or uranium to reach critical mass, the mass at which the nuclear reactions going on inside the material can make up for the neutrons leaving the material through its outside surface. Usually the plutonium or uranium in a bomb is separated into parts so that critical mass is not reached until the bomb is set to explode. At that point, a set of chemical explosives or some other mechanism drives all the different pieces of uranium or plutonium together to produce a critical mass. After this occurs, there are enough neutrons bouncing around in the material to create a chain reaction of fissions. In the fission reactions, collisions between neutrons and uranium or plutonium atoms cause the atoms to split into pairs of nuclear fragments, releasing energy and more neutrons. Once the reactions begin, the neutrons released by each reaction hit other atoms and create more fission reactions until all the fissile material is exhausted or scattered. NUCLEAR BOMBS, represent a giant increase in explosive power over conventional bombs. Nuclear bombs have only been used against another country twice, when American B-29 bombers dropped them on the Japanese cities of Hiroshima and Nagasaki in August 1945 during World War II. In addition to blast and heat, nuclear bombs also release radioactive substances, often in the form of radioactive fallout. Radioactivity can cause radiation sickness that can lead to death months or years later and pollute the target for centuries.There are two types of nuclear weapons: atomic and hydrogen (often referred to as A-bombs and H-bombs). Atomic bombs release huge amounts of energy by splitting the nuclei of atoms in a process called fission. High-energy neutrons bang into atoms of uranium or plutonium and cause the atoms to split, releasing more neutrons. In the Hiroshima bomb nicknamed “Little Boy,” a device similar to a gun fired two masses of enriched uranium at each other. When they merged, the two pieces made up a supercritical mass, in which enough neutrons were present to cause a chain reaction that had the force of 12.5 kilotons tons of TNT (12,500 tons). The Nagasaki bomb (nicknamed “Fat Man”) contained two hemispheres of radioactive plutonium wrapped in a sphere of explosive. The explosive squeezed the plutonium spheres together, creating a supercritical mass and an explosion equal to 22 kilotons (22,000 tons) of TNT. NUCLEAR WEAPONS, explosive devices designed to release nuclear energy on a large scale, used primarily in military applications. The first atomic bomb (or A-bomb), which was tested on July 16, 1945, at Alamogordo, New Mexico, represented a completely new type of explosive. All explosives prior before that time derived their power from the rapid burning or decomposition of some chemical compound. Such chemical processes release only the energy of the outermost electrons in the atom. The atomic bombings of Hiroshima and Nagasaki were nuclear attacks during World War II against the Empire of Japan by the United States of America under US President Harry S. Truman. On August 6, 1945, the nuclear weapon "Little Boy" was dropped on the city of Hiroshima, followed on August 9, 1945 by the explosion of the "Fat Man" nuclear bomb over Nagasaki. These are the only uses of nuclear weapons in warfare.

Several factors make it difficult to estimate casualty numbers due to the bombings. The population before the bombings is only roughly known, because of formal and informal withdrawls, and unknown numbers of forced laborers. Some victims were burned beyond recognition or their bodies disposed in mass cremations. Records of military personnel were destroyed, and entire families perished, leaving nobody to report the casualties. According to most estimates, the bombing of Hiroshima killed approximately 70,000 people due to immediate effects of the blast. Estimates of total deaths by the end of 1945 range from 90,000 to 140,000, due to burns, radiation, and subsequent disease, were made severe by lack of medical resources. Some estimates state up to 200,000 may have died by 1950, due to cancer and other long-term effects.The numbers for Nagasaki are comparatively lower, because the valley terrain reduced the impact of the bomb, with immediate deaths estimates ranging from 40,000 to 75,000. In both cities, the majority of the deaths were civilians.The role of the bombings in Japan's surrender, as well as the effects and justification of them, has been subject to much debate.On August 15, 1945 Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2 which officially ended World War II. Further, the experience of bombing led post-war Japan to adopt Three Non-Nuclear Principles, which prevents Japan from nuclear armament.