Enriched uranium: Uranium that has been treated to increase the proportion of the isotope U-235, a form more readily split than U-238, the dominant isotope. In nature, 99.3 percent of all uranium atoms are U-238; only 0.7 percent are U-235.

Plutonium: A highly radioactive element made by bombarding U-238 with neutrons. Plutonium does not exist in nature; it is made in nuclear reactors.

Fission: Splitting atoms into two smaller atoms, a process that releases energy.

Fusion: Combining atoms into one larger atom, releasing energy. Fusion is what keeps stars burning. In nuclear weapons, a fission device is used to fuse deuterium and tritium, two isotopes of hydrogen.

Critical mass: The amount of nuclear "fuel" needed for a nuclear explosion. "Little Boy" required 60 kilograms (132 pounds) of U-235. "Fat Man" needed only 6.2 kilograms (13.6 pounds) of plutonium.

Nuclear chain reaction: A reaction that results when neutrons split the nuclei of uranium or plutonium atoms, releasing two or more neutrons. These neutrons split additional nuclei, releasing yet more neutrons to continue the process. As the nuclei split, energy is released.

Kiloton, megaton: Units of measure for the force of nuclear weapons. A yield of 1 kiloton equals the explosive force of 1,000 tons of TNT. A 1-megaton device equals 1 million tons of TNT. The most powerful hydrogen bomb in the US arsenal had a yield of between 15 and 20 megatons.

Little Boy: Nickname for the first atomic bomb used in war. The United States dropped Little Boy on Hiroshima, Japan, on Aug. 6, 1945. It exploded with the force of 15 kilotons. High explosives shot a small quantity of U-235 into a larger chunk of U-235 to create a critical mass.

Fat Man: The bomb dropped on Nagasaki, Japan, on Aug. 9, 1945. It exploded with the force of 23 kilotons. Critical mass was achieved by using a spherical shell of high explosives to compress a subcritical core of plutonium.

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