Following are some of the technical terms and their definitions used in nuclear energy:

CHAIN REACTION: When uranium atoms split, they emit neutrons that split other uranium atoms in a continuing process. When the number of neutrons emitted is sufficient to keep the chain reaction going, the reactor is said to have reached criticality or has reached critical mass.

CLADDING: The material that coats or surrounds the nuclear fuel.

CONDENSER: The heat exchanger in which steam is transformed into (liquid) water by removing heat.

CONTROL RODS: Carbon that, when inserted into the nuclear core, neutralizes the fission, causing the reaction to slow down or stop.

COOLANT: The fluid that removes the nuclear-generated heat from the core; in most plants, water.

PRIMARY COOLANT SYSTEM: The entire circuit through which the fluid passes including piping, vessels and components; may include the reactor vessel, coolant pump and steam generator.

COOLING TOWER: An hourglass shaped tower in which heat from the reactor is released to the air by evaporating water. This process does not release radioactivity.

CONTAINMENT BUILDING: The thick-walled building that surrounds the reactor vessel. It is the second line of defense against radioactivity being released to the atmosphere.

CORE: The center of a nuclear reactor that contains the fissionable fuel that, when activated, splits atoms of uranium and thus produces heat. The heat in turn converts water in nearby generators into steam that operates the turbines that produce electricity.

DECAY HEAT: The heat produced by radioactive decay of materials that are primarily the remnants of the chain reaction.

DOME: The top of the structure that houses the core. The core of the Three Mile Island dome structure has8.4-inch high-strength carbon-steel walls. It is housed in the "containment building" with walls 4-feet-thick made of prestressed concrete and steel reinfforcing rods.

FISSION: The splitting of the nucleus of an atom enabling the creation of nuclear energy.

FUEL PELLET: The basic form in which the uranium is contained.

FUEL RODS: Hollow pipes containing uranium fuel pellets that fuel the reactor to produce heat.

MELTDOWN: The overheating of a plant's nuclear fuel to such a degree that it melts the protective shell around the nuclear reactor core, resulting in release of radioactive contamination.

MILLIREM: The term used to measure absorption of radiation by humans. The average Americann is exposed to 100 to 200 milirems of radiation per year, including radiation from x-rays to cosmic rays. A normal chest x-ray exposes a person to between 20 and 30 millirems.

NUCLEAR RADIATION: The release of nuclear energy which, when absorbed by the human body in sufficient quantities, can damage or kill human cells. The dangers of radiation include death, latent cancers, genetic damage and contamination of the environment.

REACTOR VESSEL: The pressure tank that surrounds the core, control rods and related equipment. It is the first line of defense against the release of radioactivity to the outside.

URANIUM: The element-a metal, with radioactive properties-used as fuel because of its ability to undergo continuous fission.