Japan crisis: Glossary of nuclear terms

A glossary of some of the nuclear terms being used as Japan’s nuclear plant crisis unfolds.

Cladding: The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents both corrosion of the fuel by the coolant and the release of fission products into the coolant. Aluminum, stainless steel, and zirconium alloys are common cladding materials.

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Watch how the crisis at the Fukushima Daiichi power plant unfolded.

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Track the status of the nuclear crisis in Japan.

Containment building: The air-tight building, which houses a nuclear reactor and its pressurized, reactor coolant pumps, steam generator, and other equipment or piping that might otherwise release fission products to the atmosphere in the event of an accident. Such buildings are usually made of steel-reinforced concrete.

Core: The central portion of a nuclear reactor, which contains the fuel assemblies, control rods and support structures. The reactor core is where fission takes place.

Control rod: A rod, plate, or tube containing a material such as hafnium or boron used to control the power of a nuclear reactor. By absorbing neutrons, a control rod prevents the fission reactions from cascading further than desired.

Criticality: The normal operating condition of a reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.

Curie: One of the three units to measure the intensity of radioactivity. The value refers to the amount of highly-charged radiation released when an element such as uranium spontaneously emits energy as a result of radioactive decay. The curie is named after Marie and Pierre Curie, who discovered the element radium in 1898.

Depleted uranium: Uranium with a percentage of uranium-235 lower than the 0.7 percent (by mass) contained in natural uranium. Depleted uranium is typically found in spent fuel elements or byproduct tailings or residues. Depleted uranium can be blended with highly-enriched uranium, such as that from weapons, to make reactor fuel.

Dose: A general term, which may be used to refer to the amount of energy absorbed by an object or person. Known as the “absorbed dose,” this reflects the amount of energy that radioactive sources deposit in materials through which they pass, and is measured in units of radiation-absorbed dose (rad). The biological dose, given in rems or sieverts, is a measure of the biological damage to living tissue as a result of radiation exposure.

Fission: The splitting of an atom, which releases a considerable amount of energy (usually in the form of heat) that can be used to produce electricity. Fission may be spontaneous, but is usually caused by the nucleus of an atom becoming unstable (or “heavy”) after capturing or absorbing a neutron. During fission, the heavy nucleus splits into roughly equal parts, producing the nuclei of at least two lighter elements. In addition to energy, this reaction usually releases very high-energy gamma radiation.

Fuel pin: A long slender tube made of a zirconium alloy and containing fuel pellets. Several fuel pins are bundled together to form a fuel element.