A Glossary of Terms From a Nuclear Accident

Following is a glossary of terms related to nuclear power:

Alpha particles: Emitted from naturally occurring materials (such as uranium, thorium and radium) and man-made elements (such as plutonium and americium). They have a very limited ability to penetrate other materials. These particles of ionizing radiation can be blocked by a sheet of paper, skin or even a few inches of air, but are potentially dangerous if they are inhaled or swallowed.

Background radiation: The radiation in the natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals. The usually quoted average individual exposure from background radiation is 300 millirems per year.

Beta particles: These are lighter than alpha particles, and they generally have a greater ability to penetrate other materials. As a result, these particles can travel a few feet in the air and can penetrate skin. Nonetheless, a thin sheet of metal or plastic or a block of wood can stop beta particles. As a result, like alpha particles, they do not usually make things radioactive but can cause biological harm.

Biological half-life: The time required for a biological system, such as that of a human, to eliminate, by natural processes, half of the amount of a substance (such as a radioactive material) that has entered it.

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

Containment: The gas‑tight shell or other enclosure around a reactor to confine fission products that otherwise might be released into the atmosphere in the event of an accident.

Coolant: A substance circulated through a nuclear reactor to remove or transfer heat. The most commonly used coolant in the U.S. is water. Other coolants include air, carbon dioxide and helium.

Core: The central portion of a nuclear reactor containing the fuel elements and control rods.

Decay heat: The heat produced by the decay of radioactive fission products after the reactor has been shut down.

Decontamination: The reduction or removal of contaminating radioactive material from a structure, area, object or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination; (2) letting the material stand so the radioactivity is decreased by natural decay; and (3) covering the contamination to shield the radiation emitted.

Dosimeter: A small, portable instrument to measure and record the total accumulated personal dose of ionizing radiation.

Emergency feed-water system: Backup feed-water supply used during nuclear plant start-up and shutdown; also known as auxiliary feed-water system.

Feed water: Water supplied to the steam generator that removes heat from the fuel rods by boiling and becoming steam. The steam then becomes the driving force for the turbine generator.

Fuel rod: A long, slender tube that holds uranium fuel pellets that fuel nuclear reactors. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.

Gamma rays and X-rays: High-energy waves that can travel great distances at the speed of light and generally have a great ability to penetrate other materials. For that reason, gamma rays (such as those from cobalt-60) are often used in medical applications to treat cancer and sterilize medical instruments. Exposure to uncontrolled gamma rays can cause biological harm.

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 a release of radioactive contamination.

Nuclear reactor: A device in which nuclear fission may be sustained and controlled in a self‑supporting nuclear reaction. There are several varieties but all incorporate certain features, such as fissionable material or fuel, a moderating material (to control the reaction), a reflector to conserve escaping neutrons, provisions for removal of heat, measuring and controlling instruments, and protective devices.

Pressure vessel: A strong‑walled container housing the core of most types of power reactors.

Pressurizer: A tank or vessel that controls the pressure in a certain type of nuclear reactor.

Primary system: The cooling system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine.

Radiation: Particles (alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an unstable atom as a result of radioactive decay.

Reactor coolant system: (see primary system)

SCRAM: An immediate emergency shutdown of a reactor where control rods are automatically inserted into the core to halt the heat-generating fission.

Secondary system: The steam generator tubes, steam turbine, condenser and associated pipes, pumps and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation.

Steam generator: The heat exchanger used in some reactor designs to transfer heat from the primary (reactor coolant) system to the secondary (steam) system. This design permits heat exchange with little or no contamination of the secondary system equipment.

Turbine: A rotary engine made with a series of curved vanes on a rotating shaft. Usually turned by water or steam, turbines are considered to be the most economical means to turn large electrical generators.