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.

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.

Fuel rod: Long, slender, zirconium metal tubes containing pellets of fissionable material, which provide fuel for nuclear reactors. Fuel rods are assembled into bundles called fuel assemblies, which are loaded individually into the reactor core.

Half-life: The time in which one half of the atoms of a particular radioactive substance disintegrate into another nuclear form. Measured half-lives vary from millionths of a second to billions of years. Also called physical or radiological half-life.

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

Ion: An atom that has too many or too few electrons, causing it to have an electrical charge, and therefore, be chemically active. Also, an electron that is not associated (in orbit) with a nucleus.

Ionizing radiation: A form of radiation that include gamma rays, x-rays, high-speed electrons and other particles capable of producing ions. Compared to non-ionizing radiation, such as radio- or microwaves, or visible, or ultraviolet light, ionizing radiation has considerably more energy. When ionizing radiation passes through material such as air, water, or living tissue, it deposits enough energy to produce ions by breaking molecular bonds and displace (or remove) electrons from atoms or molecules. This electron displacement may lead to changes in living cells.

Irradiation: Exposure to ionizing radiation. Irradiation may be intentional, such as in cancer treatments or in sterilizing medical instruments. It may also be accidental, such as being exposed to an unshielded source. It does not usually result in radioactive contamination, but damage can occur, depending on the dose received.

Isotopes: Two or more forms of a given element that have identical atomic numbers (the same number of protons in their nuclei) and the same or very similar chemical properties. But they have different atomic masses (different numbers of neutrons in their nuclei) and distinct physical properties. Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element carbon, and the numbers denote the approximate atomic masses. Among their distinct physical properties, some isotopes are radioactive because their nuclei emit radiation as they strive toward a more stable nuclear configuration. For example, carbon-12 and carbon-13 are stable, but carbon-14 is unstable and radioactive.

Meltdown: Severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping.

Mixed oxide fuel: A type of nuclear reactor fuel (often called “MOX”) that contains plutonium oxide mixed with either natural or depleted uranium oxide, in ceramic pellet form. This differs from conventional nuclear fuel, which is made of pure uranium oxide. Using plutonium reduces the amount of highly enriched uranium needed to produce a controlled reaction in commercial light-water reactors. However, plutonium exists only in trace amounts in nature and, therefore, must be produced by neutron irradiation of uranium-238 or obtained from other manufactured sources.

Nuclear fuel: Fissionable material that has been enriched to a composition that will support a self-sustaining fission chain reaction when used to fuel a nuclear reactor, thereby producing energy (usually in the form of heat or useful radiation) for use in other processes.

Probabilistic risk analysis: A systematic method for addressing the risks inherent in a complex system to understand likely outcomes, sensitivities, areas of importance, system interactions, and areas of uncertainty. The “risk triplet” is the set of three questions that the NRC uses to define “risk”: (1) What can go wrong? (2) How likely is it? and (3) What are the consequences?

Rad: One of the two units used to measure the amount of radiation absorbed by an object or person. A rad measures the amount of energy that radioactive sources deposit in materials they pass through.

Radiation, nuclear: Energy given off in the form of tiny fast-moving particles or pulsating electromagnetic rays or waves emitted from the nuclei of unstable radioactive atoms. All matter is composed of atoms, which are made up of various parts; the nucleus contains minute particles called protons and neutrons, and the atom’s outer shell contains other particles called electrons. The nucleus carries a positive electrical charge, while the electrons carry a negative electrical charge. These forces work toward a strong, stable balance by getting rid of excess atomic energy (radioactivity).

Radioactivity: The property possessed by some elements (such as uranium) of spontaneously emitting energy in the form of radiation as a result of the decay (or disintegration) of an unstable atom. Radioactivity is also the term used to describe the rate at which radioactive material emits radiation. Radioactivity is measured in curies, or disintegrations per second.

Radiation sickness: The complex of symptoms that result from excessive exposure of the body to ionizing radiation. The earliest of these symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed by loss of hair, hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, death may occur within two to four weeks. Those who survive six weeks after the receipt of a single large dose of radiation to the whole body may generally be expected to recover.

Radioactive decay: The spontaneous transformation of one radioactive isotope into one or more different isotopes (known as “decay products” or “daughter products”), accompanied by a decrease in radioactivity. This transformation takes place over a defined period of time (known as a “half-life”). Each isotope in the sequence (known as a “decay chain”) decays to the next until it forms a stable, less energetic end product.

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

Scram: The sudden shutting down of a reactor, typically by the rapid insertion of control rods. It may occur either automatically or manually by the reactor operator. It originally stood for safety control rod axe man, a title given to personnel who were assigned to insert the emergency rod at the original Manhattan Project experiment.

Sievert: A dosage unit that attempts to measure the biological effects of exposure to radioactive materials. It is named after Rolf Sievert, a Swedish medical physicist renowned for research into the biological effects of radiation.

Spent fuel pool: An underwater storage and cooling facility for depleted fuel assemblies that have been removed from a reactor.

Millisievert: One-thousandth of a sievert, a measurement of the effect of radiation on a body. The level of millisieverts determines how much radiation is present in an environment.

Spent nuclear fuel: Nuclear reactor fuel that has been used to the extent that it can no longer effectively sustain a chain reaction.

Uranium: A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (which comprises 0.7 percent of natural uranium and can begin a natural fission chain reaction) and uranium-238 (99.3 percent of natural uranium that can begin splitting after absorbing one neutron.

Marc Kaufman

Sources: Nuclear Regulatory Commission, Merriam-Webster Dictionary, Scientific Digital Images