Acute Exposure: A single exposure to a substance which results in biological harm or death. Usually characterized by a brief exposure lasting no more than a day, as compared to longer, continuing exposure over a period of time (chronic exposure).

Agreement State: A state that has signed an agreement with the Nuclear Regulatory Commission allowing the state to regulate the use of by-product radioactive material within that State.

ALARA: Acronym for "As Low As Reasonably Achievable." It means making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical, consistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in relation to state of technology and in relation to benefits to the public health and safety, and other societal and socioeconomic considerations.

Alpha particle: A positively charged particle ejected spontaneously from the nuclei of some radioactive elements. It has low penetrating power and a short range (a few centimeters in air). The most energetic alpha particle will generally fail to penetrate the dead layers of cells covering the skin and can be easily stopped by a sheet of paper. Alpha particles are hazardous when an alpha-emitting isotope is inside the body.

Atom: The smallest unit of an element that cannot be divided or broken up by chemical means. It consists of a central core of protons and neutrons (except hydrogen which has no neutrons), called the nucleus. Electrons revolve in orbits in the region surrounding the nucleus.

Atomic energy: Energy released in nuclear reactions. Of particular interest is the energy released when a neutron initiates the breaking up of an atom's nucleus into smaller pieces (fission), or when two nuclei are joined together under millions of degrees of heat (fusion). It is more correctly called nuclear energy.

Atomic Energy Commission: Federal agency created in 1946 to manage the development, use, and control of nuclear energy for military and civilian applications. Abolished by the Energy Reorganization Act of 1974 and succeeded by the Energy Research and Development Administration (now part of the U.S. Department of Energy) and the U.S. Nuclear Regulatory Commission.

Atoms for Peace: President Eisenhower's 1954 initiative to allow the peaceful uses of atomic energy to be available to other nations.

Background radiation: Radiation from cosmic sources and terrestrial sources, including radon. It does not include radiation from source or byproduct nuclear materials regulated by the Nuclear Regulatory Commission. The average individual exposure from background radiation is about 300 millrems per year.

Beta particle: A charged particle emitted from a nucleus during radioactive decay, with a mass equal to 1/1837 that of a proton. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Large amounts of beta radiation may cause skin burns, and beta emitters are harmful if they enter the body. Beta particles may be stopped by thin sheets of metal or plastic.

Biological effectiveness factor: Neutrons and alpha particles do more harm per unit dose than photons or beta particles. An experimentally determined value for this difference is referred to as the relative biological effectiveness (RBE) and is mostly restricted to uses in the field of radiobiology. Each species tested, each target organ within that species, and each radionuclide chosen might give a different RBE. For humans, a conservative upper limit of the RBE, called the quality factor (Q) or the radiation weighting factor (WR), is used to determine the dose equivalent.

Carcinogen: A cancer-causing substance.

Chain reaction: A reaction that initiates its own repetition. In a fission chain reaction, a fissionable nucleus absorbs a neutron and fissions spontaneously, releasing additional neutrons. These, in turn, can be absorbed by other fissionable nuclei, releasing still more neutrons. A fission chain reaction is self-sustaining when the number of neutrons released in a given time equals or exceeds the number of neutrons lost by absorption in nonfissionable material or by escape from the system.

Charged particle: An ion. An elementary particle carrying a positive or negative electric charge.

Chronic exposure: Exposure to a substance over a long period of time resulting in adverse health effects.

Compact: A group of two or more states formed to dispose of low-level radioactive waste on a regional basis. Forty-four states have formed ten compacts.

Contamination: The deposition of unwanted radioactive material on the surfaces of structures, areas, objects, or people. It may also be airborne, external, or internal (inside components or people).

Cooling tower: A heat exchanger designed to aid in the cooling of water that is used to cool exhaust steam exiting the turbines of a power plant. Cooling towers transfer exhaust heat into the air instead of into a body of water.

Core: The central portion of a nuclear reactor containing the fuel elements, moderator, neutron poisons, and support structures.

Core melt accident: An event or sequence of events that result in the melting of part of the fuel in a nuclear reactor core.

Cosmic radiation: Ionizing radiation, both particulate and electromagnetic, originating in outer space.

Criticality: A term used in reactor physics to describe the state when the number of neutrons released by fission is exactly balanced by the neutrons being absorbed and escaping the reactor core. A reactor is said to be "critical" when it achieves a self-sustaining nuclear chain reaction, as when the reactor is operating.

Cumulative dose: The total dose to an individual resulting from repeated exposures of ionizing radiation to the same portion of the body, or to the whole body, over a period of time.

Curie (Ci): The basic unit used to describe the intensity of radioactivity in a sample of material. The curie is equal to 37 billion (3 X 1010) disintegrations per second, which is approximately the activity of 1 gram of radium. A curie is also a quantity of any radionuclide that decays at a rate of 37 billion disintegrations per second. It is named for Marie and Pierre Curie, who discovered radium in 1898.

Decay, radioactive: The decrease in the amount of any radioactive material with the passage of time due to the spontaneous emission of radiation from the atomic nuclei (either alpha or beta particles, often accompanied by gamma radiation).

Decommission: The process of closing down a nuclear facility and reducing radioactivity at the facility to a level safe for unrestricted use.

Decontamination: The reduction or removal of contaminated 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 that the radioactivity is decreased as a result of natural radioactive decay, or (3) covering the contamination to limit the radiation emitted.

Dose, absorbed: Represents the amount of energy absorbed from the radiation in a gram of any material. It is expressed numerically in rads.

Dose equivalent (also called biological dose): A measure of the biological damage to living tissue from the radiation exposure. It takes into account the type of radiation and the absorbed dose. For example when considering beta, X-ray, and gamma ray radiation, the equivalent dose (expressed in rems) is equal to the absorbed dose (expressed in rads). For alpha radiation, the equivalent dose is assumed to be twenty times the absorbed dose. It is expressed numerically in rem.

Dose rate: The ionizing radiation dose delivered per unit time. For example, rem per hour.

Dosimeter: A small portable instrument (such as a film badge, thermoluminescent, or pocket dosimeter) for measuring and recording the total accumulated personnel dose of ionizing radiation.

Electromagnetic radiation: Radiation consisting of electric and magnetic waves. A traveling wave motion resulting from changing electric or magnetic fields. It ranges from X-rays (and gamma rays) with short wavelength, through the ultraviolet, visible, and infrared regions, to radar and radio waves with relatively long wave length.

Electron: An elementary particle with a negative charge and a mass 1/1,837 that of the proton. Electrons surround the positively charged nucleus and determine the chemical properties of the atom.

Element: One of the 103 known chemical substances that cannot be broken down further without changing its chemical properties. Some examples include, hydrogen, nitrogen, gold, lead, and uranium.

Entomb: A method of decommissioning a nuclear facility in which radioactive contaminants are encased in long-lived material, such as concrete. The entombment structure is maintained and monitored until the radioactivity decays to a level allowing decommissioning and ultimately, safe unrestricted use of the property.

Epidemiological studies: Studies of the distribution of disease and other health issues as related to age, sex, race, ethnicity, occupation, economic status, or other factors.

Fallout, nuclear: The slow decent of minute particles of radioactive debris in the atmosphere following a nuclear explosion.

Film badge: Photographic film used for measurement of ionizing radiation exposure for personnel monitoring purposes. The film badge may contain two or three films of differing sensitivities, and it may also contain a filter that shields part of the film from certain types of radiation.

Fissile material: Although sometimes used as a synonym for fissionable material, this term has acquired a more restricted meaning: namely, any material fissionable by thermal (slow) neutrons. The three primary fissile materials are uranium-233, uranium- 235, and plutonium-239.

Fission (fissioning): The splitting of a nucleus into at least two other nuclei and the release of a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation. Fissioning is also referred to as burning.

Fuel cycle: The series of steps involved in supplying and managing fuel used in nuclear power reactors. It can include mining, milling, isotopic enrichment, fabrication of fuel elements, use in a reactor, reenrichment of the fuel material, refabrication into new fuel elements, and waste disposal.

Fuel rod: A long, slender tube that holds fissionable material and managing (fuel) used in nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.

Fusion: A reaction in which at least one heavier, more stable nucleus is produced from two lighter, less stable nuclei. Reactions of this type are responsible for enormous release of energy, as in the energy of stars, for example.

Gamma radiation: High-energy, short wavelength, electromagnetic radiation emitted from the nucleus. Gamma radiation frequently accompanies alpha and beta emissions. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead. Gamma rays are similar to X-rays.

Geiger counter (or Geiger-Mueller counter): A radiation detection and measuring instrument. It consists of a gas-filled tube containing electrodes, between which there is an electrical voltage, but no current flowing. When ionizing radiation passes through the tube, a short, intense pulse of current passes from the negative electrode to the positive electrode and is measured or counted. The number of pulses per second measures the intensity of the radiation field. It is the most commonly used portable radiation instrument.

Half-life: The time in which one half of the atoms of a particular radioactive substance decay into another nuclear form. Half-lives vary from millionths of a second to billions of years.

Hazardous Waste: By-products that can pose a substantial or potential hazard to human health or the environment when improperly managed. Hazardous waste has at least one of four characteristics — ignitable, corrosive, reactive, or toxic, or is listed in regulations as hazardous.

High-level waste: Highly radioactive material resulting from the reprocessing of spent nuclear fuel and other highly radioactive material that, under current law, must be permanently isolated.

Ion: (1) An atom that has too many or too few electrons, causing it to have an electrical charge, and therefore, be chemically active. (2) An electron that is not associated (in orbit) with a nucleus.

Ionization: The process of adding one or more electrons to, or removing one or more electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, or nuclear radiation can cause ionization.

Ionizing radiation: Any radiation capable of displacing electrons from atoms or molecules, thereby producing ions. Some examples are alpha, beta, gamma, and X-rays. High doses of ionizing radiation may produce severe skin or tissue damage.

Irradiation: Exposure to radiation.

Isotope: One of two or more atoms with the same number of protons, but different numbers of neutrons in their nuclei. For example, carbon-12, carbon-13, and carbon-14 are isotopes of the element carbon, the numbers denote the approximate atomic weights. Isotopes have very nearly the same chemical properties, but often different physical properties (for example, carbon-12 and -13 are stable, carbon-14 is radioactive).

Linear- no-threshold-hypothesis: The theory that the number of cancers and other effects of exposure to low levels of radiation are proportionate to the number of cancers from exposure to high levels of radiation. The precise effects are uncertain because it is very difficult to directly measure the effects of low levels of radiation.

Manhatten Project: The U.S. government program to develop the first atomic weapons during World War II.

Mill-tailings: Naturally radioactive residue from the processing of uranium ore. Although the milling process recovers about 93 percent of the uranium, the residues, or tailings, contain several naturally-occurring radioactive elements, including uranium, thorium, radium, polonium, and radon.

Molecule: A group of atoms held together by chemical forces. A molecule is the smallest unit of a compound that can exist by itself and retain all of its chemical properties.

Neutron: An uncharged elementary particle with a mass slightly greater than that of the proton, and found in the nucleus of every atom heavier than hydrogen.

Non-ionizing radiation: Radiation that has lower energy levels and longer wavelengths. It is not strong enough to affect the structure of atoms it contacts, but it does heat tissue and can cause harmful biological effects. Examples include radio waves, microwaves, visible light, and infrared from a heat lamp.

NARM/NORM: Naturally Occurring and Accelerator-Produced Radioactive Materials (NARM) include by-products of petroleum production, coal ash, phosphate fertilizer production, drinking water treatment, and other industrial processes. NORM is a subset of NARM and includes everything in NARM except accelator-produced materials. The federal government has not developed a comprehensive policy for NORM/NARM disposal.

Nuclear energy: The heat energy produced by the process of nuclear reaction (fission or fusion) within a nuclear reactor or by radioactive decay.

Nuclear power plant: An electrical generating facility using a nuclear reactor as its power (heat) source. The coolant that removes heat from the reactor core is normally used to boil water. The steam produced by the boiling water drives turbines that rotate electrical generators.

Nuclear tracers: Radioisotopes that give doctors the ability to "look" inside the body and observe soft tissues and organs, in a manner similar to the way X-rays provide images of bones. A radioactive tracer is chemically attached to a compound that will concentrate naturally in an organ or tissue so that a picture can be taken.

Nucleus: The small, central, positively charged region of an atom that carries the atom's nuclei. All atomic nuclei contain both protons and neutrons (except for ordinary hydrogen, which has a single proton). The number of protons determines the total positive charge, or atomic number.

Nuclide: A general term referring to all known isotopes, both stable (279) and unstable (about 5,000), of the chemical elements.

Photon: A quantum (or packet) of energy emitted in the form of electromagnetic radiation. Gamma rays and X-rays are examples of photons.

Picocurie: One trillionth of a curie.

Plutonium: A very heavy element formed when uranium-238 absorbs neutrons. Like uranium, it has two principal isotopes that are fissile.

Poison, neutron: In reactor physics, a material other than fissionable material, in the vicinity of the reactor core that will absorb neutrons. The addition of poisons, such as control rods or boron, into the reactor is said to be an addition of negative reactivity.

Positron: Particle equal in mass, but opposite in charge, to the electron (a positive electron).

Power reactor: A reactor designed to produce heat for electric generation, as distinguished from reactors used for research, for producing radiation or fissionable materials, or for reactor component testing.

Proton: An elementary nuclear particle with a positive electric charge located in the nucleus of an atom.

Quality factor: The factor by which the absorbed dose (rad) is multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem) to an exposed individual. It is used because some types of radiation, such as alpha particles, are more biologically damaging internally than other types.

Rad: The unit of absorbed dose, which is the amount of energy from any type of ionizing radiation (e.g., alpha, beta, gamma, etc.) deposited in any medium (e.g., water, tissue, air). A dose of one rad means the absorption of 100 ergs (a small but measurable amount of energy) per gram of absorbing tissue.

Radiation: Energy in the form of waves or particles sent out over a distance.

Radiation sickness (or syndrome): The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure (greater than 200 rads) of the whole body (or large part) 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, where the radiation exposure has been approximately 1,000 rad or more, death may occur within two to four weeks.

Radiation standards: Exposure limits, permissible concentrations, rules for safe handling, regulations for transportation, and regulations controlling the use of radiation and radioactive material.

Radiation warning symbol: An officially prescribed symbol (a magenta or black trefoil) on a yellow background that must be displayed where certain quantities of radioactive materials are present or where certain doses of radiation could be received.

Radioactive contamination: Deposition of radioactive material in any place where it may harm persons, equipment, or the environment.

Radioactivity: The emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable isotope. Also, the rate at which radioactive material emits radiation.

Radioisotope: An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. Approximately 5,000 natural and artificial radioisotopes have been identified.

Radionuclide: A radioactive nuclide. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.

Radiology: The branch of medicine dealing with the diagnostic and therapeutic applications of radiation, including X-rays and radioisotopes.

Radon (Rn): A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is found naturally in soil and rocks and is formed by the radioactive decay of radium.

Reactor, nuclear: A device in which nuclear fission may be sustained and controlled in a self-supporting nuclear reaction. There are different designs.

Recycling: The reuse of slightly contaminated materials.

Rem: The unit of measurement of dose equivalent. The rem value takes into account both the amount, or dose, of radiation and the biological effect of the specific type of radiation. Rem equals the absorbed dose multiplied by the quality factor. (100 rem = 1 sievert)

Reprocessing: The mechanical and chemical process of separating out usable products (like uranium and plutonium) from spent or depleted reactor fuel then re-enriching and re-fabricating them into new fuel elements.

Risk: In many health fields, risk means the probability of incurring injury, disease, or death. Risk can be expressed as a value that ranges from zero (no injury or harm will occur) to one hundred percent (harm or injury will definitely occur).

Risk assessment: Qualitative and quantitative evaluation of the risk posed to human health and/or the environment by the actual or potential presence of hazards.

Roentgen: A unit of exposure to ionizing radiation. It is the amount of gamma or xrays required to produce ions resulting in a charge of 0.000258 coulombs/kilogram of air under standard conditions.

Somatic effects of radiation: Effects of radiation limited to the exposed individual, as distinguished from genetic effects, which may also affect subsequent unexposed generations.

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

Subatomic particles: The matter that makes up atoms. It includes particles such as neutrons, protons, electrons, and many more.

Superfund: The program operated under the authority of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Superfund Amendments and Reauthorization Act (SARA) that funds and carries out EPA hazardous waste emergency and long-term removal and remedial activities.

Terrestrial radiation: Radiation that is emitted by naturally occurring radioactive materials in the Earth, such as uranium, thorium, and radon.

Thermoluminescent dosimeter: A small device used to measure radiation dose by measuring the amount of visible light emitted from a crystal in the detector. The amount of light emitted is proportional to the radiation dose received.

Thermonuclear: An adjective referring to the process in which very high temperatures are used to bring about the fusion of light nuclei, such as those of the hydrogen isotopes deuterium and tritium, with the accompanying liberation of energy.

Ultraviolet radiation: Radiation of a wavelength between the shortest visible violet rays and low energy xrays.

Unstable isotope: A radioactive isotope.

Uranium: The heaviest element normally found in nature. The principal fuel material used in today's nuclear reactors is the fissile isotope uranium-235.

Uranium mill-tailings: See Mill-tailings. Waste, radioactive: Solid, liquid, and gaseous materials from nuclear operations or TENORM activities that are radioactive or become radioactive and for which there is no further use.

Whole-body exposure: An exposure of the body to radiation, in which the entire body, rather than an isolated part, is irradiated.

Xrays: One type of electromagnetic radiation which arises as electrons are deflected from their original paths or inner orbital electrons change their energy levels around the atomic nucleus. Like gamma rays, xrays require more shielding to reduce their intensity than do beta or alpha particles.

Sources:

• Glossary of Nuclear Terms, Nuclear Regulatory Commission, http://www.nrc.gov/reading-rm/basic-ref/glossary.html
• Fact Sheet, Health Physics Society, http://www.hps.org/publicinformation/radfactsheets/index.html
• Glossary of Nuclear Terms, http://ie.lbl.gov/education/glossary/glossaryf.htm, Lawrence Berkely Laboratory
• Glossary of Nuclear Terms, Frontline, PBS, http://www.pbs.org/wgbh/pages/frontline/shows/reaction/etc/terms.html
• Terms of Environment, Environmental Protection Agency, http://www.epa.gov/OCEPAterms/intro.htm.

Understanding Radiation | NSC Home | Comments

December 3, 2002