Glossary of Nuclear Terms




Glossary of Nuclear Terms

Access hatchAn airtight door system that preserves the pressure integrity of a reactor containment structure while allowing access to personnel and equipment.
Activation The process of making a radioisotope by bombarding a stable element with neutrons or protons.
Active fuel length The end-to-end dimension of fuel material within a fuel element.
Agreement State A state that has signed an agreement with the Nuclear Regulatory Commission under which the state regulates the use of by-product, source and small quantities of special nuclear material within that state.
Air sampling The collection of samples of air to measure the radioactivity or to detect the presence of radioactive material, particulate matter, or chemical pollutants in the air.
Airborne radioactivity area A room, enclosure, or area in which airborne radioactive materials, composed wholly or partly of licensed material, exist in concentrations that: (1) Exceed the derived air concentration limits, or (2) Would result in an individual present in the area without respiratory protection exceeding, during the hours the individual is present in the area, 0.6 percent of the annual limit on intake or 12 derived air concentration-hours (see 10 CFR §20.1003 Definitions).
ALARA Acronym for "As Low As Reasonably Achievable," 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, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and in relation to utilization of nuclear energy and licensed materials in the public interest (see 10 CFR 20.1003).
Alpha particle A positively charged particle ejected spontaneously from the nuclei of some radioactive elements. It is identical to a helium nucleus that has a mass number of 4 and an electrostatic charge of +2. 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.
Anion A negatively charged ion.
Annual limit on intake (ALI) The derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. ALI is the smaller value of intake of a given radionuclide in a year by the reference man that would result in a committed effective dose equivalent of 5 rems(0.05 sievert) or a committed dose equivalent of 50 rems (0.5 sievert) to any individual organ or tissue. (see 10 CFR 20.1003.)
Atom The smallest particle of an element that cannot be divided or broken up by chemical means. It consists of a central core of protons and 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 or fissioning 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.
Atomic number The number of positively charged protons in the nucleus of an atom.
Attenuation The process by which the number of particles or photons entering a body of matter is reduced by absorption and scattered radiation.
Auxiliary feedwater Backup water supply used during nuclear plant startup and shutdown to supply water to the steam generators during accident conditions for removing decay heat from the reactor.
Average planar linear heat generation rate (APLGHR) The average value of the linear heat generation rate of all the control rods at any given horizontal plane along a fuel bundle.
Background radiation Radiation from cosmic sources; naturally occurring radioactive materials, including radon (except as a decay product of source or special nuclear material) and global fallout as it exists in the environment from the testing of nuclear explosive devices. It does not include radiation from source, byproduct, or special nuclear materials regulated by the Nuclear Regulatory Commission. The typically quoted average individual exposure from background radiation is 360 millirems per year.
Becquerel (Bq) The unit of radioactive decay equal to 1 disintegration per second. 37 billion (3.7x1010) becquerels = 1 curie (Ci).
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 bums, and beta emitters are harmful if they enter the body. Beta particles may be stopped by thin sheets of metal or plastic.
Binding energy The minimum energy required to separate a nucleus into its component neutrons and protons.
Bioassay The determination of kinds, quantities or concentrations, and in some cases, the locations, of radioactive material in the human body, whether by direct measurement (in vivo counting) or by analysis and evaluation of materials excreted or removed (in vitro) from the human body.
Biological halflife 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.
Biological shield A mass of absorbing material placed around a reactor or radioactive source to reduce the radiation to a level safe for humans.
Boiling water reactor (BWR) A reactor in which water, used as both coolant and moderator, is allowed to boil in the core. The resulting steam can be used directly to drive a turbine and electrical generator, thereby producing electricity.
Bone seeker A radioisotope that tends to accumulate in the bones when it is introduced into the body. An example is strontium-90, which behaves chemically like calcium.
Breeder A reactor that produces more nuclear fuel than it consumes. A fertile material, such as uranium-238, when bombarded by neutrons, is transformed into a fissile material, such as plutonium-239, which can be used as fuel.
British thermal unit (Btu) A British thermal unit. The amount of heat required to change the temperature of one pound of water one degree Fahrenheit at sea level.
Byproduct Byproduct is (1) any radioactive material (except special nuclear material) yielded in, or made radioactive by, exposure to the radiation incident to the process of producing or using special nuclear material (as in a reactor); and (2) the tailings or wastes produced by the extraction or concentration of uranium or thorium from ore (see 10 CFR 20.1003).
Calibration The adjustment, as necessary, of a measuring device such that it responds within the required range and accuracy to known values of input.
Capability The maximum load that a generating station can carry under specified conditions for a given period of time without exceeding approved limits of temperature and stress.
Capacity factor (gross) The ratio of the gross electricity generated, for the period of time considered, to the energy that could have been generated at continuous full-power operation during the same period.
Capacity factor (net) The ratio of the net electricity generated, for the period of time considered, to the energy that could have been generated at continuous full-power operation during the same period.
Cask A heavily shielded container used to store and/or ship radioactive materials. Lead and steel are common materials used in the manufacture of casks.
Cation A positively charged ion.
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.
Chemical recombination Following an ionization event, the positively and negatively charged ion pairs may or may not realign themselves to form the same chemical substance they formed before ionization. Thus, chemical recombination could change the chemical composition of the material bombarded by ionizing radiation.
Cladding The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents 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.
Cleanup system A system used for continuously filtering and demineralizing a reactor coolant system to reduce contamination levels and to minimize corrosion.
Coastdown An action that permits the reactor power level to decrease gradually as the fuel in the core is depleted.
Cold shutdown The term used to define a reactor coolant system at atmospheric pressure and at a temperature below 200 degrees Fahrenheit following a reactor cooldown.
Collective dose The sum of the individual doses received on a given period of time by a specified population from exposure to a specified source of radiation.
Committed dose equivalent This is the dose to some specific organ or tissue that is received from an intake of radioactive material by an individual during the 50-year period following the intake (see 10 CFR 20.1003).
Committed effective dose equivalent The committed dose equivalent for a given organ multiplied by a weighting factor (see 10 CFR 20.1003).
Compact A group of two or more States formed to dispose of low-level radioactive waste on a regional basis. Forty-two States have formed nine compacts.
Compound A chemical combination of two or more elements combined in a fixed and definite proportion by weight.
Condensate Water that has been produced by the cooling of steam in a condenser.
Condenser A large heat exchanger designed to cool exhaust steam from a turbine below the boiling point so that it can be returned to the heat source as water. In a pressurized water reactor, the water is returned to the steam generator. In a boiling water reactor, it returns to the reactor core. The heat removed from the steam by the condenser is transferred to a circulating water system and is exhausted to the environment, either through a cooling tower or directly into a body of water.
Construction recapture The maximum number of years that could be added to the license expiration date to recover the period from the construction permit to the date when the operating license was granted. A licensee is required to submit an application for such a change.
Contamination Undesired radioactive material that is deposited on the surface of or inside structures, areas, objects or people.
Containment structure A gaslight shell or other enclosure around a nuclear reactor to confine fission products that otherwise might be released to the atmosphere in the event of an accident.
Control rod A rod, plate, or tube containing a material such as hafnium, boron, etc., used to control the power of a nuclear reactor. By absorbing neutrons, a control rod prevents the neutrons from causing further fissions.
Controlled area At a nuclear facility, an area outside of a restricted area but within the site boundary, access to which can be limited by the licensee for any reason.
Control room The area in a nuclear power plant from which most of the plant power production and emergency safety equipment can be operated by remote control.
Coolant A substance circulated through a nuclear reactor to remove or transfer heat. The most commonly used coolant in the United States is water. Other coolants include heavy water, air, carbon dioxide, helium, liquid sodium, and a sodium-potassium alloy.
Cooldown The gradual decrease in reactor fuel rod temperature caused by the removal of heat from the reactor coolant system after the reactor has been shutdown.
Cooling tower A heat exchanger designed to aid in the cooling of water that was 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 the reactor core.
Cosmic radiation Penetrating ionizing radiation, both particulate and electromagnetic, originating in outer space. Secondary cosmic rays, formed by interactions in the earth's atmosphere, account for about 45 to 50 millirem of the 360 millirem background radiation that an average individual receives in a year.
Counter A general designation applied to radiation detection instruments or survey meters that detect and measure radiation. The signal that announces an ionization event is called a count.
Critical mass The smallest mass of fissionable material that will support a self-sustaining chain reaction.
Critical organ That part of the body that is most susceptible to radiation damage under the specific conditions under consideration.
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 (by the fuel and poisons) 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.
Crud A colloquial term for corrosion and wear products (rust particles, etc.) that become radioactive (i.e., activated) when exposed to radiation. The term is actually an acronym for Chalk River Unidentified Deposits, the Canadian plant at which the activated deposits were first discovered.
Cumulative dose The total dose resulting from repeated exposures of ionizing radiation to an occupationally exposed worker to the same portion of the body, or to the whole body, over a period of time (see 10 CFR 20.1003).
Curie (Ci) The basic unit used to describe the intensity of radioactivity in a sample of material. The curie is equal to 37 billion (3.7X1010) 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.
Daughter products Isotopes that are formed by the radioactive decay of some other isotope. In the case of radium-226, for example, there are 10 successive daughter products, ending in the stable isotope, lead-206.
Decay heat The heat produced by the decay of radioactive fission products after a reactor has been shut down.
Decay, radioactive The decrease in the amount of any radioactive material with the passage of time due to the spontaneous emission from the atomic nuclei of either alpha or beta particles, often accompanied by gamma radiation.
Declared pregnant woman A woman who is also an occupational radiation worker and has voluntarily informed her employer, in writing, of her pregnancy and the estimated date of conception (see 10 CFR 20.1003, 20.1208).
Decommission The process of closing down a facility followed by reducing residual radioactivity to a level that permits the release of the property for unrestricted use (see 10 CFR 20.1003).
Decon A method of decommissioning in which the equipment, structures, and portions of a facility and site containing radioactive contaminants are removed and safety buried in a low-level radioactive waste landfill or decontaminated to a level that permits the property to be released for unrestricted use shortly after cessation of operations.
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 that the radioactivity is decreased as a result of natural radioactive decay, or (3) covering the contamination to shield or attenuate the radiation emitted (see 10 CFR 20.1003 and §20.1402).
Defense-in-depth A design and operational philosophy with regard to nuclear facilities that calls for multiple layers of protection to prevent and mitigate accidents. It includes the use of controls, multiple physical barriers to prevent release of radiation, redundant and diverse key safety functions, and emergency response measures.
Departure from nucleate boiling (DNB) The point at which the heat transfer from a fuel rod rapidly decreases due to the insulating effect of a steam blanket that forms on the rod surface when the temperature continues to increase.
Departure From Nuclear Boiling Ratio (DNBR) The ratio of the heat flux to cause departure from nucleate boiling to the actual local heat flux or a fuel rod.
Depleted uranium Uranium having a percentage of uranium-235 smaller than the 0.7 percent found in natural uranium. It is obtained from spent (used) fuel elements or as byproduct tails, or residues, from uranium isotope separation.
Derived air concentration (DAC) The concentration of radioactive material in air and the time of exposure to that radionuclide, in hours. An NRC licensee may take 2,000 hours to represent one ALI, equivalent to a committed effective dose equivalent of 5 rems (0.05 sievert).
Design-basis accident A postulated accident that a nuclear facility must be designed and built to withstand without loss to the systems, structures, and components necessary to assure public health and safety.
Design-basis phenomena Earthquakes, tornadoes, hurricanes, floods, etc., that a nuclear facility must be designed and built to withstand without loss of systems, structures, and components necessary to assure public health and safety.
Detector A material or device that is sensitive to radiation and can produce a response signal suitable for measurement or analysis. A radiation detection instrument.
Deterministic effect The health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Radiation-induced cataract formation is an example of a deterministic effect (also called a non-stochastic effect) (see 10 CFR 20.1003).
Deuterium An isotope of hydrogen with one proton and one neutron in the nucleus.
Deuteron The nucleus of deuterium. It contains one proton and one neutron. See also heavy water.
Differential pressure (dp or P) The difference in pressure between two points of a system, such as between the inlet and outlet of a pump.
Doppler coefficient Another name used for the fuel temperature coefficient of reactivity.
Dose The absorbed dose, given in rads (or the international system of units, grays), that represents the energy absorbed from the radiation in a gram of any material. Furthermore, the biological dose or dose equivalent, given in rem or sieverts, is a measure of the biological damage to living tissue from the radiation exposure.
Dose, absorbed The amount of energy deposited in any substance by ionizing radiation per unit mass of the substance. It is expressed numerically in rads or grays.
Dose equivalent The product of absorbed dose in tissue multiplied by a quality factor, and then sometimes multiplied by other necessary modifying factors at the location of interest. It is expressed numerically in rems or sieverts (see 10 CFR 20.1003).
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.
Dosimetry The theory and application of the principles and techniques involved in the measurement and recording of ionizing radiation doses.
Dose rate The ionizing radiation dose delivered per unit time. For example, rem or sieverts per hour.
Drywell The containment structure enclosing a boiling water reactor vessel and its recirculation system. The drywell provides both a pressure suppression system and a fission product barrier under accident conditions.
Earthquake, operating basis An earthquake that could be expected to affect the reactor plant site, but for which the plant power production equipment is designed to remain functional without undue risk to public health and safety.
Effective halflife The time required for the amount of a radioactive element deposited in a living organism to be diminished 50% as a result of the combined action of radioactive decay and biological elimination.
Efficiency, plant The percentage of the total energy content of a power plant's fuel that is converted into electricity. The remaining energy is lost to the environment as heat.
Electrical generator An electromagnetic device that converts mechanical (rotational) energy into electrical energy. Most large electrical generators are driven by steam or water turbine systems.
Electromagnetic radiation A traveling wave motion resulting from changing electric or magnetic fields. Familiar electromagnetic radiation range from x-rays (and gamma rays) of short wavelength, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wave length.
Electron An elementary particle with a negative charge and a mass 1/1837 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. See the periodic table of elements.
Emergency classifications Response by an offsite organization is required to protect local citizens near the site. A request for assistance from offsite emergency response organizations may be required.
Emergency core cooling systems (ECCS) Reactor system components (pumps, valves, heat exchangers, tanks, and piping) that are specifically designed to remove residual heat from the reactor fuel rods should the normal core cooling system (reactor coolant system) fail.
Emergency feedwater Another name that may be used for auxiliary feedwater.
Entomb A method of decommissioning in which radioactive contaminants are encased in a structurally long-lived material, such as concrete. The entombment structure is appropriately maintained, and continued surveillance is carried out until the radioactivity decays to a level permitting decommissioning and ultimate unrestricted release of the property.
Exclusion area That area surrounding the reactor, in which the reactor licensee has the authority to determine all activities, including exclusion or removal of personnel and property from the area.
Excursion A sudden, very rapid rise in the power level of a reactor caused by supercriticality. Excursions are usually quickly suppressed by the negative temperature coefficient, the fuel temperature coefficient or the void coefficient (depending upon reactor design), or by rapid insertion of control rods.
Exposure Being exposed to ionizing radiation or to radioactive material.
External radiation Exposure to ionizing radiation when the radiation source is located outside the body.
Extremities The hands, forearms, elbows, feet, knee, leg below the knee, and ankles (permissible radiation exposures in these regions are generally greater than in the whole body because they contain less blood forming organs and have smaller volumes for energy absorption) (see 10 CFR 20.1003).
Fast fission Fission of a heavy atom (such as uranium-238) when it absorbs a high energy (fast) neutron. Most fissionable materials need thermal (slow) neutrons in order to fission.
Fast neutron A neutron with kinetic energy greater than its surroundings when released during fission.
Feedwater Water supplied to the reactor pressure vessel (in a BWR) or the steam generator (in a PWR) that removes heat from the reactor fuel rods by boiling and becoming steam. The steam becomes the driving force for the plant turbine generator.
Fertile material A material, which is not itself fissile(fissionable by thermal neutrons), that can be converted into a fissile material by irradiation in a reactor. There are two basic fertile materials, uranium-238 and thorium-232. When these fertile materials capture neutrons, they are converted into fissile plutonium-239 and uranium-233, respectively.
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.
Fission gases Those fission products that exist in the gaseous state. In nuclear power reactors, this includes primarily the noble gases, such as krypton and xenon.
Fission products The nuclei (fission fragments) formed by the fission of heavy elements, plus the nuclide formed by the fission fragments' radioactive decay.
Fissionable material Commonly used as a synonym for fissile material, the meaning of this term has been extended to include material that can be fissioned by fast neutrons, such as uranium-238.
Flux A term applied to the amount of some type of particle (neutrons, alpha radiation, etc.) or energy (photons, heat, etc.) crossing a unit area per unit time. The unit of flux is the number of particles, energy, etc., per square centimeter per second.
Fuel assembly A cluster of fuel rods (or plates). Also called a fuel element. Many fuel assemblies make up a reactor core.
Fuel cycle The series of steps involved in supplying fuel for nuclear power reactors. It can include mining, milling, isotopic enrichment, fabrication of fuel elements, use in a reactor, chemical reprocessing to recover the fissionable material remaining in the spent fuel, reenrichment of the fuel material, refabrication into new fuel elements, and waste disposal.
Fuel reprocessing The processing of reactor fuel to separate the unused fissionable material from waste material.
Fuel rod A long, slender tube that holds fissionable material (fuel) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.
Fuel temperature coefficient of reactivity The change in reactivity per degree change in the fuel temperature. The physical property of fuel pellet material (uranium-238) that causes the uranium to absorb more neutrons away from the fission process as fuel pellet temperature increases. This acts to stabilize power reactor operations. This coefficient is also known as the doppler coefficient.
Fusion reaction 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.
Gap The space inside a reactor fuel rod that exists between the fuel pellet and the fuel rod cladding.
Gamma radiation High-energy, short wavelength, electromagnetic radiation emitted from the nucleus. Gamma radiation frequently accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead or depleted uranium. Gamma rays are similar to x-rays.
Gas-cooled reactor A nuclear reactor in which a gas is the coolant.
Gases Normally, formless fluids that completely fill the space, and take the shape of, their container.
Gaseous Diffusion Plant A facility where uranium hexafluoride gas is filtered, uranium-235 is separated from uranium-238, increasing the percentage of uranium-235 from 1 to about 3 percent. The process requires enormous amounts of electric power.
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 was named for Hans Geiger and W. Mueller, who invented it in the 1920s. It is sometimes called simply a Geiger counter or a G-M counter, and is the most commonly used portable radiation instrument.
Generation (gross) The total amount of electric energy produced by a generating station as measured at the generator terminals.
Generation (net) The gross amount of electric energy produced less the electric energy consumed at a generating station for station use.
Generation (net) The gross amount of electric energy produced less the electric energy consumed at a generating station for station use.
Graphite A form of carbon, similar to the lead used in pencils, used as a moderator in some nuclear reactors.
Gigawatt One billion (109) watts.
Gigawatthour One billion (109) watt-hours.
Gray (Gy) The new international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and replaces the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad.
Halflife The time in which one half of the atoms of a particular radioactive substance disintegrates into another nuclear form. Measured halflives vary from millionths of a second to billions of years. Also called physical or radiological halflife
Halflife, biological The time required for the body to eliminate one half of the material taken in by natural biological means.
Halflife, effective The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination.
Half-thickness The thickness of any given absorber that will reduce the intensity of its original beam of ionizing radiation to one-half of its initial value.
Head, reactor vessel The removable top section of a reactor pressure vessel. It is bolted in place during power operation and removed during refueling to permit access of fuel handling equipment to the core.
Health physics The science concerned with the recognition, evaluation, and control of health hazards which may arise from the use and application of ionizing radiation.
Heat exchanger Any device that transfers heat from one fluid (liquid or gas) to another fluid or to the environment.
Heat sink Anything that absorbs heat. It is usually part of the environment, such as the air, a river, or a lake.
Heatup The rise in temperature of the reactor fuel rods resulting from an increase in the rate of fission in the core.
Heavy water (D20) Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.
Heavy water moderated reactor A reactor that uses heavy water as its moderator. Heavy water is an excellent moderator and thus permits the use of unenriched uranium as a fuel.
High-enriched uranium Uranium enriched to 20 percent or greater in the isotope uranium-235.
High-level waste High-level radioactive waste (HLW) means (1) irradiated (spent) reactor fuel; (2) liquid waste resulting from the operation of the first cycle solvent extraction system, and the concentrated wastes from subsequent extraction cycles, in a facility for reprocessing irradiated reactor fuel; and (3) solids into which such liquid wastes have been converted. HLW is primarily in the form of spent fuel discharged from commercial nuclear power reactors. It also includes some reprocessed HLW from defense activities, and a small quantity of reprocessed commercial HLW (see 10 CFR Part 60).
High Radiation Area Any area with dose rates greater than 100 millirems (1 millisievert) in one hour 30 centimeters from the source or from any surface through which the ionizing radiation penetrates. Areas at licensee facilities must be posted as "high radiation areas" and access into these areas is maintained under strict control.
Hot A colloquial term meaning highly radioactive.
Hot spot The region in a radiation/contamination area in which the level of radiation/contamination is significantly greater than in neighboring regions in the area.
Kilo- A prefix that multiplies a basic unit by 1,000 or 103.
Kilovolt The unit of electrical potential equal to 1,000 volts.
Kinetic energy The energy that a body possesses by virtue of its mass and velocity. Also called the energy of motion.
Lethal dose (Lethal dose 50/30) The dose of radiation expected to cause death to an exposed population within 30 days to 50 percent (LD 50/30) of those exposed. Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5 sieverts) received over a very short period of time.
Light water Ordinary water (H20) as distinguished from heavy water (D20).
Light water reactor A term used to describe reactors using ordinary water as coolant, including boiling water reactors (BWRs) and pressurized water reactors (PWRs), the most common types used in the United States.
Limiting condition for operation The section of Technical Specifications that identifies the lowest functional capability or performance level of equipment required for safe operation of the facility.
Limiting safety system settings Settings for automatic protective devices related to those variables having significant safety functions. Where a limiting safety system setting is specified for a variable on which a safety limit has been placed, the setting will assure that automatic protective action will correct the abnormal situation before a safety limit is exceeded.
Linear heat generation rate The heat generation rate per unit length of fuel rod, commonly expressed in kilowatts per foot of fuel rod (kw/ft).
Loop In a pressurized water reactor, the coolant flow path through piping from the reactor pressure vessel to the steam generator, to the reactor coolant pump, and back to the reactor pressure vessel. Large PWRs may have as many as four separate loops.
Loss of coolant accident (LOCA) Those postulated accidents that result in a loss of reactor coolant at a rate in excess of the capability of the reactor makeup system from breaks in the reactor coolant pressure boundary, up to and including a break equivalent in size to the double-ended rupture of the largest pipe of the reactor coolant system.
Low-level waste Low-level radioactive waste (LLW) is a general term for a wide range of wastes. Industries, hospitals and medical, educational, or research institutions; private or government laboratories; and nuclear fuel cycle facilities (e.g., nuclear power reactors and fuel fabrication plants) using radioactive materials generate low-level wastes as part of their normal operations. These wastes are generated in many physical and chemical forms and levels of contamination (see 10 CFR Part 61).
Low population zone (LPZ) An area of low population density often required around a nuclear installation before being built. The number and density of residents is of concern in emergency planning so that certain protective measures (such as notification and instructions to residents) can be accomplished in a timely manner.
Mass-energy equation The equation developed by Albert Einstein, which is usually given as E = mc2, showing that, when the energy of a body changes by an amount E (no matter what form the energy takes), the mass, m, of the body will change by an amount equal to E/c2. The factor c2, the square of the speed of light in a vacuum (3x108 meters/second), may be regarded as the conversion factor relating units of mass and energy. The equation predicted the possibility of releasing enormous amounts of energy by the conversion of mass to energy. It is also called the Einstein Equation.
Mass number The number of nucleons (neutrons and protons) in the nucleus of an atom. Also known as the atomic weight of an atom.
Maximum dependable capacity (gross) Dependable main-unit gross generating capacity, winter or summer, whichever is smaller. The dependable capacity varies because the unit efficiency varies during the year due to temperature variations in cooling water. It is the gross electrical output as measured at the output terminals of the turbine generator during the most restrictive seasonal conditions (usually summer).
Maximum dependable capacity (net) Gross maximum dependable generating capacity less the normal station service loads.
Mega- A prefix that multiplies a basic unit by 1,000,000 or 106.
Megacurie One million (106) curies.
Megawatt (MW) One million (106) watts.
Megawatt hour (MWh) One million (106) watt-hours.
Metric ton Approximately 2,200 pounds in the English system of measurements (Note: in the international system of measurements, 1 metric ton = 1000 kg.)
Micro- A prefix that divides a basic unit into one million parts (10-6).
Microcurie One millionth (10-6) of a curie.
Mill tailings Naturally radioactive residue from the processing of uranium ore into yellowcake in a mill. 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.
Milli- A prefix that divides a basic unit by 1,000.
Millirem One thousandth of a rem. (1 mrem = 10-3 rem)
Milliroentgen One thousandth of a roentgen, R. (1 mR = 10-3R)
Mixed oxide fuel A mixture of uranium oxide and plutonium oxide used to fuel a reactor. Often it is abbreviated as "MOX." Conventional fuel is made of pure uranium oxide.
Moderator A material, such as ordinary water, heavy water, or graphite, that is used in a reactor to slow down high-velocity neutrons, thus increasing the likelihood of fission.
Moderator temperature coefficient of reactivity The change in reactivity per degree change in moderator temperature due to the property of the reactor moderator to slow down fewer neutrons as its temperature increases. This acts to stabilize power reactor operations.
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.
Monitoring Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in an occupied region, as a safety measure, for the purpose of health protection.
Nano- A prefix that divides a basic unit by one billion (10-9).
Nanocurie One billionth (10-9) of a curie.
Natural circulation The circulation of the coolant in the reactor coolant system without the use of the reactor coolant pumps. The circulation is due to the natural convection resulting from the different densities of relative cold and heated portions of the system.
Natural uranium Uranium as found in nature. It contains 0.7 percent uranium-235, 99.3 percent uranium-238, and a trace of uranium-234.
Net summer capability The steady hourly output that generating equipment is expected to supply to system load exclusive of auxiliary power, as demonstrated by tests at the time of summer peak demand.
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.
Neutron capture The process in which an atomic nucleus absorbs or captures a neutron.
Neutron chain reaction A process in which some of the neutrons released in one fission event cause other fissions to occur. There are three types of chain reactions: (1) Nonsustaining chain reaction--An average of less than one fission is produced by the neutrons released by each previous fission (reactor subcriticality). (2) Sustaining chain reaction--An average of exactly one fission is produced by the neutrons released by each previous fission (reactor criticality). (3) Multiplying chain reaction--An average of more than one fission is produced by the neutrons released by precious fission (reactor supercriticality).
Neutron flux
Neutron generation The release, thermalization, and absorption of fission neutrons by a fissile material and the fission of that material producing a second generation of neutrons. In a typical nuclear power reactor system, there are about 40,000 generations of neutrons every second.
Neutron leakage Neutrons that escape from the vicinity of the fissionable material in a reactor core. Neutrons that leak out of the fuel region are no longer available to cause fission and must be absorbed by shielding placed around the reactor pressure vessel for that purpose.
Neutron source A radioactive material (decays by neutron emission) that can be inserted into a reactor to ensure that a sufficient quantity of neutrons are available to register on neutron detection equipment for power level indication.
Neutron, thermal A neutron that has (by collision with other particles) reached an energy state equal to that of its surroundings, typically on the order of 0.025 eV (electron volts).
Noble gas A gaseous chemical element that does not readily enter into chemical combination with other elements. An inert gas. Examples are helium, argon, krypton, xenon and radon.
Nonpower reactor Reactors used for research, training, and test purposes, and for the production of radioisotopes for medical and industrial uses.
Non-stochastic effect The health effects, the severity of which vary with the dose and for which a threshold is believed to exist. Radiation-induced cataract formation is an example of a deterministic effect (also called a deterministic effect) (see 10 CFR 20.1003).
Non-vital plant systems Systems at a nuclear facility that may or may not be necessary for the operation of the facility (i.e., power production), but that would have little or no effect on public health and safety should they fail. These systems are not safety related.
Nozzle As used in PWRs and BWRs, the interface for fluid (inlet and outlet) between reactor plant components (pressure vessel, coolant pumps, steam generators, etc.) and their associated piping systems.
Nuclear energy The energy liberated by a nuclear reaction (fission or fusion) or by radioactive decay.
Nuclear force A powerful short-ranged attractive force that holds together the particles inside an atomic nucleus.
Nuclear power plant An electrical generating facility using a nuclear reactor as its power (heat) source.
Nuclear steam supply system The reactor and the reactor coolant pumps (and steam generators for a pressurized water reactor) and associated piping in a nuclear power plant used to generate the steam needed to drive the turbine generator unit.
Nuclear waste see High-level waste and Low-level waste.
Nucleon Common name for a constituent particle of the atomic nucleus. At present, applied to protons and neutrons, but may include any other particles found to exist in the nucleus.
Nucleus The small, central, positively charged region of an atom that carries the atom's nuclei. Except for the nucleus of ordinary (light) hydrogen, which has a single proton, all atomic nuclei contain both protons and neutrons. The number of protons determines the total positive charge, or atomic number. This is the same for all the atomic nuclei of a given chemical element. The total number of neutrons and protons is called the mass number.
Nuclide A general term referring to all known isotopes, both stable (279) and unstable (about 5,000), of the chemical elements.
Operable A system, subsystem, train, component, or device is operable or has operability when it is capable of performing its specified functions, and when all necessary attendant instrumentation, controls, electrical power, cooling or seal water, lubrication or other auxiliary equipment that are required for the system, subsystem, train, component or device to perform its functions are also capable of performing their related support functions.
Operational mode An operational mode corresponds to any one inclusive combination of core reactivity condition, power level, and average reactor coolant temperature.
Parent A radionuclide that upon radioactive decay or disintegration yields a specific nuclide (the daughter).
Parts per million (ppm) Parts (molecules) of a substance contained in a million parts of another substance (or water).
Pellet, fuel As used in pressurized water reactors and boiling water reactors, a pellet is a small cylinder approximately 3/8-inch in diameter and 5/8-inch in length, consisting of uranium fuel in a ceramic form--uranium dioxide, UO2. Typical fuel pellet enrichments in nuclear power reactors range from 2.0 percent to 3.5 percent uranium-235.
Performance-based Regulation Required results or outcome of performance rather than a prescriptive process, technique, or procedure.
Periodic Table An arrangement of chemical elements in order of increasing atomic number. Elements of similar properties are placed one under the other, yielding groups or families of elements. Within each group, there is a variation of chemical and physical properties, but in general, there is a similarity of chemical behavior within each group. See an online periodic table.
Personnel monitoring The use of portable survey meters to determine the amount of radioactive contamination on an individual, or the use of dosimetry to determine an individual's occupational radiation dose.
Photon A quantum (or packet) of energy emitted in the form of electromagnetic radiation. Gamma rays and x-rays are examples of photons.
Pico- A prefix that divides a basic unit by one trillion (10-12).
Picocurie One trillionth (10-12) of a curie.
Pig A colloquial term describing a container (usually lead or depleted uranium) used to ship or store radioactive materials. The thick walls of this shielding device protect the person handling the container from radiation. Large containers used for spent fuel storage are commonly called casks.
Pile A colloquial term describing the first nuclear reactors. It is called a pile because the earliest reactors were "piles" of graphite and uranium blocks.
Planned special exposure An infrequent exposure to radiation, separate from and in addition to, the annual dose limits (see 10 CFR 20.1003 and §20.1206).
Plutonium (Pu) A heavy, radioactive, manmade metallic element with atomic number 94. Its most important isotope is fissile plutonium-239, which is produced by neutron irradiation of uranium-238. It exists in only trace amounts in nature.
Pocket dosimeter A small ionization detection instrument that indicates ionizing radiation exposure directly. An auxiliary charging device is usually necessary.
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.
Pool reactor A reactor in which the fuel elements are suspended in a pool of water that serves as the reflector, moderator, and coolant. Popularly called a "swimming pool reactor," it is used for research and training, not for electrical generation.
Positron Particle equal in mass, but opposite in charge, to the electron (a positive electron).
Power coefficient of reactivity The change in reactivity per percent change in power. The power coefficient is the summation of the moderator temperature coefficient of reactivity, the fuel temperature coefficient of reactivity, and the void coefficient of reactivity.
Power defect The total amount of reactivity added due to a given change in power. It can also be expressed as the integrated power coefficient over the range of the power change.
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.
Pressure vessel A strong-walled container housing the core of most types of power reactors. It usually also contains the moderator, neutron reflector, thermal shield, and control rods.
Pressurized water reactor (PWR)
A power reactor in which heat is transferred from the core to an exchanger by high temperature water kept under high pressure in the primary system. Steam is generated in a secondary circuit. Many reactors producing electric power are pressurized water reactors.
Pressurizer A tank or vessel that acts as a head tank (or surge volume) to control the pressure in a pressurized water reactor.
Primary system A term that may be used for referring to the reactor coolant system.
Proportional counter A radiation instrument in which an electronic detection system receives pulses that are proportional to the number of ions formed in a gas-filled tube by ionizing radiation.
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 or gray) is to be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or sievert) to an exposed individual. It is used because some types of radiation, such as alpha particles, are more biologically damaging internally than other types.
Quantum theory The concept that energy is radiated intermittently in units of definite magnitude, called quanta, and absorbed in a like manner.
Rad The special unit for radiation absorbed dose, which is the amount of energy from any type of ionizing radiation (e.g., alpha, beta, gamma, neutrons, 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 (100 rad = 1 gray).
Radiation, nuclear Particles (alpha, beta, neutrons) or photons(gamma) emitted from the nucleus of unstable radioactive atoms as a result of radioactive decay.
Radiation area Any area with radiation levels greater than 5 millirems (0.05 millisievert) in one hour at 30 centimeters from the source or from any surface through which the radiation penetrates.
Radiation detection instrument A device that detects and displays the characteristics of ionizing radiation.
Radiation shielding Reduction of radiation by interposing a shield of absorbing material between any radioactive source and a person, work area, or radiation-sensitive device.
Radiation sickness (syndrome) The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure (greater than 200 rads or 2 gray) 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 (epilation), 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 (10 gray) or more, death may occur within two to four weeks. Those who survive 6 weeks after the receipt of a single large dose of radiation to the whole body may generally be expected to recover.
Radiation source Usually a sealed source of radiation used in teletherapy and industrial radiography, as a power source for batteries (as in use in space craft), or in various types of industrial gauges. Machines, such as accelerators and radioisotope generators, and natural radionuclides may be considered sources.
Radiation standards Exposure standards, permissible concentrations, rules for safe handling, regulations for transportation, regulations for industrial control of radiation, and control of radioactive material by legislative means.
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 or equipment.
Radioactive series A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results. The first member is called the parent, the intermediate members are called daughters, and the final stable member is called the end product.
Radioactivity The spontaneous 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. Measured in units of becquerels or disintegrations per second.
Radiography The making of a shadow image on photographic film by the action of ionizing 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.
Radiological sabotage Any deliberate act directed against a nuclear facility licensed by the NRC or against a component of such a facility which could endanger the public health and safety by exposure to radiation.
Radiological survey The evaluation of the radiation hazards accompanying the production, use, or existence of radioactive materials under a specific set of conditions. Such evaluation customarily includes a physical survey of the disposition of materials and equipment, measurements or estimates of the levels of radiation that may be involved, and a sufficient knowledge of processes affecting these materials to predict hazards resulting from expected or possible changes in materials or equipment.
Radiology That branch of medicine dealing with the diagnostic and therapeutic applications of radiant energy, including x-rays and radioisotopes.
Radionuclide A radioisotope.
Radiosensitivity The relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation.
Radium (Ra) A radioactive metallic element with atomic number 88. As found in nature, the most common isotope has a mass number of 226. It occurs in minute quantities associated with uranium in pitchblende, camotite, and other minerals.
Radon (Rn) A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is a daughter of radium.
Reaction Any process involving a chemical or nuclear change.
Reactivity A term expressing the departure of a reactor system from criticality. A positive reactivity addition indicates a move toward supercriticality (power increase). A negative reactivity addition indicates a move toward subcriticality (power decrease).
Reactor coolant system The system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine.
Reactor, nuclear A device in which nuclear fission may be sustained and controlled in a self-supporting nuclear reaction. The varieties are many, but all incorporate certain features, including fissionable material or fuel, a moderating material (unless the reactor is operated on fast neutrons), a reflector to conserve escaping neutrons, provisions of removal of heat, measuring and controlling instruments, and protective devices. The reactor is the heart of a nuclear power plant.
Reference man A person with the anatomical and physiological characteristics of an average individual which is used in calculations assessing internal dose (also may be called "Standard Man").
Reflector A layer of material immediately surrounding a reactor core that scatters back (or reflects) into the core many neutrons that would otherwise escape. The returned neutrons can then cause more fissions and improve the neutron economy of the reactor. Common reflector materials are graphite, beryllium, water, and natural uranium.
REM (Roentgen Equivalent Man) - a standard unit that measures the effects of ionizing radiation on humans.
Restricted area Any area to which access is controlled for the protection of individuals from exposure to radiation and radioactive materials.
Risk-informed regulation Incorporating an assessment of safety significance or relative risk in NRC regulatory actions. Making sure that the regulatory burden imposed by individual regulations or processes is commensurate with the importance of that regulation or process to protecting public health and safety and the environment.
Roentgen (R) A unit of exposure to ionizing radiation. It is the amount of gamma or x-rays required to produce ions resulting in a charge of 0.000258 coulombs/kilogram of air under standard conditions. Named after Wilhelm Roentgen, the German scientist who discovered x-rays in 1895.
Rubblization A decommissioning technique involving demolition and burial of formerly operating nuclear facilities. All equipment from buildings is removed and the surfaces are decontaminated. Above-grade structures are demolished into rubble and buried in the structure's foundation below ground. The site surface is then covered, regraded and landscaped for unrestricted use.
Safeguards The protection of special nuclear material(SNM) to prevent its theft, loss, or sabotage.
Safe shutdown earthquake A design-basis earthquake.
Safety injection The rapid insertion of a chemically soluble neutron poison (such as boric acid) into the reactor coolant system to ensure reactor shutdown.
Safety limit A limit placed upon important process variables which are found to be necessary to reasonably protect the integrity of the physical barriers which guard against the uncontrolled release of radioactivity.
Safety related The managerial controls, administrative documents, operating procedures, systems, structures, and components that have been designed to mitigate the consequences of postulated accidents that could cause undue risk to public health and safety.
SAFSTOR A method of decommissioning in which the nuclear facility is placed and maintained in such condition that the nuclear facility can be safely stored and subsequently decontaminated to levels that permit release for unrestricted use.
Scattered radiation Radiation that, during its passage through a substance, has been changed in direction. It may also have been modified by a decrease in energy. It is one form of secondary radiation.
Scintillation detector The combination of phosphor, photomultiplier tube and associated electronic circuits for counting light emissions produced in the phosphor by ionizing radiation.
Scram The term used to mean the sudden shutting down of a nuclear reactor, usually by rapid insertion of control rods, either automatically or manually by the reactor operator. May also be called a reactor trip. It is actually an acronym for "safety control rod axe man," the man assigned to insert the emergency rod on the first reactor (the Chicago pile) in the U.S.
Sealed source Any special nuclear material or byproduct encased in a capsule designed to prevent leakage or escape of the material.
Secondary radiation Radiation originating as the result of absorption of other radiation in matter. It may be either electromagnetic or particulate in nature.
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. Most commonly used in reference to pressurized water reactors.
Seismic category I A term used to define structures, systems, and components that are designed and built to withstand the maximum potential earthquake stresses for the particular region where a nuclear plant is sited.
Shielding Any material or obstruction that absorbs radiation and thus tends to protect personnel or materials from the effects of ionizing radiation.
Shutdown A decrease in the rate of fission (and heat production) in a reactor (usually by the insertion of control rods into the core).
Shutdown margin The instantaneous amount of reactivity by which the reactor is subcritical or would be subcritical from its present condition assuming all full-length rod cluster assemblies (shutdown and control) are fully inserted except for the single rod cluster assembly of highest reactivity worth which is assumed to be fully withdrawn.
Sievert (Sv) The new international system (SI) unit for dose equivalent equal to 1 Joule/kilogram. 1 sievert = 100 rem.
Somatic effects of radiation Effects of radiation limited to the exposed individual, as distinguished from genetic effects, which may also affect subsequent unexposed generations.
Source material Uranium or thorium, or any combination thereof, in any physical or chemical form or ores which contain by weight one-twentieth of one percent (0.05%) or more of: (1) uranium, (2) thorium or (3) any combination thereof. Source material does not include special nuclear material.
Special nuclear material Includes plutonium, uranium-233, or uranium enriched in the isotopes uranium-233 or uranium-235.
Spent (depleted) fuel Nuclear reactor fuel that has been used to the extent that it can no longer effectively sustain a chain reaction.
Spent fuel pool An underwater storage and cooling facility for fuel elements that have been removed from a reactor.
Spent nuclear fuel Fuel that has been removed from a nuclear reactor because it can no longer sustain power production for economic or other reasons.
Stable isotope An isotope that does not undergo radioactive decay.
Standard Technical Specifications NRC staff guidance on model technical specifications for an operating license. (See also Technical Specifications.)
Startup An increase in the rate of fission (and heat production) in a reactor (usually by the removal of control rods from the core).
Stay time The period during which personnel may remain in a restricted area in a reactor before accumulating some permissible occupational dose.
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.
Stochastic effects Effects that occur by chance, generally occurring without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. In the context of radiation protection, the main stochastic effects are cancer and genetic effects.
Subcriticality The condition of a nuclear reactor system when the rate of production of fission neutrons is lower than the rate of production in the previous generation due to increased neutron leakage and poisons.
Supercritical reactor A reactor in which the power level is increasing with time.
Supercriticality The condition for increasing the level of operation of a reactor. The rate of fission neutron production exceeds all neutron losses, and the overall neutron population increases.
Supercritical reactor A reactor in which the power level is increasing with time.
Superheating The heating of a vapor, particularly steam, to a temperature much higher than the boiling point at the existing pressure. This is done in some power plants to improve efficiency and to reduce water damage to the turbine.
Survey meter Any portable radiation detection instrument especially adapted for inspecting an area or individual to establish the existence and amount of radioactive material present.
Technical Specifications Part of an NRC license authorizing the operation of a nuclear production or utilization facility. A Technical Specification establishes requirements for items such as safety limits, limiting safety system settings, limiting control settings, limiting conditions for operation, surveillance requirements, design features, and administrative controls. (See also Standard Technical Specifications.)
Terrestrial radiation The portion of the natural background radiation that is emitted by naturally occurring radioactive materials, such as uranium, thorium, and radon in the earth.
Thermal breeder reactor A breeder reactor in which the fission chain reaction is sustained by thermal neutrons.
Thermalization The process undergone by high-energy (fast) neutrons as they lose energy by collision.
Thermal power The total core heat transfer rate to the reactor coolant.
Thermal reactor A reactor in which the fission chain reaction is sustained primarily by thermal neutrons. Most current reactors are thermal reactors.
Thermal shield A layer, or layers, of high-density material located within a reactor pressure vessel or between the vessel and the biological shield to reduce radiation heating in the vessel and the biological shield.
Thermoluminescent dosimeter A small device used to measure radiation by measuring the amount of visible light emitted from a crystal in the detector when exposed to ionizing radiation.
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.
Transient A change in the reactor coolant system temperature and/or pressure due to a change in power output of the reactor. Transients can be caused by adding or removing neutron poisons, by increasing or decreasing electrical load on the turbine generator, or by accident conditions.
Trip, reactor A term that is used by pressurized water reactors for a reactor scram.
Tritium A radioactive isotope of hydrogen (one proton, two neutrons). Because it is chemically identical to natural hydrogen, tritium can easily be taken into the body by any ingestion path. It decays by beta emission. It has a radioactive halflife of about 12.5 years.
Turbine A rotary engine made with a series of curved vanes on a rotating shaft, usually turned by water or steam. Turbines are considered the most economical means to turn large electrical generators.
Turbine generator (TG) A steam (or water) turbine directly coupled to an electrical generator. The two devices are often referred to as one unit.
Ultraviolet Electromagnetic radiation of a wavelength between the shortest visible violet and low energy x-rays.
Unrestricted area The area outside the owner-controlled portion of a nuclear facility (usually the site boundary). An area in which a person could not be exposed to radiation levels in excess of 2 millirems in any one hour from external sources (see 10 CFR 20.1003).
Unstable isotope A radioactive isotope.
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 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons and is fertile. Natural uranium also includes a minute amount of uranium-234.
Uranium fuel fabrication facility A facility that (1) manufactures reactor fuel containing uranium for any of the following (i) preparation of fuel materials; (ii) formation of fuel materials into desired shapes; (iii) application of protective cladding; (iv) recovery of scrap material; and (v) storage associated with such operations; or (2) conducts research and development activities.
Uranium hexafluoride production facility A facility that receives natural uranium in the form of ore concentrate; enriches it, either by gaseous diffusion or gas centrifuge methods; and converts it into uranium hexafluoride (UF6).
Vapor The gaseous form of substances that are normally in liquid or solid form.
Void An area of lower density in a moderating system (such as steam bubbles in water) that allows more neutron leakage than does the more dense material around it.
Very high radiation area An area in which radiation levels exceed 500 rad (5 gray) in one hour at 1 meter from the source or from any surface that the radiation penetrates (see 10 CFR 20.1003).
Viability assessment A Department of Energy decision making process to judge the prospects for geologic disposal of high-level radioactive wastes at Yucca Mountain based on (1) specific design work on the critical elements of the repository and waste package, (2) a total system performance assessment that will describe the probable behavior of the repository, (3) a plan and cost estimate for the work required to complete a license application, and (4) an estimate of the costs to construct and operate the repository (see 10 CFR Part 60).
Void coefficient of reactivity The change in reactivity per percent change in void content due to an increase in the neutron leakage as the density of the moderator decreases with an increasing void content.
Waste, radioactive Solid, liquid, and gaseous materials from nuclear operations that are radioactive or become radioactive and for which there is no further use. Wastes are generally classified as high-level (having radioactivity concentrations of hundreds of thousands of curies per gallon or foot), low-level (in the range of 1 microcurie per gallon or foot), or intermediate level (between these extremes) (see 10 CFR Parts 60 and 61).
Watt An electrical unit of power. 1 watt = 1 Joule/second.
Watt-hour An electrical energy unit of measure equal to 1 watt of power supplied to, or taken from, an electrical circuit steadily for 1 hour.
Weighting factor (Weighting factor, WT) Multipliers of the equivalent dose to an organ or tissue used for radiation protection purposes to account for different sensitivities of different organs and tissues to the induction of stochastic effects of radiation (see 10 CFR 20.1003).
Well-logging A technique used in oil and gas exploration to help predict the commercial viability of new or existing wells. It involves lowering a well-logging tool, including a sealed source of radioactive material, into a well on a wire. This device sends data on the well's underground characteristics to the surface, where it is plotted on a chart.
Wheeling Service The movement of electricity from one system to another over transmission facilities of intervening systems. Wheeling service contracts can be established between two or more systems.
Whole-body counter A device used to identify and measure the radioactive material in the body of human beings and animals. It uses heavy shielding to keep out naturally existing background radiation and ultrasensitive radiation detectors and electronic counting equipment.
Whole-body exposure An exposure of the body to radiation, in which the entire body, rather than an isolated part, is irradiated. Where a radioisotope is uniformly distributed throughout the body tissues, rather than being concentrated in certain parts, the irradiation can be considered as whole-body exposure.
Wipe sample A sample made for the purpose of determining the presence of removable radioactive contamination on a surface. It is done by wiping, with slight pressure, a piece of soft filter paper over a representative type of surface area. It is also known as a "swipe or smear" sample.
X-rays Penetrating electromagnetic radiation (photon) having a wavelength that is much shorter than that of visible light. These rays are usually produced by excitation of the electron field around certain nuclei. In nuclear reactions, it is customary to refer to photons originating in the nucleus as x-rays.
Yellowcake A solid uranium-oxygen compound (U3O8) that takes its name from its color and texture. It is a product of the uranium milling process and is the feed material used for fuel enrichment and fuel pellet fabrication.

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