Home > Electronic Reading Room > Basic References > Glossary > Full Text

Full-Text Glossary

Access hatch

An 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.)

Anticipated Transient Without Scram (ATWS)

ATWS is one of the "worst case" accidents, consideration of which frequently motivates the NRC to take regulatory action. The accident could happen if the system that provides a highly reliable means of shutting down the reactor (scram system) fails to work during a reactor event (anticipated transient). They types of events considered are those used for designing the plant.

Assumptions (For IPEs, IPEEs & PRAs)

In the context of PRAs, assumptions are those parts of the mathimatical models which the analyst expects will hold true for the range of solutions used for making decisions. Without assumptions, even the most powerful computers may not be able to provide useful solutions for the models.

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.

Bayesian Estimation

A mathematical formulation, using Bayes' Theorem, by which the likelihood of an event can be estimated talking explicit consideration of certain contextual features (such as amount of datam nature of decision, etc.).

Bayesian Prior

A way to express the context of a Bayesian estimation in which initial data is updated as new data becomes available.).

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.

Beyond design-basis accidents

This term is used as a technical way to discuss accident sequences that are possible but were not fully considered in the design process because they were judged to be too unlikely. As the regulatory process strives to be as thorough as possible, “beyond design basis” accident sequences are analyzed so as to fully understand the capability of a design.

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.

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.

Contamination

Undesired radioactive material that is deposited on the surface of or inside structures, areas, objects or people.

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.

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.

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.

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 Damage Frequency

An expression of the likelihood that, given the way a reactor is designed and operated, an accident could cause the fuel in the reactor to be damaged.

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 toradiation. Because the activated deposits were first discovered at Chalk River, a Canadian nuclear plant, "crud" has been used as shorthand for Chalk River Unidentified Deposits.

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 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.

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.

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 threat

A profile of the type, composition, and capabilities of an adversary. The NRC and its licensees use the design basis threat (DBT) as a basis for designing safeguards systems to protect against acts of radiological sabotage and to prevent the theft of special nuclear material. The DBT is described in detail in Title 10, Section 73.1(a), of the Code of Federal Regulations [10 CFR 73.1(a)]. This term is applied to clearly identify for a licensee the expected capability of its facility to withstand a threat.

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 (probabilistic)

Consistent with the principles of "determinism," which holds that specific causes completely and certainly determine effects of all sorts. As applied in nuclear technology, it generally deals with evaluating the safety of a nuclear power plant in terms of the consequences of a predetermined bounding subset of accident sequences. The term "probabilistic" is associated with an evaluation that explicitly accounts for the likelihood and consequences of possible accident sequences in an integrated fashion.

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 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).

Dose rate

The ionizing radiation dose delivered per unit time. For example, rem or sieverts per hour.

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.

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.

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.

Environmental qualification

A process for ensuring that equipment will be capable of withstanding the ambient conditions that could exist when the specific function to be performed by the equipment is actually called upon to be performed under accident conditions.

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.

Fiscal Year

The 12-month period, from October 1 through September 30, used by the Federal Government in budget formulation and execution. The fiscal year is designated by the calendar year in which it ends.

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.

Formula quantity

Strategic special nuclear material in any combination in a quantity of 5000 grams or more computed by the formula, grams=(grams contained U - 235)+2.5 (grams U - 233+grams plutonium). This class of material is sometimes referred to as a Category I quantity of material (see 10 CFR 70.4).

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.

Full-time equivalent

A measurement equal to one staff person working a full-time work schedule for 1 year.

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.

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.

Gap

The space inside a reactor fuel rod that exists between the fuel pellet and the fuel rod cladding.

Gas Centrifuge

A uranium enrichment process that uses a large number of rotating cylinders in a series. These series of centrifuge machines, called trains, are interconnected to form cascades. In this process, UR6 gas is placed in a rotating drum or cylinder and rotated at high speed. This rotation creates a strong gravitational field so that the heavier gas molecules (containing U-238) move toward the outside of the cylinder and the lighter gas molecules (containing U-235) collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. Significantly more U-235 enrichment can be obtained from a single unit gas centrifuge than from a single unit gaseous diffusion barrier.

No gas centrifuge plants are operating in the United States, however, Louisiana Energy Services (LES) and the U.S. Enrichment Corporation (USEC) have plans to submit license applications in 2002 and 2004, respectively. See Gas Centrifuge Enrichment for details.

Gas-cooled reactor

A nuclear reactor in which a gas is the coolant.

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.

Gases

A substance possessing perfect molecular mobility and the property of indefinite expansion, as opposed to a solid or liquid; any such fluid or mixture of fluids other than air. Normally, these formless substances completely fill the space, and take the shape of, their container.

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.

Gigawatt

One billion watts.

Gigawatthour

One billion watt-hours.

Graphite

A form of carbon, similar to the lead used in pencils, used as a moderator in some nuclear reactors.

Gray (Gy)

The international system (SI) unit of absorbed dose. One gray is equal to an absorbed dose of 1 Joule/kilogram (one gray equals 100 rads) (see 20.1004).

Half-thickness

Any given absorber that will reduce the intensity of an original beam of ionizing radiation to one-half of its initial value.

Halflife

The time in which one half of the atoms of a particular radioactive substance disintegrates into another nuclear form. Measured half-lives 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.

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 hazards to health and the environment that may arise from the use and application of ionizing radiation.

Heap leach

A method of extracting uranium from ore using a leaching solution. Small ore pieces are placed in a heap on an impervious material (plastic, clay, asphalt) with perforated pipes under the heap. Acidic solution is then sprayed over the ore, dissolving the uranium. The solution in the pipes is collected and transferred to an ion-exchange system for concentration of the uranium.

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 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.

High-enriched uranium

Uranium enriched to 20 percent or greater in the isotope uranium-235.

High-level waste

Radioactive materials at the end of a useful life cycle that should be properly disposed of, including--

1. The highly radioactive material resulting from the reprocessing of spent nuclear fuel, including liquid waste directly in reprocessing and any solid material derived from such liquid waste that contains fission products in concentrations;
   
   
2. Irradiated reactor fuel; and
   
   
3. Other highly radioactive material that the Commission, consistent with existing law, determines by rule require permanent isolation.

High-level waste (HLW) is primarily in the form of spent fuel discharged from commercial nuclear power reactors. It also includes HLW from activities and a small quantity of reprocessed commercial HLW (see 10 CFR 63.2 and Nuclear waste; Waste, radioactive; and Low-level waste).

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.

In situ leach

A process using a leaching solution to extract uranium from underground ore bodies in place (in other words, in situ). The leaching agent, which contains an oxidant such as oxygen with sodium carbonate, is injected through wells into the ore body in a confined aquifier to dissolve the uranium. This solution is then pumped via other wells to the surface for processing.

In vitro

From the Latin, in glass, isolated from the living organism and artificially maintained, as in a test tube.

In vivo

From the Latin, in one that is living, occuring within the living.

Individual Plant Examination

As requested by the NRC in Generic Letter 88-20, “Individual Plant Examination for Severe Accident Vulnerabilities” (11/23/1988), a risk analysis that considers the unique aspects of a particular nuclear power plant, identifying the specific vulnerabilities to severe accident of that plant.

Individual Plant Examination for External Events

While the “individual plant examination” took into account events that could challenge the design from things that could go awry internally (in the sense that equipment might fail because components do not work as expected), the “individual plant examination for external events” considered challenges, such as earthquakes, internal fires, and high winds.

Integrated plant evaluation

An evaluation that considers the plant as a whole rather than system by system.

Iodine Spiking Factor

The magnitude of a rapid, short-term increase in the appearance rate of radioiodine in the reactor coolant system. This increase is generally caused by a reactor transient that results in a rapid drop in reactor coolant system pressure relative to the fuel rod internal pressure.

Ion-exchange

A common method for concentrating uranium from a solution. The uranium solution is passed through a resin bed where the uranium-carbonate complex ions are transferred to the resin by exchange with a negative ion like chloride. After build-up of the uranium complex on the resin, the uranium is eluted with a salt solution and the uranium is precipitated in another process.

Isotope

Any two or more forms of an element having identical or very closely related chemical propeties and the same atomic number but different atomic weights or mass numbers.

Kilo-

A greek prefix meaning "thousand" in the momendature of the metric system. This prefix multiplies a basic unit by 1,000.

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

The dose of radiation expected to cause death to an exposed population within 30 days to 50 percent (LD 50/30) of those exposed (Lethal dose (LD) 50/30). 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.

Licensing basis

The collection of documents or technical criteria that provides the basis upon which the NRC issues a license to possess radioactive materials, conduct operations involving emission of radiation, utilize special nuclear materials, or dispose of radioactive waste.

Light water

Ordinary water as distinguished from heavy water.

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 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.

Low-level waste

A general term for a wide range of wastes having low levels of radioactivity. 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) that use 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 61.2).

Low-level radioactive wastes containing source, special nuclear, or byproduct material are acceptable for disposal in a land disposal facility. For the purposes of this definition, low-level waste has the same meaning as in the Low-level Waste Policy Act, that is, radioactive waste not classified as high-level radioactive waste, transuranic waste, spent nuclear fuel, or byproduct material as defined in section 11e.(2) of the Atomic Energy Act (uranium or thorium tailings and waste). See also High-level waste and Low-level waste.

Mass number

The number of nucleons (neutrons and protons) in the nucleus of an atom. Also known as the atomic weight of an atom.

Mass-energy equation

The equation developed by Albert Einstein, which is usually given as E = mc squared, 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/c squared. The factor c squared, the speed of light in a vacuum (3x10 to the 8th power), 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. This equation is also called the Einstein Equation.

Maximum dependable capacity (gross)

In a nuclear power reactor, 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)

In a nuclear power reactor, gross maximum dependable generating capacity less the normal station service loads.

Mega-

A prefix that multiplies a basic unit by 1,000,000 (10 to the sixth power).

Megacurie

One million curies.

Megawatt (MW)

One million watts.

Megawatt hour (MWh)

One million 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 (0.000001).

Microcurie

One millionth of a curie. That amount of radioactive material that disintegrates (decays) at the rate of 37 thousand atoms per second.

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 (0.001 rem).

Milliroentgen

One thousandth of a roentgen, R. (1 mR = 10-3 R = 0.001 R).

Mixed oxide fuel

A mixture of uranium oxide and plutonium oxide used to fuel a reactor. Mixed oxide fuel is often abbreviated as "MOX." Conventional nuclear fuel is made of pure uranium oxide. See also Mixed Oxide Fuel Fabrication Facility Licensing and the Mixed Oxide Fuel Fact Sheet.

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

As the moderator (water) increases in temperature, it becomes less dense and slows down fewer neutrons, which results in a negative change of reactivity. This negative temperature coefficient acts to stabilize an atomic 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 of Radiation

Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in a region, as a safety measure, for the purpose of health or environmental protection. Monitoring is done for air, surface, & ground water, soil & sediment, equipment surfaces, and personnel (for example, bioassay or alpha scans).

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 by weight. In terms of the amount of radioactivity, it contains approximately 2.2 percent uranium-235, 48.6 percent uranium-238, and 49.2 percent 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 reaction that occurs when a nucleus captures a neutron. The probability that a given material will capture a neutron is proportional to its neutron capture cross section and depends on the energy of the neutrons and the nature of the material.

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 previous fission (reactor supercriticality).

Neutron flux

A measure of the intensity of neutron radiation in neutrons/cm²-sec. It is the number of neutrons passing through 1 square centimeter of a given target in 1 second. Expressed as nv, where n = the number of neutrons per cubic centimeter and v = their velocity in centimeters per second.

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

Any material that emits neutrons, such as a mixture of radium and beryllium, that can be inserted into a reactor to ensure a neutron flux large enough to be distinguished from background to register on neutron detection equipment.

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.

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 non-stochastic 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.

Nonpower reactor

Reactors used for research, training, and test purposes, and for the production of radioisotopes for medical and industrial uses.

Nozzle

As used in power water reactors and boiling water reactors, the interface (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 heat source to provide steam to a turbine generator.

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

A particular type of radioactive waste, that is produced as part of the nuclear fuel cycle, i.e., those activities needed to produce nuclear fission, or splitting of the atom. These include extraction of uranium from ore, concentration of uranium, processing into nuclear fuel, and disposal of byproducts. Radioactive waste is a broader terms that includes all waste that contains radioactivity. Residues from water treatment, contaminated equipment from oil drilling, and tailings from the processing of metals such as vanadium and copper, also contain radioactivity but are not "nuclear waste" because they are produced outside of the nuclear fuel cycle. NRC generally regulates only those wastes produced in the nuclear fuel cycle (uranium mill tailings, depleted uranium, spent fuel rods, etc).

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. Except for the nucleus of ordinary hydrogen, which has only a proton, all atomic nuclei contain both protons and neutrons. The number of protons determines the total positive charge or atomic number. This number 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 2,700), 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

In a nuclear power reactor, 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.

Performance-based regulatory action

Licensee attainment of defined objectives and results without detailed direction from the NRC on how these results are to be obtained. (See the Communication Plan for Performance-Based Regulation by using Accession ML021120533 in ADAMS.

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).

Plausible accidents

Postulated events that meet a probability test rather than the more challenging test represented by a design basis event.

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).

Possession-only license

A form of license that allows possession but not operation.

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.

Probabilistic risk analysis

A systematic method for addressing the risk triplet as it relates to the performance of a complex system to understand likely outcomes, sensitivities, areas of importance, system interactions, and areas of uncertainty. The risk triplet is the set of three questions that the NRC uses to define “risk”: (1) What can go wrong?, (2) How likely is it?, and (3) What are the consequences?. NRC identifies important scenarios from such an assessment.

Production expense

Production expenses are a component of generation expenses that includes costs associated with operation, maintenance, and fuel.

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.

Proprietary Information

Privately owned knowledge or data, such as that protected by a registered patent, copyright, or trademark.

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 (ionizing radiation)

Alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions. Radiation, as used in 10 CFR Part 20, does not include non-ionizing radiation, such as radio- or microwaves, or visible, infrared, or ultraviolet light (see also 10 CFR 20.1003).

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.

Radiation, nuclear

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

Radioactive contamination

Deposition of radioactive material in any place where it may harm persons or equipment.

Radioactive decay

Large unstable atoms can become more stable by emitting radiation. This process is called radioactive decay. This radiation can be emitted in the form of a positively charged ALPHA particle, a negatively charged BETA particle, or GAMMA RAYS or X-RAYS.

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 plant or transport in which an activity licensed pursuant to 10 CFR Part 73 of NRC's regulations is conducted, or against a component of such a plant or transport which could directly or indirectly 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.

Reasonable

Rational, sensible, or resulting from sound judgement.

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

The acronym for Roentgen Equivalent Man is a standard unit that measures the effects of ionizing radiation on humans. The dose equivalent in rems is equal to the absorbed dose in rads multiplied by the quality factor of the type of radiation (see 10 CFR 20.1004).

Restricted area

Any area to which access is controlled for the protection of individuals from exposure to radiation and radioactive materials.

Risk

The combined answers to (1) What can go wrong? (2) How likely is it? and What are the consequences?

Risk-based decisionmaking

An approach to regulatory decisionmaking in which such decisions are made solely based on the results of a probabilistic risk analysis.

Risk-informed decisionmaking

An approach to decisionmaking in which insights from probabilistic risk analyses are considered with other engineering insights.

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.

Risk-significant

When used to qualify an object, such as a system, structure, component, accident sequence, or cut set, this term identifies that object as exceeding a predetermined criterion related to its contribution to the risk from the facility being addressed. One that is associated with a level of risk that exceeds a predetermined significance criterion.

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.

Safe shutdown earthquake

Is the maximum earthquake potential for which certain structures, systems, and components, important to safety, are designed to sustain and remain functional.

Safeguards

As used in regulation of domestic nuclear facilities and materials, the use of material control and accounting programs verify that all special nuclear material is properly controlled and accounted for, and the physical protection (also referred to as physical security) equipment and security forces.

As used by the International Atomic Energy Agency (IAEA), verifying that the "peaceful use" commitments made in binding non-proliferation agreements, both bilateral and multilateral, are honored.

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 restriction or range placed upon important process variables that are necessary to reasonably protect the integrity of the physical barriers that guard against the uncontrolled release of radioactivity.

Safety related

In the regulatory arena, this term applies to systems, structures, components, procedures and controls of a facility or process that are relied upon to remain functional during and following design basis events. Their functionality ensures that key regulatory criteria, such as levels of radioactivity released, are met. Examples of safety related functions include shutting down a nuclear reactor and maintaining it in a safe shutdown condition.

Safety-significant

When used to qualify an object, such as a system, structure, component, accident sequence, or cut set, this term identifies that object as having an impact on safety, whether determined through risk analysis or other means, that exceeds a predetermined significance criterion.

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 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 radioactive 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

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.

Severe accident

A type of accident that may challenge safety systems at a level much higher than expected.

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 that 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

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 spent (used) 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 Review Plan

A document that provides guidance to the staff for reviewing an application to obtain an NRC license to construct or operate a nuclear facility or to possess or use nuclear materials.

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 owing 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.

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.

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.

Thermalization

The process undergone by high-energy (fast) neutrons as they lose energy by collision.

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 (1) by adding or removing neutron poisons, (2) by that is increasing or decreasing electrical load on the turbine generator, or (3) by accident conditions.

Transuranic Element

An artificially made, radioactive element that has an atomic number higher than uranium in the Periodic Table of Elements such as neptunium, plutonium, americium, and others.

Transuranic Waste

Material contaminated with transuranic elements that is produced primarily from reprocessing spent fuel and from use of plutonium in fabrication of nuclear weapons.

Trip, reactor

A term that is used by pressurized water reactors for a reactor scram (see 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.

Uncertainty range

Defines an interval within which a numerical result is expected to lie within a specified level of confidence. The interval often used is the 5-95 percentile of the distribution reporting the uncertainty.

Unnecessary regulatory burden

Regulatory criteria that go beyond the levels that would be reasonably expected to be imposed on licensees given that regulations apply to conditions that incorporate normal operation and design-basis conditions.

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 (see also stable 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; processes the concentrate and converts it into uranium hexafluoride (UF6).

Vapor

The gaseous form of substances that are normally in liquid or solid form.

Very high radiation area

An area accessible to individuals, 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).

The viability assessment was required by the Energy and Water Development Appropriations Act, 1997 (Public Law 104-206). After the viability assessment was completed, site-specific environmental standards at 40 CFR Part 197 and implementing regulations (see 10 CFR Part 63).

Void

In a nuclear power reactor, 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.

Void coefficient of reactivity

A rate of change in the reactivity of a water reactor system resulting from a formation of steam bubbles as the power level and temperature increase.

Waste, radioactive

Radioactive materials at the end of a useful life cycle or in a product that is no longer useful and should be properly disposed of.

Watt

An electrical unit of power. 1 watt = 1 Joule/second. It is equal to the power in a circuit in which a current of one ampere flows across a potential difference of one volt.

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 for complete information).

Well-logging

All operations involving the lowering and raising of measuring devices or tools that contain licensed material or are used to detect licensed materials in wells for the purpose of obtaining information about the well or adjacent formations that may be used in oil, gas, mineral, groundwater, or geological exploration (see 10 CFR 39.2).

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

For purposes of whole body exposure includes at least the external exposure, head, trunk, arms above the elbow, or legs above the knee. 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 (see also 10 CFR 20.1003).

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

Yellowcake is the product of the uranium extraction (milling) process; early production methods resulted in a bright yellow compound, hence the name yellowcake. The material is a mixture of uranium oxides that can vary in proportion and in color from yellow to orange to dark green (blackish) depending at which temperature the material was dried (level of hydration and impurities). The higher drying temperatures produce a darker, less soluble material. Yellowcake is commonly referred to as U3O8 and is assayed as pounds U3O8 equivalent. This fine powder is packaged in drums and sent to a conversion plant that produces uranium hexafluoride (UF6) as the next step in the manufacture of nuclear fuel.

Privacy Policy | Site Disclaimer
Last revised Tuesday, March 09, 2004