Radiation Safety Glossary

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A

A - A symbol for mass number.

Absolute Risk - An expression of excess risk based on the assumption that the excess risk from exposure to radiation adds to the underlying (baseline) risk by an increment dependent on dose but independent of the underlying natural risk.

Absorbed Dose - Absorbed dose is the amount of energy deposited in any material by ionizing radiation. It is a measure of energy absorbed per gram of material. The SI unit of absorbed dose is the gray. The special unit of absorbed dose is the rad.  

Absorbed Dose Rate - Absorbed dose divided by the time it takes to deliver that dose. High dose rates are usually more damaging to humans and animals than low dose rates. This is because repair of damage is more efficient when the dose rate is low.

Absorption - The phenomenon by which radiation imparts some or all of its energy to any material through which it passes. 

Accelerator - A device that uses electrostatic or electromagnetic fields to increase the kinetic energy of a particle.

Access Hatch - An airtight door system that preserves the pressure integrity of a reactor containment structure while allowing access to personnel and equipment.

Actinides - Series of elements beginning with actinium (element number 89) and continuing through lawrencium (element number 103). These elements occupy the same position in the periodic table. The series includes uranium (element 92) and all transuranic elements.

Action Level - The level of radioactive contamination or biologic uptake at which additional measures or cleaning must be performed. The action level varies depending upon the type of survey performed. 

Activation - The process of making a radioisotope by bombarding a stable element with neutrons, protons, or other types of radiation. 

Activation Analysis - Use of radioactivity produced by nuclear bombardment for chemical analysis.

Activation Product - A nuclide formed through the process of activation.

Active Fuel Length - The length of fuel within a fuel element.

Activity - The number of nuclear disintegrations occurring in a given quantity of material per unit time. 

Acute Dose - The amount of a substance that produces an acute exposure to a biological system. For radiation it is the amount of energy deposited in the biological system or a specific tissue or component of that system.

Acute Exposure - The absorption of a relatively large amount of radiation (or intake of radioactive material) over a short period of time. 

Acute Health Effects - Prompt radiation effects (those that would be observable within a short period of time) for which the severity of the effect varies with the dose, and for which a practical threshold exists. 

Adaptive Response - The ability of cells to respond to low doses of radiation with the induction of a series of genes and to reduce the level of radiation-induced damage when challenged with a subsequent high dose of radiation.

After Heat - Heat produced by the decay of radioactive materials in a reactor that has been shut down.

Agreement State - A state that has signed an agreement with the Nuclear Regulatory Commission under which the state regulates the use of byproduct, source, and small quantities of special nuclear material within that state. 

Air Sampling - The collection of samples to detect the presence of, and/or to measure the quantity of volatile or solid radioactive material, nonradioactive particulate matter, or various chemical pollutants in the air. 

Airborne Contamination - Radioactive material found in sufficient quantities in the air to require that actions be taken to reduce the potential of receiving an internal dose.  

Airborne Radioactivity Area - A room, enclosure, or area in which airborne radioactive materials, composed wholly or partly of licensed material, exist. It exists 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. 

ALARA - Acronym for "As Low As Reasonably Achievable." It means making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical. Be 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. These means are in relation to utilization of nuclear energy and licensed materials in the public interest. 

Alkali Metals - The set of chemical elements that form group IA in the periodic chart. The elements making up the alkali metals include lithium (Li, Z=3), sodium (Na, Z=11), potassium (K, Z=19), rubidium (Rb, Z=37), cesium (Cs, Z=55), and francium (Fr, Z=87).

Alkaline Earth Metals - The set of chemical elements that form group IIA in the periodic chart. The elements making up the alkaline earth metals beryllium (Be, Z=2), magnesium (Mg, Z=12), calcium (Ca, Z=20), strontium (Sr, Z=38), barium (Ba, Z=56), and radium (Ra, Z=88).

Alpha Decay - Radioactive disintegration by emission of an alpha particle.

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 electric 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 represent much more of a health risk when emitted by radionuclides deposited inside the body. 

Ambient Air - The air that surrounds us.

Anion - A negatively charged ion.

Annihilation Radiation - Radiation produced by the annihilation of a positron and an electron. For particles at rest, two photons with an energy of 511 keV each are produced.  

Annual Dose Limit - The maximum dose that may be received in a calendar year.

Annual Limit on Intake (ALI) - The derived limit for the permissible amount of radioactive material taken into the body of an adult radiation worker by inhalation or ingestion in a year. The ALI is the smaller value of intake of a given radionuclide in a year by the reference man that would result in either 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. 

Area Monitoring - Routine monitoring of the level of radiation or radioactive contamination of any particular area, building, room or equipment.

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 at millions of degrees of heat (fusion). It is more correctly called nuclear energy. 

Atomic Energy Commission - A 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 US Department of Energy) and the US Nuclear Regulatory Commission. 

Atomic Mass Number - The total number of protons and neutrons in the nucleus of an atom.

Atomic Mass Unit (amu) - 1 amu is equal to one twelfth of the mass of a carbon-12 atom.

Atomic Nucleus - The central part of an atom that contains protons and neutrons and which accounts for much of the atomic mass.

Atomic Number - The number of positively charged protons in the nucleus of an atom. 

Atomic Weight - The mass of an atom, expressed in atomic mass units.

Attenuation - Attenuation is the process by which the number of particles or photons entering a body of matter is reduced by absorption and scattering. 

Attenuation Coefficient - The rate at which the intensity of a radiation decreases or is absorbed in matter. If μ is the attenuation coefficient, and I₀ is the initial intensity of the radiation, then the intensity at some absorber thickness d is given by I = I₀ e^-μd.

Attenuation Factor - The degree to which a beam of radiation has been attenuated. It is equal to I/I₀ or e^-μx, where I₀ is the intensity of the unattenuated beam, I is the intensity of the beam after passing through an absorber with a thickness of x and an attenuation coefficient μ.

Auxiliary Building - Building at a nuclear power plant, frequently located adjacent to the reactor containment structure, that houses most of the reactor auxiliary and safety systems, such as radioactive waste systems, chemical and volume control systems, and emergency cooling water systems.

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.

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B

Background Count - A measurement performed to estimate the intensity of background radiation observable by a radiation detection system. Background counts are usually performed to remove the contribution of background radiation from a second measurement.

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. 

Backscattering - The process of scattering or deflecting into the sensitive volume of a measuring instrument radiation that originally had no motion in that direction. The process is dependent on the nature of the mounting material, the shield surrounding the sample and the detector, the nature of the sample, the type of energy of the radiation, and the geometry.

Barn - Unit of reaction probability defined as 10^-28 m² (10^-24 cm²).

Beam Collimation - Diameter of the laser beam at which power per unit area of the beam is 1/e times that of the peak power per unit area.  

Becquerel (Bq) - The unit of radioactive decay equal to one disintegration per second. The Becquerel is the basic unit of radioactivity used in the international system of radiation units, referred to as the “SI” units. 37 billion (3.7×10¹⁰) becquerels = 1 curie (Ci). 

Beta Decay - Decay by emission of a beta particle. Three types of beta decay are common in nuclei: Neutron rich nuclei tend to decay by emitting a β^- particle. An antineutrino is also emitted in this type of b decay and the it results in the nucleus converting a neutron into a proton. Neutron deficient nuclei tend to decay by positron emission or electron capture. Positron emission refers to the emission of a positron (β⁺), which is the antiparticle of the electron. A neutrino is emitted in the process and this results in the nucleus converting a proton into a neutron. Electron capture is usually classified as a type of beta decay and involves an orbital electron being absorbed by a nucleus, effectively converting a proton into a neutron.

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. Exposure to large amounts of beta radiation from external sources may cause skin burns (erythema). Beta emitters can also be harmful if they enter the body. Thin sheets of metal or plastic may stop beta particles. 

Binding Energy - The energy that would be released if a nuclide were formed by combining an appropriate number of hydrogen atoms and neutrons. For the nuclide ⁴⁰K, it is the energy that would be released by combining 19 hydrogen atoms and 21 neutrons.

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 Dosimetry (Biodosimetry) - The laboratory or clinical methods used to measure or estimate the dose of ionizing radiation energy absorbed by an individual. Biodosimetry tools measure the dose to internal organs and tissues from external exposure and internal contamination.

Biological Effects of Ionizing Radiation (BEIR) Reports - Reports of the National Research Council's committee on the Biological Effects of Ionizing Radiation. 

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

Biological Shield -  A mass of absorbing material placed around a reactor or radioactive source to reduce the radiation to a level safe for humans.

Body Burden - The amount of radioactive material which if deposited in the total body will produce the maximum permissible dose rate to the critical organ. 

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 - An element or chemical that if ingested migrates to the bones. An example is ⁹⁰Sr, which behaves chemically like calcium and is deposited in the bones.

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.

Bremsstrahlung Radiation - Produced by the sudden deceleration of an electrically charged particle when passing through an intense electrical field.  

British Thermal Unit (Btu) - A unit of energy define as 1055.87 J. It is the amount of energy required to change the temperature of one pound of water by one degree Fahrenheit at sea level.

Burnup - A measure of nuclear reactor fuel consumption defined as the ratio of the fissile material consumed to that originally present.

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. 

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C

Calibration - Determination of variation from standard, or accuracy, of a measuring instrument to ascertain necessary correction factors. The check or correction of the accuracy of a measuring instrument to assure proper operational characteristics. 

Calibration Factor - A normalization factor that converts between the value of a measured parameter and the actual value of that parameter in a system. The need for calibration factors arises because detectors and sensors do not directly sample a parameter but instead produce a response that scales with the value of the parameter. An example is the measurement of a gamma-ray energy by a radiation detector that produces an electrical pulse whose total charge is proportional to the energy deposited by a gamma ray.

Capacity Factor - A ratio of the average power output of a power plant to its rated capacity.

Capture Cross Section - A measure of the probability that an incident particle or photon will be absorbed by a target nuclide.

Carrier - A quantity of non-radioactive or non-labeled material of the same chemical composition as its corresponding radioactive or non-labeled counterpart. When mixed with the corresponding radioactive labeled mixture, the carrier permits chemical (and some physical) manipulation of the mixture with less label or radioactivity loss than would be true for the undiluted label or radioactivity.

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.

Cave - A heavily-shielded room for storing or handling highly-radioactive materials. Remote manipulators allow personnel to safely work with the radioactivity.

Cerium Earths - Elements with atomic numbers 57 through 62. The set of cerium earths includes the following elements: 57 lanthanum (La); 58 cerium (Ce); 59 praseodymium (Pr); 60 neodymium (Nd); 61 promethium (Pm); 62 samarium (Sm).

Chain Reaction - A reaction that is self-sustaining. In a fission chain reaction, the nucleus of a fissionable atom absorbs a neutron and fissions (i.e., breaks apart), 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 non-fissionable material or by escape from the system. 

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

Chart of Nuclides - A map showing the known nuclides. The number of protons in a nucleus is shown on the vertical axis and the number of neutrons is shown on the horizontal axis. Nuclide charts are often colored to visualize a particular property and the nuclide boxes often contain a brief summary of nuclear properties.

Chemical Atomic Weight - The weighted mean of the masses of the neutral atoms of an element expressed in atomic mass units.

Chronic Dose - A dose delivered over a period of greater than one day.

Chronic Exposure - One or more exposures that occur over a period longer than one day.

Chronic Radiation Dose - A single or multiple doses delivered over a period of greater than one day. Chronic radiation doses can be low enough to cause few effects and to permit recovery, and they can be severe enough to cause radiation sickness and death.

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 continuous water filtration and demineralization system for reactor coolant systems. It serves to reduce contamination levels in the water and to reduce corrosion.

Coincidence - Of or pertaining to events that are coincident.

Cold - Not radioactive.

Cold Neutron - A neutron with an energy greater than or equal to 5x10^-5 eV and less than or equal to 0.025 eV.

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 in a given time period by a specified population from exposure to a specified source of radiation. 

Committed Dose Equivalent - The dose to a specific organ or tissue that is received from an intake of radioactive material by an individual over a specified time after the intake. For radiation protection purposes, the specified time is to the age of 70, which is normally taken to be 50 years for a radiation worker and 70 years for a member of the public. 

Committed Effective Dose Equivalent - The committed dose equivalent for a given organ multiplied by a weighting factor. 

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

Compton Scattering - Elastic scattering of photons in materials, resulting in a loss of some of the photon's energy.  

Concentration - The ratio of the amount of a specific substance in a given volume or mass of solution to the mass or volume of solvent.

Condensate - Water formed by cooling steam in a condenser.

Conference of Radiation Control Program Directors (CRCPD) - An organization whose members represent state radiation protection programs. 

Confinement Systems - Those systems, including ventilation, that act as barriers between areas containing radioactive substances and the environment.

Containment - (1) (general) The process of confining radioactive nuclides within a vessel. (2) (nuclear reactors) The process of confining radioactive nuclides to a reactor vessel. It also applies to the reactor building which is designed to confine fission products in the event of a major accident. (3) (waste disposal) Confinement of radioactive waste within a designated boundary. (4) (fusion research) The process of confining a plasma.

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

Continuum Region Neutron - A neutron with an energy between 0.01 MeV and 25 MeV.

Controlled Area - An area, outside of a restricted area but inside the site boundary, access to which can be limited by the licensee for any reason. 

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 a restricted area but within the site boundary, access to which the licensee can limit 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 - Cooling of the fuel rods following reactor shutdown by circulation of coolant through the reactor core.

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.

Cosmic Radiation - Penetrating ionizing radiation, both particulate and electromagnetic, that originates in outer space. Secondary cosmic rays, formed by interactions in the earth's atmosphere, account for about 45 to 50 millirem of the 300 millirem of natural background radiation that an average member of the US public receives in a year. 

Coulomb - The SI unit of electric charge defined as 1.0 A-s.

Count (Radiation Measurements) - The indication of a device detecting ionizing radiation events. It may refer to a single detected event or to the total registered in a given period of time, or be expressed as a count rate (e.g., counts per minute (cpm) or counts per second (cps)).  

Counter - A general designation applied to radiation detection instruments or survey meters that detect and measure radiation. The signal that indicates an ionization event has been detected is called a count. 

Counting System - A radiation detection system consisting of one or more detectors, electronics for processing the detector signals, output devices for displaying results, and often computer hardware and software for recording and processing data.

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

Critical Mass - That minimum mass of fissionable material that supports a sustained fission chain reaction.

Critical Organ - That part of the body that is most susceptible to radiation damage resulting from the specific exposure conditions under consideration, taking into account the dose the various parts of the body receive under the exposure conditions. 

Cross Section - A measure of the probability that a nuclear reaction will occur. It is the apparent or effective area presented by a target nucleus or particle to an oncoming radiation. The barn is the standard unit for the cross section.

Crud - A colloquial term for corrosion and wear products (rust particles, etc.) that become radioactive (i.e., activated) when exposed to radiation. 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 the same portion of the body, or to the whole body, over a period of time. 

Curie (Ci) - The original unit used to express the decay rate of a sample of radioactive material. The curie is equal to that quantity of radioactive material in which the number of atoms decaying per second is equal to 37 billion (3.7×10¹⁰). It was based on the rate of decay of atoms within one gram of radium. It is named for Marie and Pierre Curie who discovered radium in 1898. The curie is the basic unit of radioactivity used in the system of radiation units in the United States, referred to as "traditional" units. 

Cutaneous Radiation Syndrome (CRS) - The complex syndrome resulting from radiation exposure of more than 200 rads to the skin. The immediate effects can be reddening and swelling of the exposed area (like a severe burn), blisters, ulcers on the skin, hair loss, and severe pain. Very large doses can result in permanent hair loss, scarring, altered skin color, deterioration of the affected body part, and death of the affected tissue (requiring surgery).

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D

Daughter Nuclide - A radionuclide produced by the decay of a parent nuclide.  

Dead Time - The time that the instrument is busy processing an input signal and is not able to accept another input; often expressed as a percentage.  

Decay Chain - A series of nuclides linked in a chain by radioactive decay. Each nuclide in the chain decays to the next until a stable nuclide is reached.

Decay Constant - The fraction of a number of atoms of a radionuclide that disintegrates in a unit of time. The decay constant is inversely proportional to the radioactive half-life.

Decay Heat - Heat produced through radioactive decay. It is especially important for fission products.

Decay Mode - A particular type of radioactive decay. Examples include β^- decay, β⁺ decay, a decay, internal transition, and spontaneous fission.

Decay Products - Decay products are also called "daughter products". They are radionuclides that are formed by the radioactive decay of parent radionuclides. In the case of radium-226, for example, nine successive different radioactive decay products are formed in what is called a "decay chain." The chain ends with the formation of lead-206, which is a stable nuclide. 

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 a radiation worker and has voluntarily informed her employer, in writing, of her pregnancy and the estimated date of conception. 

Decommission - The process of closing down a facility followed by reducing the residual quantities of radioactive material to a level that permits the release of the property for unrestricted use. 

Decontamination - The reduction or removal of contaminated radioactive material from a structure, area, object, or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination or (2) letting the material stand to permit the quantity of radioactive material decrease as a result of radioactive decay. 

Decontamination Factor - The ratio of the amount of undesired radioactive material initially present to the amount remaining after a suitable processing step has been completed. Decontamination factors may refer to the reduction of some particular type of radiation or to the gross measurable radioactivity.

Deep Dose Equivalent - Applies to external whole-body exposure and is the dose equivalent at a tissue depth of one centimeter (1000 mg/cm2). 

Delayed Health Effects - Radiation health effects which are manifested long after the relevant exposure. The vast majority are stochastic, that is, the severity is independent of dose and the probability is assumed to be proportional to the dose, without threshold. 

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 that will result in an annual limit of intake if an individual breathes that air for a year. For a radiation worker, it is assumed air is breathed for 2,000 hours in one year in the workplace. 

Design-Basis Accident - A standard to which a nuclear facility is designed. It is a postulated accident that the facility must withstand without loss to the components, systems, and structures.

Detection Gas - A gas sensitive to radiation and that produces an electric, electromagnetic or other signal which can be more easily detected.

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 - Health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Deterministic effects generally result from the receipt of a relatively high dose over a short time period. Skin erythema (reddening) and radiation-induced cataract formation is an example of a deterministic effect (formerly called a nonstochastic effect). 

Dirty Bomb - Commonly refers to a device that spreads radioactive material by exploding a conventional (non-nuclear) explosive, such as dynamite. Because they do not involve the sophisticated technology required to create a nuclear explosion, dirty bombs are much simpler to make than a true nuclear bomb.  

Disintegration - A spontaneous nuclear transformation (radioactivity) characterized by the emission of energy and/or mass from the nucleus. In one microcurie, there are 2,220,000 disintegrations emitted every minute.  

Disintegrations Per Minute - The number of decays that occur per minute for a population of a nuclide or a sample containing radioactive material.

Disintegrations Per Second - The number of decays that occur per second for a population of a nuclide or a sample containing radioactive material.

Dose - A general term used to refer to the effect on a material that is exposed to radiation. It is used to refer either to the amount of energy absorbed by a material exposed to radiation or to the potential biological effect in tissue exposed to radiation. 

Dose Coefficient - The factor used to convert radionuclide intake to dose. Usually expressed as dose per unit intake (e.g., sieverts per becquerel).

Dose Commitment - The dose equivalent received by a person to a specified organ during one calendar year due to uptake of a radionuclide.

Dose Equivalent - The product of absorbed dose in tissue multiplied by a quality factor, and then sometimes multiplied by other necessary modifying factors, to account for the potential for a biological effect resulting from the absorbed dose. It is expressed numerically in rems (traditional units) or sieverts (SI units). 

Dose Limits - The permissible upper bounds of radiation doses.

Dose Rate - The radiation dose delivered per unit time. 

Dose Ratemeter - Any instrument which measures radiation dose rate.

Dose reconstruction - A scientific study that estimates doses to people from releases of radioactivity or other pollutants. The dose is reconstructed by determining the amount of material released, the way people came in contact with it, and the amount they absorbed.

Dosimeter - A small portable instrument (such as a film badge, thermoluminescent or pocket dosimeter) for measuring and recording the total accumulated 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.

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E

Effective Dose - A measure of dose in which the type of radiation and the sensitivity of tissues and organs to that radiation is taken into account.

Effective Dose Equivalent - The sum of the dose equivalents to the organ or tissue (H_T) and the weighting factors (W_T) applicable to each of the body organs or tissues that are irradiated.

Effective Half-Life - The time required for the amount of a radionuclide deposited in a living organism to be diminished 50 percent as a result of the combined action of radioactive decay and biological elimination. 

Efficiency (Radiation Detection Instrument) - A measure of the probability that a count will be recorded when radiation is incident on a detector. Usage varies considerably so be aware of which factors (window, transmission, sensitive volume, energy dependence, etc.) are included in a given case.  

Electromagnetic Radiation - A traveling wave motion resulting from changing electric or magnetic fields. Familiar types of 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 wavelength. Only the higher-energy (higher frequency/shorter wavelength) forms of electromagnetic radiation are ionizing. Radiation in the lower-energy ranges, such as visible, infrared, radar, and radio waves, are nonionizing. 

Electron - An elementary particle with a negative charge and a mass 1/1837 that of the proton. Electrons surround the positively charged nucleus of the atom.

Electron Capture - A mode of radioactive decay involving the capture of an orbital electron by its nucleus. X-rays are emitted as a consequence of the rearrangement of the orbital electrons.  

Electron Volt - A unit of energy defined as 1.60919x10^-19 joules. It is the energy required to raise an electron through a potential difference of 1 volt. The electron volt is not an SI unit but its use is valid unit within the International System of units.

Electroscope - Instrument for detecting the presence of electric charges by the deflection of charged bodies.

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

Emergency Core Cooling System - A backup coolant system for reactors that automatically activates when the primary coolant system fails. It is designed to limit the temperature of the reactor core and thereby prevent damage. Upon activation of the backup coolant system, the reactor scrams, reducing the heat output of the core. Latent heat produced from the decay of fission products in the core requires continued cooling after the reactor has been scrammed.

Energy - The capacity for doing work.

Enriched Uranium - Uranium in which the proportion of the isotope uranium-235 has been increased. 

Epidemiology - The study of the distribution and determinants of health-related states or events in specified populations; and the application of this study to the control of health problems.

Equilibrium - Any state where the creation or addition of a component equals the consumption or removal of that component. The term is often used in nuclear science in the context of the activity of a radioactive nuclide that is fed by a radioactive parent. The system is said to be in equilibrium when the rate of decay of the parent equals the rate of decay of the daughter.

Equivalent Dose - The absorbed dose expressed in terms of the potential injury that it could cause. It is the product of the absorbed dose, the quality factor, and all other necessary modifying factors at the location of interest. The SI standard uni for dose equivalent is the sievert.

Excited State - The state of molecule, atom, or nucleus when it possesses more than its ground state energy. Excess molecular or atomic energy may be reduced through emission of photons or heat. Excess nuclear energy may be reduced through emission of gamma rays or conversion electrons or by further decay of a radionuclide.  

Exposure - A general term used loosely to express what a person receives as a result of being exposed to ionizing radiation. 

Exposure Pathway - A route by which a radionuclide or other toxic material can enter the body. The main exposure routes are inhalation, ingestion, absorption through the skin, and entry through a cut or wound in the skin.

Exposure Rate - A measure of the ionization produced in air by x-rays or gamma rays per unit of time (frequently expressed in roentgens per hour).

External Dose - That portion of the dose equivalent received from radiation sources outside the body. 

External Radiation - The situation in which the source of exposure is external to, that is, outside the body. 

Extremities - The hands, forearms, elbows, feet, knees, legs 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). 

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F

Fallout, Nuclear - The slow descent of minute particles of radioactive debris in the atmosphere following a nuclear explosion.  

Fast Neutron - A neutron with kinetic energy greater than its surroundings when released during fission.

Feedwater - Water used to remove heat from a reactor and produce steam to drive the turbine generators. This water is supplied directly to the reactor pressure vessel in a boiling water reactor. It is supplied to the steam generator in a pressurized water reactor.

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 contain a filter that shields part of the film from certain types of radiation. 

Fissile - A nuclide capable of undergoing neutron-induced fission. The nuclides ²³⁵U and ²³⁹Pu are classified as fissile because they have very large cross sections for neutron-induced fission.

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

Fission (Fissioning) - The splitting of the nucleus of an atom (generally of a heavy element) 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. 

Fissionable - Capable of undergoing spontaneous fission or induced fission.

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 nuclides formed by the subsequent decay products of the radioactive fission fragments. 

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. 

Fluence - The number of particles per unit area. It is a time-integrated flux.

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.

Fractionated Exposure - Exposure to radiation that occurs in several small acute exposures, rather than continuously as in a chronic exposure.

Fuel Assembly - A cluster of fuel rods (or plates). Also called a fuel element. Many fuel assemblies make up a reactor core.

Fuel Cladding - Material used to construct reactor components and designed to maintain a separation between their contents and the coolant. An example is the cladding of a fuel pin that separates the fuel pellets from the coolant. Zirconium and zirconium alloys (e.g., Zircaloy) are common cladding materials.

Fuel Cycle - The series of steps involved in supplying fuel for nuclear power reactors. The fuel cycle can include mining, milling, isotopic enrichment, fabrication of fuel elements, use in a reactor, re-enrichment of the fuel material, re-fabrication into new fuel elements, and waste disposal.  

Fuel Element - A cluster of fuel pins mounted into a single assembly.

Fuel Pellet - A small cylinder of enriched uranium, typically as UO₂. Several fuel pellets are assembled into a fuel pin which forms a component of a fuel element.

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.  

Fusion Reaction - A reaction in which at least one heavier, more-stable nucleus is produced by the combination of two lighter, less-stable nuclei. Reactions of this type are responsible for enormous releases of energy, for example, the heat from the sun. 

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G

Gamma Radiation - High-energy, short wavelength, electromagnetic radiation emitted from the nucleus of an atom. Gamma radiation frequently accompanies the emission of alpha and beta particles and always accompanies fission. 

Gamma Rays - Very penetrating and are best stopped or shielded by dense materials, such as lead or uranium. Gamma rays are similar to x rays. 

Gaseous Diffusion - A process of separating isotopes as gases by their slight difference in velocity. Lighter isotopes diffuse faster through a porous membrane or vessel than do heavier isotopes.

Gaseous Diffusion Plant - A facility where uranium hexafluoride gas is filtered, uranium-235 is separated from uranium-238, increasing the percentage of uranium-235. The process requires enormous amounts of electric power. 

Gas Flow Counter - A gas ionization detector that does not have a window between the sample and the detection medium. Samples are placed inside the detector and the detection volume is flushed with a suitable detection gas.

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 and ionizes the gas within 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 is an indication of the rate at which ionizing events are occurring within the tube. 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. 

Generator (Cow): A device in which radioactive progeny is eluted from an ion exchange column containing a parent radionuclide that is long-lived compared to the progeny.

Genetic Effects - Effects from radiation that are seen in offspring of the individual who received the radiation. The radiation must be encountered pre-conception.

Geometry - The detector to sample distance, the sizes and shapes of the detector, the sample, and any shielding, all of which affect the radiation seen by the detector. The geometry helps define the efficiency of the detector.

Gray (Gy) - The 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 has replaced the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad. 

Ground State - The state of a nucleus, atom or molecule at its lowest energy level.  

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H

Half-Life - The time in which one-half of the activity of a particular radioactive substance is lost due to radioactive decay. Measured half-lives vary from millionths of a second to billions of years. Also called physical or radiological half-life. 

Half-Life (Biological) - The time required for the body to eliminate, by biological processes, one-half of the material originally taken in. 

Half-Life (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-Life (Radioactive) - Time required for a radioactive substance to lose 50% of its activity by radioactive decay. Each radionuclide has a unique half-life (also known as physical half-life).

Half Thickness - That absorber thickness required to attenuate the intensity of a radiation beam by one half.

Half Value Layer - The thickness of any specified material necessary to reduce the intensity of an x-ray or gamma ray beam to one-half its original value. 

Hazardous Waste - Waste products that can pose a substantial or potential hazard to human health or the environment when improperly managed. Hazardous waste is regulated at the federal level under the Resource Conservation and Recovery Act. A waste may be hazardous because it has at least one of four characteristics - ignitability, corrosivity, reactivity, or toxicity--or it may be included on one of several lists of waste groups that are known to be hazardous.  

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 to permit the safe use and application of ionizing radiation. 

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 licensed facilities must be posted as "high radiation areas" and access into these areas is maintained under strict control. 

High Enriched Uranium - The isotope uranium-235 enriched to 20 percent or greater in total concentration. 

High-Level Radioactive 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. 

Hot - Radioactive.

Hot Spot - The region in a radiation/contamination area in which the level of radiation/contamination is noticeably greater than in neighboring regions in the area. 

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I

Individual Monitoring Devices - Devices designed to be worn by a single individual for the assessment of dose equivalent such as film badges, thermoluminescence dosimeters (TLDs), pocket ionization chambers, and personal (lapel) air sampling devices.

Ingestion - 1) the act of swallowing; 2) in the case of radionuclides or chemicals, swallowing radionuclides or chemicals by eating or drinking.

Inhalation - 1) the act of breathing in; 2) in the case of radionuclides or chemicals, breathing in radionuclides or chemicals.

In Situ Counting - Measurement and analysis of radioactivity performed at the sample's location.  

Intake - Quantity of material introduced into the body by inhalation, ingestion or through the skin (absorption, puncture, etc.) 

Internal Radiation - Radiation produced by the decay of radioactive material in the body.

Inverse Square Law - The intensity of radiation at any distance from a point source varies inversely as the square of that distance. For example: if the radiation exposure is 100 R/hr at 1 inch from a source, the exposure will be 0.01 R/hr at 100 inches. 

In Vivo Counting - In vivo counting refers to directly measuring and analyzing radionuclide activity levels in a living body.  

Ion - An atom or molecule that has become electrically charged by having lost or gained one or more electrons. Examples of an ion are an alpha particle, which is a helium atom minus its two electrons, and a proton, which is a hydrogen atom minus its single electron. 

Ionization - Of or pertaining to the process of ionizing an atom, ion or molecule.

Ionization Chamber - An instrument designed to measure the quantity of ionizing radiation in terms of the charge of electricity associated with ions produced within a defined volume. 

Ionization Detector - A device for detecting radiations that consists of a chamber filled with an appropriate gas and containing electrodes maintained at a high potential. Ions are produced by the interaction of radiation with the gas. These ions drift to the electrodes, producing a measurable electric current.

Ionizing Radiation - Any electromagnetic or particulate radiation capable of producing ionization in matter. Examples of ionizing radiation include alpha particles, beta particles, gamma rays x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions. This definition excludes non-ionizing radiation, such as radio- or microwaves, or visible, infrared, or ultraviolet light.

Ionizing Radiation Warning Symbol - A radiation warning symbol, to supplement the existing trefoil symbol, has been published by ISO as Standard #21482 - Ionizing-Radiation Warning—Supplementary Symbol. The new symbol is a universal radiation warning symbol with the message of "Danger-Stay Away". It is intended for IAEA Category 1, 2, and 3 sources defined as dangerous sources capable of causing death or serious injury. It should be placed on the device housing the source, as a warning not to dismantle the device or to get any closer. Where practical, it should be placed under the device cover such that it is not visible under normal use but would be visible if anyone attempts to disassemble the device. The symbol is not intended for doors or shipping containers. 

Irradiate - To expose to radiation.

Irradiation - Exposure to radiation.  

Isotope - One of two or more atoms with the same number of protons, but different numbers of neutrons in their nuclei. Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element, carbon, the numbers denoting the mass number of each isotope. Isotopes have very nearly the same chemical properties, but often have different physical properties. For example, carbon-12 and carbon-13 are stable; carbon-14 is unstable, that is, it is radioactive. 

Isotopic Abundance - The concentration of one isotope in a sample relative to the other isotopes of that element.

Isotopic Tracer - The isotopic or non-natural mixture of isotopes of an element which may be incorporated into a sample to make possible observation of the course of that element, alone or in combination, through a chemical, biological or physical process. The observations may be made by measurement of radioactivity or isotopic abundance.

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J

Joule - An SI unit of energy defined as 1.0 kg m²/s² or 1.0 N m.

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K

Kerma - The initial kinetic energy of the primary ionizing particles (photoelectrons, Compton electrons, positron/negatron pairs from photon radiation, and scattered nuclei from fast neutrons) produced by the interaction of the incident uncharged radiation, per unit mass of interacting medium. Unit of measure is gray.

Kiloelectron Volt (keV) - One thousand electron volts.

Kilovolt - A unit of electrical potential defined as 1,000 volts.

Kilowatt - A unit of power equivalent to 1,000 watts.

Kilowatt Hour - A unit of energy defined as exactly 3.6×10⁶ J.

Kilo - A prefix that multiplies a basic unit by 1,000 or 10³. 

Kinetic Energy - Energy possessed by a particle, nucleon, nucleus, atom or other body by virtue of its motion.

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L

Labeled Compound - A compound consisting, in part, of labeled molecules. By observations of radioactivity or isotopic composition this compound or its fragments may be followed through physical, chemical or biological processes. 

Lanthanides - Elements with atomic numbers 58 through 71 (Z=57-71), the elements that follow lanthanum in the periodic chart. The set of lanthanides includes the following elements: cerium (Ce, Z=58); praseodymium (Pr, Z=59); neodymium (Nd, Z=60); promethium (Pm, Z=61); samarium (Sm, Z=62); europium (Eu, Z=63); gadolinium (Gd, Z=64); terbium (Tb, Z=65); dysprosium (Dy, Z=66); holmium (Ho, Z=67); erbium (Er, Z=68); thulium (Tm, Z=69); ytterbium (Yb, Z=70); lutetium (Lu, Z=71).

Latent Period - The time between exposure to a toxic material and the appearance of a resultant health effect.

Lethal Dose - That dose of radiation which is likely to cause death.

Licensed Material - Source material, special nuclear material, or byproduct material received, possessed, used, transferred or disposed of under a general or specific license issued by the Nuclear Regulatory Commission. 

Licensee - The holder of the license. 

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.

Limits - the permissible upper bounds of radiation exposures, contamination or releases. 

Linear Energy Transfer - The rate at which a particle loses energy as it passes through matter.

Loose Surface Contamination - Contamination that is easily removed. Loose contamination must be controlled to minimize internal contamination, spread of contamination to the environment, or contamination of experiments.  

Loss of Coolant Accident - Inadvertent escape of water from the primary coolant system.

Low Enriched Uranium - A sample of uranium that has a ²³⁵U concentration greater than its natural isotopic abundance (0.711%) and less than 20%.

Low-Level Radioactive 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. 

Low Population Zone - The area immediately surrounding a reactor in which the population density is kept low for safety concerns.

Low Specific Activity - Radioactive material with limited specific activity that satisfies the descriptions and limits set forth in the three low specific activity categories. Shielding materials surrounding the low specific activity material may not be considered in determining the estimated average specific activity of the package contents.

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M

Mass Number - The total number of protons and neutrons in a nucleus. The total number of nucleons. It is generally used with the chemical symbol to distinguish between nuclides. Tritium contains one proton and two neutrons and is represented by the symbol ³H, where the mass number is 3.

Maximum Permissible Concentration (MPC) - The concentration limit for a given radionuclide in air or water in determining possible inhalation, ingestion or absorption for health physics controls. 

Mega - Prefix that multiplies a basic unit by 1,000,000 or 10⁶. 

Megacurie - One million (10⁶) curies.

Megawatt (MW) - One million watts.

Megawatt Hour (MWh) - One million watt-hours.

Metalloids - The set of chemical elements that form a sort of boundary in the periodic chart between the metals and non-metals. The elements making up the metalloids include boron (B, Z=5), silicon (Si, Z=14), germanium (Ge, Z=32), arsenic (As, Z=33), antimony (Sb, Z=51), tellurium (Te, Z=52), and polonium (Po, Z=84).

Metastable Isotope - A long-lived energy state of a particular nuclide that is not its ground state. Some nuclides have more than one isomeric state. An isomeric state has the same mass number and atomic number as the ground state, but possesses different radioactive properties.  

Micro (µ) - A prefix that divides a basic unit into one million parts (10^-6). 

Microcurie (µCi) - One millionth (10^-6) of a curie. 

Milli (m) - A unit prefix equivalent to 10^-3.

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 (10^-3). 

Millirem (mrem) - One thousandth of a rem. (1 mrem = 10^-3 rem) 

MilliRoentgen (mR) - A sub multiple of the Roentgen equal to one-thousandth (1/1000th) of a Roentgen. 

Mixed Waste - Waste which contains both hazardous waste (as defined by RCRA and its amendments) and radioactive waste (as defined by AEA and its amendments). It is jointly regulated by NRC or NRC's Agreement States and EPA or EPA's RCRA Authorized States. The fundamental and most comprehensive statutory definition is found in the Federal Facilities Compliance Act (FFCA) where Section 1004(41) was added to RCRA: "The term 'mixed waste' means waste that contains both hazardous waste and source, special nuclear, or by-product material subject to the Atomic Energy Act of 1954." 

Mixed Transuranic Waste - Waste which contains both hazardous waste and transuranic waste. 

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.

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. 

Monitor - A device that measures radiation levels. It can also record results or trip an alarm if a set point has been exceeded.

Monitoring - The measurement of radiation levels, concentrations, surface area concentrations or quantities of radioactive material and the use of the results of these measurements to evaluate potential exposures and doses. 

Mrem - Millirem, one thousandth of a rem. 

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N

n - A symbol for neutron.

Nano - A prefix that divides a basic unit by one billion (10^-9). 

Nanocurie - One billionth (10^-9) of a curie. 

Natural Background Radiation - Ionizing radiation produced by cosmic rays, the interaction of cosmic rays with matter, and from the decay of non-anthropogenic radioactive materials.

Natural Uranium - Uranium as found in nature. It contains about 0.7 percent uranium-235, 99.3 percent uranium-238, and a trace of uranium-234. 

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 Activation - The process of inducing radioactivity in a sample using the neutron capture reaction.

Neutron Activation Analysis (NAA) - The process of activating materials by neutron absorption then measuring the emission of characteristic photons on decay to determine the relative abundance of elements in an object. 

Neutron Capture - Process in which an atomic nucleus absorbs or captures a neutron. 

Neutron Generation - A population of neutrons produced during one link in the fission chain reaction. Neutrons in one generation that are successfully moderated and that induce further fission reactions produce a new generation of neutrons.

Neutron Moderator - A substance used to reduce the energy of neutrons through inelastic scattering. Good moderators have a large scattering cross section and a small absorption cross section. Graphite, water, and heavy water are examples of good neutron moderators.

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-Removable Contamination - Contamination adhering to the surface of structures, areas, objects or personnel and will not readily be picked up or wiped up by physical or mechanical means during the course of a survey or during decontamination efforts. 

Nonionizing Radiation - Electromagnetic or particulate radiation that lacks sufficient energy to remove electrons from the outer shells of atoms.

Nonstochastic Effect - Health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Nonstochastic effects generally result from the receipt of a relatively high dose over a short time period. Skin erythema (reddening) and radiation-induced cataract formation is an example of a nonstochastic effect. This term has been replaced with Deterministic Effect. 

NORM - An acronym for Naturally Occurring Radioactive Material. Naturally occurring radioactive materials are common in virtually all rocks, minerals, and soils. They naturally contain small amounts of uranium, thorium, and a radioactive isotope of potassium. Plants and animals are also naturally radioactive; they contain small (but measurable) levels of radioactive potassium as well as radioactive carbon (C-14) and hydrogen (tritium, or H-3) that are formed by cosmic ray interactions in the atmosphere. 

Nuclear Energy - The energy liberated by a nuclear reaction (fission or fusion) or by radioactive decay. 

Nuclear Facility - A facility for producing, processing, using, storing, or handling special nuclear material. This includes irradiated material that is of national security significance.

Nuclear Force - A powerful short-ranged attractive force that holds together the particles inside an atomic nucleus. 

Nuclear Fuel - Fissile material used in a reactor.

Nuclear 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 reactors, chemical reprocessing to recover the fissionable material remaining in the spent fuel, re enrichment of the fuel material refabrication into new fuel elements and waste disposal.  

Nuclear Fusion - The process of forming a heavier nucleus from two lighter ones.

Nuclear Incident - An unexpected event involving a nuclear weapon, facility, or component, resulting in any of the following, but not constituting a nuclear weapon(s) accident: (1) an increase in the possibility of explosion or radioactive contamination; (2) errors committed in the assembly, testing, loading, or transportation of equipment, and/or the malfunctioning of equipment and materiel which could lead to an unintentional operation of all or part of the weapon arming and/or firing sequence, or which could lead to a substantial change in yield, or increased dud probability; and (3) any act of God, unfavorable environment, or condition resulting in damage to the weapon, facility, or component.

Nuclear Material - (1) Any source material or any special nuclear material. This definition is similar to that for Radioactive Material but 10CFR part 75 does not specifically include by-product material. 
(2) The following nuclides, chemical elements, and materials: ²H, ³H, ⁶Li, ²³³U, ²³⁷Np, ²⁵²Cf, ^238-242Pu and ^239-241Pu [sic.]; americium, berkelium, curium, and thorium; depleted uranium, normal uranium, and enriched uranium. [DOE Order 5633.3B] (3) A collective term for materials subject to the provisions of DOE Order 5660.1B.

Nuclear Power - Electric power generated using nuclear reactors.

Nuclear Power Plant - An electrical generating facility using a nuclear reactor as its power (heat) source. 

Nuclear Radiation - Particulate and electromagnetic radiation emitted from atomic nuclei in various nuclear processes. The important nuclear radiations, from the weapon standpoint, are alpha and beta particles, gamma rays, and neutrons. All nuclear radiations are ionizing radiations, but the reverse is not true; x-rays for example, are included among ionizing radiations, but they are not nuclear radiations since they do not originate from atomic nuclei.

Nuclear Reaction - A reaction involving an atomic nucleus. It is usually initiated by bombarding a target nucleus with a radiation, called a projectile. The interaction of the radiation with the nucleus may cause the emission of other radiations, called ejectiles. In the reaction ¹⁴N + n → ¹⁴C + p, the target nucleus is ¹⁴N, the neutron is the projectile, and the proton is the ejectile. This reaction can also be written as ¹⁴N(n,p)¹⁴C.

Nuclear Reactor - (1) A device that produces a sustained, controlled fission chain reaction. (2) A device in which a sustained fission reaction can be maintained. The core is made of a fissile material such as uranium enriched in the isotope ²³⁵U. It is usually surrounded by water which moderates neutrons and removes heat from the core.

Nuclear Regulatory Commission - NRC is an independent regulatory agency created out of the Atomic Energy Commission in 1975 to regulate the civilian uses of nuclear material. It has five Commissioners, who are nominated by the President and confirmed by the Senate; the President designates one of the Commissioners as Chairman. The NRC is responsible for ensuring that a civilian nuclear activities are carried out with adequate protection of the public health and safety, the environment, and national security: operation of nuclear power and fuel cycle plants nuclear medicine industrial use of radioactive materials use of radioactive materials in research regulation of all AEA materials. Except in a few cases, NRC does not regulate the Department of Energy's radioactive materials.  

Nuclear Safeguards - The general topic of maintaining control and accountability of special nuclear materials.  

Nuclear Safety - Aspects of safety that encompass activities and systems that present the potential for (1) uncontrolled releases of fission products or other radioactive materials to the environment or (2) for inadvertent criticality.

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

Nuclei - Plural of nucleus. A population of one or more nuclides, either specified or left unspecified.

Nucleon - A proton or a neutron. the nuclides ³H and ³He both have three nucleons.

Nucleus - The small, central, positively charged central core of an atom. 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, which in turn determines the chemical element that a given atom represents. That is to say, all atoms of a given chemical element have the same number of protons in their nuclei. The total number of neutrons and protons is called the mass number.

Nuclide - A general term that refers to any known isotope, either stable or unstable, of any element. Whereas a single element can have isotopes, when referring to the isotopes of more than one element, the proper term is nuclide. A radionuclide is an unstable nuclide. 

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O

Occupational Dose - An individual’s ionizing radiation dose (external and internal) as a result of that individual's work assignment. Occupational dose does not include doses received as a medical patient or doses resulting from background radiation or participation as a subject in medical research programs.

Operational Decontamination - Decontamination carried out by an individual and/or a unit, restricted to specific parts of operationally essential equipment, materiel and/or working areas, in order to minimize contact and transfer hazards and to sustain operations. This may include decontamination of the individual beyond the scope of immediate decontamination, as well as decontamination of mission-essential spares and limited terrain decontamination.

Orphan Sources - Refers to sealed sources of radioactive material contained in a small volume (but not radioactively contaminated soils and bulk metals) in any one or more of the following conditions (taken from the NRC Orphan Source Initiative): 1) In an uncontrolled condition that requires removal to protect public health and safety from a radiological threat; 2) Controlled or uncontrolled, but for which a responsible party cannot be readily identified; 3) Controlled, but the material's continued security cannot be assured. If held by a licensee, the licensee has few or no options for, or is incapable of providing for, the safe disposition of the material; 4) In the possession of a person, not licensed to possess the material, who did not seek to possess the material; or 5) In the possession of a state radiological protection program for the sole purpose of mitigating a radiological threat because of one of the above conditions, and for which the state does not have a means to provide for the material's appropriate disposition.

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P

Packaging - The assembly of components necessary to ensure compliance with the packaging requirements in 10CFR part 71. It may consist of one or more receptacles, absorbent materials, spacing structures, thermal insulation, radiation shielding, and devices for cooling or absorbing mechanical shocks. The vehicle, tie-down system, and auxiliary equipment may be designated as part of the packaging.

Parent - A radionuclide that upon radioactive decay or disintegration yields a specific nuclide (the decay product or daughter). 

Particle Accelerator - Any machine capable of accelerating electrons, protons, deuterons, or other charged particles in a vacuum and of discharging the resultant particulate or other radiation into a medium at energies usually in excess of 1 MeV.  

Pathways - The routes by which people are exposed to radiation or other contaminants. The three basic pathways are inhalation, ingestion, and direct external exposure.

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.

Penetrating Radiation - Radiation that can penetrate the skin and reach internal organs and tissues. Photons (gamma rays and x-rays), neutrons, and protons are penetrating radiations. However, alpha particles and all but extremely high-energy beta particles are not considered penetrating radiation.

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. 

Personnel Monitoring - The use of portable survey meters to determine the presence or amount of radioactive contamination on an individual, or the use of a dosimeter to determine an individual's radiation dose. 

Phantom - An object constructed to duplicate the size, density, and composition of a human body. Phantoms are used in radiological protection studies.

Photoelectric Absorption - The process in which a photon interacts with an absorber atom, the photon disappears completely, and the atom ejects a photoelectron (from one of its bound shells) in place of the photon.  

Photoelectron - An electron released from an atom or molecule by means of energy supplied by radiation, especially light.  

Photomultiplier Tube (PMT) - A device for amplifying the flashes of light produced by a scintillator.

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 container (usually lead or plastic) used to ship or store radioactive materials. The thick walls protect the person handling the container from radiation. Large containers are commonly called casks. 

Pitchblende - A uranium-containing mineral that has concentrations of U₃O₈ on the order of 50-80%. It has a black, grayish or greenish color; a pitch-like, earth, dull, or glassy luster; and a specific gravity of 6-9.

Plume - The material spreading from a particular source and traveling through environmental media, such as air or ground water. For example, a plume could describe the dispersal of particles, gases, vapors, and aerosols in the atmosphere, or the movement of contamination through an aquifer (for example, dilution, mixing, or adsorption onto soil).

Plutonium (Pu) - A heavy, radioactive, man-made metallic element with atomic number 94. Its most important isotope is fissile plutonium-239 which is produced by neutron irradiation of uranium-238, followed by a two-step decay. It exists in only trace amounts in nature. 

Pocket Dosimeter - A small ionization detection instrument worn by an individual that directly measures the ionizing radiation exposure. 

Positron (ß⁺ ) - An elementary particle, an "anti-electron" with the mass of an electron but having a positive charge. It is emitted by some radionuclides and is also created in pair production by the interaction of high-energy gamma rays with matter. 

Pressurized Water Reactor - A reactor design in which water flows through the core at very high pressures. The water is not allowed to boil in the core but flows to a steam generator.

Primary Radiation - Radiation in a system that was not produced as a result of the action of another radiation on that 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. 

Protective Barriers - Barriers of radiation absorbing material, such as lead, concrete, plaster and plastic, that are used to reduce radiation exposure. 

Proton - An elementary nuclear particle located in the nucleus of an atom. The proton has a single positive electric charge. 

Public Dose - The dose received by a member of the public from exposure to radiation and to radioactive material released by a licensee, or to another source of radiation. It does not include occupational dose or doses received from background radiation, as a patient from medical practices, or from voluntary participation in medical research programs. 

Pulse Height Analysis (PHA) - The acquisition of energy-correlated data in the MCA. Each channel, defined as an energy window, is incremented by one count for each event that falls within the window, producing a spectrum which correlates the number of energy events as a function of their amplitude.  

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Q

Quality Factor - The factor by which the absorbed dose (rad or gray) must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or sievert) to the exposed tissue. It is used because some types of radiation, such as alpha particles, are more biologically damaging to live tissue than other types of radiation when the absorbed dose from both is equal. The term, quality factor, has now been replaced by "radiation weighting factor" in the latest system of recommendations for radiation protection. 

Quench - (1) To limit or stop a continuous discharge in a gas ionization detector. (2) To attenuate the light output in a scintillation medium.

Q-Value - The energy released in a nuclear reaction or in radioactive decay as calculated from the difference in total energy (including rest mass) of the products (product nuclei and ejectiles) and reactants (target nuclei and projectiles). When applied to radioactive decay, the target nucleus is the parent nuclide, the daughter nucleus is the product nuclide, and there is no projectile.

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R

Rad - The original unit developed for expressing 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 is equivalent to the absorption of 100 ergs (a small but measurable amount of energy) per gram of absorbing tissue. The rad has been replaced by the gray in the SI system of units (1 gray = 100 rad). 

Radiation - The emission or propagation of energy through matter or space by electromagnetic disturbances which display both wave-like and particle-like behavior. Though in this context the "particles" are known as photons, the term radiation has been extended to include streams of fast-moving particles. Nuclear radiation includes alpha particles, beta particles, gamma rays and free neutrons emitted from an atomic nucleus during 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 Injury - Injury that is a result of exposure to large doses of radiation.

Radiation Monitoring - The measurement of radiation levels, concentrations, surface area concentrations or quantities of radioactive material and the use of the results of these measurements to evaluate potential exposures and doses.

Radiation Safety Officer - An individual with responsibility for the overall radiation safety program at the facility.

Radiation Shielding - Attenuation of radiation by placing a shield of absorbing material between the radiation source and a person, or a device sensitive to radiation.

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. 

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 - Dose and dose rate limits, permissible concentrations, rules for handling, regulations for transportation, regulations for industrial control of radiation, and control of radioactive material established by legislative or regulatory means for the safe use and application of ionizing radiation. 

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 Weighting Factor - The factor by which the absorbed dose (rad or gray) must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or sievert) to the exposed tissue. It is used because some types of radiation, such as alpha particles, are more biologically damaging to live tissue than other types of radiation when the absorbed dose from both is equal. This replaces the term quality factor in the latest system of recommendations for radiation protection. 

Radiation Worker - An individual who uses radioactive materials under the licensees control. Individuals must be trained and have passed a radiation safety examination prior to beginning work with radioactive materials. 

Radioactive Contamination - Deposition of radioactive material in any place where it is not wanted. 

Radioactive Decay - Spontaneous emission by a nucleus of photons or particles.  The spontaneous transformation of one nuclide into another by emission of particles, absorption of an orbital electron, or by fission. It also refers to gamma-ray and conversion electron emission that only reduces the excitation energy of the nucleus.

Radioactive Equilibrium - A condition where the rate of production of a nuclide by radioactive decay equals the rate of decay of that nuclide.

Radioactive Isotope - A nuclide that is radioactive.

Radioactive Material - (1) (49CFR173.403y) Material with a specific activity of greater than 0.002 mCi/g. (2) (10CFR76) Source material, special nuclear material, or byproduct material, possessed, used, transferred, or disposed of.

Radioactive Nuclide - A nuclide capable of undergoing radioactive decay.

Radioactive Series - A succession of nuclides, each of which transforms by radioactive disintegration into another nuclide until a stable nuclide results. The first member is called the parent, the intermediate members are called decay (or daughter) products, and the final stable member is called the end product. 

Radioactive Standard - A sample of radioactive material, usually with a long half-life, in which the number and type of radioactive atoms at a definite time is known. It may be used as a radiation source for calibrating radiation measurement equipment.

Radioactive Waste - (1) Radioactive sources, devices or materials made radioactive by contamination or irradiation, by-products from nuclear power and the nuclear fuel cycle that are to be discarded or placed in long-term storage.  (2) High-level radioactive waste and other radioactive materials other than high-level radioactive waste that are received for emplacement in a geologic repository.  (3) Solid, liquid, or gaseous material that contains radionuclides regulated under the Atomic Energy Act of 1954, as amended, and which is of negligible economic value considering costs of recovery. Radioactive waste includes sources, devices or materials made radioactive by contamination or irradiation, by-products from nuclear power and the nuclear fuel cycle.

Radioactivity - The process of undergoing the transformation of an unstable nucleus by the spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable radionuclide. Often used also to express the rate at which radioactive material emits radiation. Measured in units of becquerels in the SI system of units or curies in the traditional system of units. 

Radiogenic - Health effects caused by exposure to ionizing radiation.

Radiography - The examination of the structure of materials by nondestructive methods, utilizing sealed sources of byproduct materials. Radiations can be used to produce images of an object either by measuring their transmission through or their interaction with the object. Medical x-rays and x-ray baggage inspection are examples of transmission measurements. A neutron baggage inspection system images an object by measuring the spatial distribution of capture gamma rays produced by the reaction of neutrons with nitrogen in the object. Autoradiography describes the process of imaging an object using radiations produced by the radioactive decay of nuclides in the object. The radionuclides can be the result of radionuclide tagging, contamination by some source, or they can be produced by irradiating the object with neutrons or other radiations.

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 - Related to radioactive materials or radiation. The radiological sciences focus on the measurement and effects of radiation.

Radiological dispersal device (RDD) - A device that disperses radioactive material by conventional explosive or other mechanical means, such as a spray. See also dirty bomb.

Radiological exposure device (RED) - Also called a "hidden sealed source." An RED is a terrorist device intended to expose people to significant doses of ionizing radiation without their knowledge. Constructed from partially or fully unshielded radioactive material, an RED could be hidden from sight in a public place (e.g., under a subway seat, in a food court, or in a busy hallway), exposing those who sit or pass close by. If the seal around the source were broken and the radioactive contents released from the container, the device could become a radiological dispersal device (RDD), capable of causing radiological contamination.

Radiological Health - The art and science of protecting human beings from injury by radiation, and promoting better health through beneficial applications of radiation.

Radiological Monitoring - Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in an occupied region as a safety measure for purposes of health protection.

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 - The branch of medicine dealing with the diagnostic and therapeutic applications of radiant energy, including x rays and radioisotopes. 

Radioluminescence - The luminescence produced by particles emitted during radioactive decay.

Radionuclide - A radioisotope. 

Radiosensitivity - The relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation. 

Radiotoxicity - Term referring to the potential of an isotope to cause damage to living tissue by absorption of energy from the disintegration of the radioactive material introduced into the body. 

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, carnotite, 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 and thorium. 

Rare Earths - The lanthanides including sometimes scandium and yttrium.

Reactivity - A measure of the departure of a reactor from criticality. It is defined as r = k_eff - 1, where k_eff is the effective multiplication factor. Reactivity is expressed in units of cent, dollar, and inhour.

Reactor Core - The region of a nuclear reactor containing the fissionable material.

Reactor Vessel - A large steel container that houses the reactor core, control rods, moderator and coolant, and other control systems to maintain safe operation of the reactor.

Reactor Vessel Head - The top section of a reactor pressure vessel. It is bolted in place during reactor operation and is removed to provide access to the core during maintenance and refueling.

Reference Man - A person assumed to have the anatomical and physiological characteristics of an average individual. These assumed characteristics are used in calculations assessing internal dose (also may be called "Standard Man"). 

Relative Biological Effectiveness (RBE) - The ratio of an absorbed dose of a radiation to that of a standard which will give comparable damage in a biological material.

Relative Risk - The ratio between the risk for disease in an irradiated population to the risk in an unexposed population. A relative risk of 1.1 indicates a 10% increase in cancer from radiation, compared with the "normal" incidence.

Rem (Roentgen Equivalent Man) - A unit in the traditional system of units that measures the effects of ionizing radiation on humans. 

Removable Contamination - Contamination deposited on the surface of structures, areas, objects or personnel that can readily be picked up or wiped up by physical or mechanical means during the course of a survey or during decontamination efforts. 

Research Reactor - A nuclear reactor designed for radionuclide production, materials testing, and training.

Restricted Area - An area, access to which is limited by the licensee for the purpose of protecting individuals against undue risks from exposure to radiation and radioactive materials. Restricted area does not include areas used as residential quarters, but separate rooms in a residential building may be set apart as a restricted area. 

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

Risk Assessment - An evaluation of the risk to human health or the environment by hazards. Risk assessments can look at either existing hazards or potential hazards.

Roentgen - The international unit of X radiation or gamma radiation. The amount of radiation producing, under ideal conditions in one cc ionization of either sign equal to one electrostatic unit of charge.

Roentgen Absorbed Dose - A unit of radiation dose defined as 100 erg/g or 0.01 J/kg. The SI unit of dose is the gray (Gy), where 1 Gy = 100 rad.

Roentgen Equivalent Man - A unit of absorbed dose defined as the number of rads times a quality factor. It represents a dose equivalent or a dose that is correlated with injury due to radiation exposure. The sievert is the SI unit of absorbed dose; 1 rem is equivalent to 0.01 sievert.

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S

Safeguards - (1) An integrated system of physical protection, material accounting, and material control measures designed to deter, prevent, detect, and respond to unauthorized possession, use, or sabotage of nuclear material. (2) Information not otherwise classified as National Security Information or Restricted Data which specifically identifies a licensee's or applicant's detailed, (1) security measures for the physical protection of special nuclear material, or (2) security measures for the physical protection and location of certain plant equipment vital to the safety of production or utilization facilities.

Safety Injection - The rapid insertion of a soluble neutron poison into the reactor coolant system to ensure reactor shutdown.

Safety Rod - A control rod used to decrease the reactor reactivity in the case of emergencies. Safety rods have a large reactivity and their motion is very fast.

Scaler - An instrument for counting of pulses from radiation detection equipment. Scalers produce pulses that are proportional in number to the number of input pulses.

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 - Light produced by the interaction of ionizing radiation with a fluor.

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 reactor, typically by the rapid insertion of control rods. It may occur either automatically or manually by the reactor operator. It originally stood for safety control rod axe man, a title given to personnel who were assigned to insert the emergency rod at the original Chicago pile.

Sealed Radioactive Source - A radioactive source specifically manufactured, obtained, or retained for the purpose of utilizing the emitted radiation. The sealed radioactive source consists of a known quantity of radioactive material contained within a sealed capsule, sealed between layers of non-radioactive material, or firmly fixed to a non-radioactive surface by electroplating or other means intended to prevent leakage or escape of the radioactive material.

Secondary Electron - An electron ejected from an atom, molecule or surface as a result of an interaction with a charged particle or photon.

Secondary Ionization - The process of producing ions by the interaction of a primary ion with matter. Secondary ionization occurs when an ion that is produced has or achieves sufficient energy to induce further ionization.

Secondary Radiation - Radiation produced by the interaction of a charged particle or other radiation with matter.

Self-Absorption - Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located; in particular, the absorption of radiation within a sample being assayed.

Sensitivity - Ability of an analytical method to detect small concentrations of radioactive material.

Shallow Dose Equivalent - Applies to the external exposure of the skin or an extremity and is taken as the dose equivalent at a tissue depth of 0.007 centimeter (7 mg/cm2) averaged over an area of one square centimeter. 

Shield - An absorber placed between a radioactive source and an object to reduce the intensity of radiation.

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).

Sievert (Sv) - The international system (SI) unit for dose equivalent equal to 1 Joule/kilogram. The sievert has replaced the rem. One sievert is equivalent to 100 rem. 

Site Boundary - that line beyond which the land or property is not owned, leased, or otherwise controlled by the licensee. 

Slow Neutron - A neutron with an energy below 1 eV.

Smear - A procedure in which a swab, e.g., filter paper or cotton tipped applicator, is rubbed on a surface and its radioactivity measured to determine if the surface is contaminated with loose (removable) radioactive material. 

Somatic Effects - Effects of radiation limited to the exposed individual, as distinguished from genetic effects which may 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 (SNM) - Includes plutonium, uranium-233, or uranium enriched in the isotopes uranium-233 or uranium-235. 

Specific Activity - Total radioactivity of a given nuclide per gram of a compound, element or radioactive nuclide. 

Spectrum - A distribution of radiation intensity as a function of energy or time.  

Spent Nuclear Fuel - (1) Fuel that has been withdrawn from a nuclear reactor following irradiation, has undergone at least one year's decay since being used as a source of energy in a power reactor, and has not been chemically separated into its constituent elements by reprocessing. Spent nuclear fuel includes the special nuclear material, by-product material, source material, and other radioactive materials associated with fuel assemblies.  (2) Fuel that has been permanently withdrawn from a nuclear reactor following irradiation, but has not been processed to remove its constituent elements. (Does not include material categorized as waste that has been removed from the DOE accountability system in accordance with DOE 5633.3A, 5633.4 and 5633.5.)

Spontaneous Fission - Fission that occurs spontaneously, not induced by an incident particle. A type of radioactive decay.

Spurious Count - In a radiation counting device, a count caused by any agent other than radiation.

Stable Isotope - Isotope that does not undergo radioactive decay.

Standard Deviation (s) - A measure of the dispersion about the mean value of a series of observations expressed in the same units as the mean value.

Stochastic Effects - Effects that occur by chance and which may occur 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 effect is cancer. 

Survey - An evaluation of the radiological conditions and potential hazards incident to the production, use, transfer, release, disposal or presence of radioactive material or other sources of radiation. When appropriate, such an evaluation includes a physical survey of the location of radioactive material and measurements or calculations of levels of radiation, or concentrations or quantities of radioactive material present.

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. 

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T

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. 

Thermoluminescent Dosimeter - A small device used to measure the radiation dose by measuring the amount of light emitted from a crystal in the detector when the crystal is heated after being exposed to the radiation. 

Total Effective Dose Equivalent (TEDE) - The sum of effective dose equivalent from external radiation and the committed effective dose equivalent from inhaled and ingested radioactive material. Quoted in units of rem.

Transuranic (TRU) - Possessing an atomic number higher than that of uranium (92).  

Trip, Reactor - A term that is used by pressurized water reactors for a reactor scram.

Tritium - A radioactive isotope of hydrogen. Tritium contains one proton and two neutrons in its nucleus. Because it is chemically identical to the natural hydrogen atoms present in water, tritium can easily be taken into the body by ingestion. It decays by beta emission and has a radioactive half-life 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.

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U

Ultraviolet - Electromagnetic radiation with a wavelength ranging from violet within the visible spectrum to low-energy x rays. 

Unrestricted Area - An area, access to which is neither limited nor controlled by the licensee. 

Unstable Isotope - A radioactive isotope. 

Uptake - Quantity of material taken up into the extracellular fluids. It is usually expressed as a fraction of the deposition in the organ from which uptake occurs. 

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. 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). 

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V

Valence Electron - An electron which is gained, lost or shared in a chemical reaction.

Very High Radiation Area - An area accessible to individuals, in which radiation levels could result in an individual receiving an absorbed dose in excess of 500 rads (5 grays) in one hour at one meter from a radiation source or from any surface that the radiation penetrates. 

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W

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). 

Weighting Factor (WT) - A multiplier that is used for converting the equivalent dose to a specific organ or tissue into what is called the “effective dose.” The goal of this process was to develop a method for expressing the dose to a portion of the body in terms of an equivalent dose to the whole body that would carry with it an equivalent risk in terms of the associated fatal cancer probability. It applies only to the stochastic effects of radiation. 

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. 

Whole Body - For purposes of external exposure, head, trunk (including male gonads), arms above the elbow, or legs above the knee. 

Whole-Body Counter - A device used to identify and measure the radioactive material in the body of human beings and animals. It typically uses heavy shielding to keep out background radiation from the ultra-sensitive 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. 

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X

X rays - Penetrating electromagnetic radiation having a range of wavelengths (energies) that are similar to those of gamma photons. X rays are usually produced by excitation of the electron field around certain nuclei. Although once formed, there is no difference in x rays and gamma photons; however, there is a difference in their origin. X rays are produced by shifts in the electrons between the rings outside the nucleus of an atom whereas gamma photons are produced by reactions within the nucleus of an atom.

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Y

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). 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.

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Z