| Activation |
The process of making a radioisotope by bombarding a stable element with
neutrons or protons. |
| Active fuel length |
The end-to-end dimension of fuel material within a fuel element. |
| Agreement State |
A state that has signed an agreement with the Nuclear Regulatory Commission
under which the state regulates the use of by-product, source and small
quantities of special nuclear material within that state. |
| Air sampling |
The collection of samples of air to measure the radioactivity or to detect the
presence of radioactive material, particulate matter, or chemical pollutants in
the air. |
| Airborne radioactivity area |
A room, enclosure, or area in which airborne radioactive materials, composed
wholly or partly of licensed material, exist in concentrations that: (1) Exceed
the derived air concentration limits, or (2) Would result in an individual
present in the area without respiratory protection exceeding, during the hours
the individual is present in the area, 0.6 percent of the annual limit on intake
or 12 derived air concentration-hours (see 10 CFR §20.1003 Definitions). |
| ALARA |
Acronym for "As Low As Reasonably Achievable," means making every
reasonable effort to maintain exposures to ionizing radiation as far below the
dose limits as practical, consistent with the purpose for which the licensed
activity is undertaken, taking into account the state of technology, the
economics of improvements in relation to state of technology, the economics
of improvements in relation to benefits to the public health and safety, and
other societal and socioeconomic considerations, and in relation to utilization
of nuclear energy and licensed materials in the public interest (see 10 CFR
20.1003). |
| Alpha particle |
A positively charged particle ejected spontaneously from the nuclei of some
radioactive elements. It is identical to a helium nucleus that has a mass
number of 4 and an electrostatic charge of +2. It has low penetrating power
and a short range (a few centimeters in air). The most energetic alpha particle
will generally fail to penetrate the dead layers of cells covering the skin and
can be easily stopped by a sheet of paper. Alpha particles are hazardous when
an alpha-emitting isotope is inside the body. |
| Anion |
A negatively charged ion. |
| Annual limit on intake (ALI) |
The derived limit for the amount of radioactive material taken into the body of
an adult worker by inhalation or ingestion in a year. ALI is the smaller value
of intake of a given radionuclide in a year by the reference man that would
result in a committed effective dose equivalent of 5 rems(0.05 sievert) or a
committed dose equivalent of 50 rems (0.5 sievert) to any individual organ or
tissue. (see 10 CFR 20.1003.) |
| Atom |
The smallest particle of an element that cannot be divided or broken up by
chemical means. It consists of a central core of protons and neutrons, called
the nucleus. Electrons revolve in orbits in the region surrounding the nucleus. |
| Atomic energy |
Energy released in nuclear reactions. Of particular interest is the energy
released when a neutron initiates the breaking up or fissioning of an atom's
nucleus into smaller pieces (fission), or when two nuclei are joined together
under millions of degrees of heat (fusion). It is more correctly called nuclear
energy. |
| Atomic Energy Commission |
Federal agency created in 1946 to manage the development, use, and control
of nuclear energy for military and civilian applications. Abolished by the
Energy Reorganization Act of 1974 and succeeded by the Energy Research
and Development Administration (now part of the U. S. Department of
Energy) and the U. S. Nuclear Regulatory Commission. |
| Atomic number |
The number of positively charged protons in the nucleus of an atom. |
| Attenuation |
The process by which the number of particles or photons entering a body of
matter is reduced by absorption and scattered radiation. |
| Auxiliary feedwater |
Backup water supply used during nuclear plant startup and shutdown to supply
water to the steam generators during accident conditions for removing decay
heat from the reactor. |
| Average planar linear heat generation rate (APLGHR) |
The average value of the linear heat generation rate of all the control rods at
any given horizontal plane along a fuel bundle. |
| Background radiation |
Radiation from cosmic sources; naturally occurring radioactive materials,
including radon (except as a decay product of source or special nuclear
material) and global fallout as it exists in the environment from the testing of
nuclear explosive devices. It does not include radiation from source,
byproduct, or special nuclear materials regulated by the Nuclear Regulatory
Commission. The typically quoted average individual exposure from
background radiation is 360 millirems per year. |
| Becquerel (Bq) |
The unit of radioactive decay equal to 1 disintegration per second. 37 billion
(3.7x1010) becquerels = 1 curie (Ci). |
| Beta particle |
A charged particle emitted from a nucleus during radioactive decay, with a
mass equal to 1/1837 that of a proton. A negatively charged beta particle is
identical to an electron. A positively charged beta particle is called a positron.
Large amounts of beta radiation may cause skin bums, and beta emitters are
harmful if they enter the body. Beta particles may be stopped by thin sheets of
metal or plastic. |
| Binding energy |
The minimum energy required to separate a nucleus into its component
neutrons and protons. |
| Bioassay |
The determination of kinds, quantities or concentrations, and in some cases,
the locations, of radioactive material in the human body, whether by direct
measurement (in vivo counting) or by analysis and evaluation of materials
excreted or removed (in vitro) from the human body. |
| Biological halflife |
The time required for a biological system, such as that of a human, to
eliminate, by natural processes, half of the amount of a substance (such as a
radioactive material) that has entered it. |
| Biological shield |
A mass of absorbing material placed around a reactor or radioactive source to
reduce the radiation to a level safe for humans. |
| Boiling water reactor (BWR) |
A reactor in which water, used as both coolant and moderator, is allowed to
boil in the core. The resulting steam can be used directly to drive a turbine and
electrical generator, thereby producing electricity. |
| Bone seeker |
A radioisotope that tends to accumulate in the bones when it is introduced into
the body. An example is strontium-90, which behaves chemically like
calcium. |
| Breeder |
A reactor that produces more nuclear fuel than it consumes. A fertile material,
such as uranium-238, when bombarded by neutrons, is transformed into a
fissile material, such as plutonium-239, which can be used as fuel. |
| British thermal unit (Btu) |
A British thermal unit. The amount of heat required to change the temperature
of one pound of water one degree Fahrenheit at sea level. |
| Byproduct |
Byproduct is (1) any radioactive material (except special nuclear material)
yielded in, or made radioactive by, exposure to the radiation incident to the
process of producing or using special nuclear material (as in a reactor); and (2)
the tailings or wastes produced by the extraction or concentration of uranium
or thorium from ore (see 10 CFR 20.1003). |
| Calibration |
The adjustment, as necessary, of a measuring device such that it responds
within the required range and accuracy to known values of input. |
| Capability |
The maximum load that a generating station can carry under specified
conditions for a given period of time without exceeding approved limits of
temperature and stress. |
| Capacity factor (gross) |
The ratio of the gross electricity generated, for the period of time considered,
to the energy that could have been generated at continuous full-power
operation during the same period. |
| Capacity factor (net) |
The ratio of the net electricity generated, for the period of time considered, to
the energy that could have been generated at continuous full-power operation
during the same period. |
| Cask |
A heavily shielded container used to store and/or ship radioactive materials.
Lead and steel are common materials used in the manufacture of casks. |
| Cation |
A positively charged ion. |
| Chain reaction |
A reaction that initiates its own repetition. In a fission chain reaction, a
fissionable nucleus absorbs a neutron and fissions spontaneously, releasing
additional neutrons. These, in turn, can be absorbed by other fissionable
nuclei, releasing still more neutrons. A fission chain reaction is self-sustaining
when the number of neutrons released in a given time equals or exceeds the
number of neutrons lost by absorption in nonfissionable material or by escape
from the system. |
| Charged particle |
An ion. An elementary particle carrying a positive or negative electric charge. |
| Chemical recombination |
Following an ionization event, the positively and negatively charged ion pairs
may or may not realign themselves to form the same chemical substance they
formed before ionization. Thus, chemical recombination could change the
chemical composition of the material bombarded by ionizing radiation. |
| Cladding |
The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It
prevents corrosion of the fuel by the coolant and the release of fission products
into the coolant. Aluminum, stainless steel, and zirconium alloys are common
cladding materials. |
| Cleanup system |
A system used for continuously filtering and demineralizing a reactor coolant
system to reduce contamination levels and to minimize corrosion. |
| Coastdown |
An action that permits the reactor power level to decrease gradually as the fuel
in the core is depleted. |
| Cold shutdown |
The term used to define a reactor coolant system at atmospheric pressure and
at a temperature below 200 degrees Fahrenheit following a reactor cooldown. |
| Collective dose |
The sum of the individual doses received on a given period of time by a
specified population from exposure to a specified source of radiation. |
| Committed dose equivalent |
This is the dose to some specific organ or tissue that is received from an intake
of radioactive material by an individual during the 50-year period following
the intake (see 10 CFR 20.1003). |
| Committed effective dose equivalent |
The committed dose equivalent for a given organ multiplied by a weighting
factor (see 10 CFR 20.1003). |
| Compact |
A group of two or more States formed to dispose of low-level radioactive
waste on a regional basis. Forty-two States have formed nine compacts. |
| Compound |
A chemical combination of two or more elements combined in a fixed and
definite proportion by weight. |
| Condensate |
Water that has been produced by the cooling of steam in a condenser. |
| Condenser |
A large heat exchanger designed to cool exhaust steam from a turbine below
the boiling point so that it can be returned to the heat source as water. In a
pressurized water reactor, the water is returned to the steam generator. In a
boiling water reactor, it returns to the reactor core. The heat removed from the
steam by the condenser is transferred to a circulating water system and is
exhausted to the environment, either through a cooling tower or directly into a
body of water. |
| Construction recapture |
The maximum number of years that could be added to the license expiration
date to recover the period from the construction permit to the date when the
operating license was granted. A licensee is required to submit an application
for such a change. |
| Contamination |
Undesired radioactive material that is deposited on the surface of or inside
structures, areas, objects or people. |
| Containment structure |
A gaslight shell or other enclosure around a nuclear reactor to confine fission
products that otherwise might be released to the atmosphere in the event of an
accident. |
| Control rod |
A rod, plate, or tube containing a material such as hafnium, boron, etc., used
to control the power of a nuclear reactor. By absorbing neutrons, a control rod
prevents the neutrons from causing further fissions. |
| Controlled area |
At a nuclear facility, an area outside of a restricted area but within the site
boundary, access to which can be limited by the licensee for any reason. |
| Control room |
The area in a nuclear power plant from which most of the plant power
production and emergency safety equipment can be operated by remote
control. |
| Coolant |
A substance circulated through a nuclear reactor to remove or transfer heat.
The most commonly used coolant in the United States is water. Other coolants
include heavy water, air, carbon dioxide, helium, liquid sodium, and a
sodium-potassium alloy. |
| Cooldown |
The gradual decrease in reactor fuel rod temperature caused by the removal of
heat from the reactor coolant system after the reactor has been shutdown. |
| Cooling tower |
A heat exchanger designed to aid in the cooling of water that was used to cool
exhaust steam exiting the turbines of a power plant. Cooling towers transfer
exhaust heat into the air instead of into a body of water. |
| Core |
The central portion of a nuclear reactor containing the fuel elements,
moderator, neutron poisons, and support structures. |
| Core melt accident |
An event or sequence of events that result in the melting of part of the fuel in
the reactor core. |
| Cosmic radiation |
Penetrating ionizing radiation, both particulate and electromagnetic,
originating in outer space. Secondary cosmic rays, formed by interactions in
the earth's atmosphere, account for about 45 to 50 millirem of the 360
millirem background radiation that an average individual receives in a year. |
| Counter |
A general designation applied to radiation detection instruments or survey
meters that detect and measure radiation. The signal that announces an
ionization event is called a count. |
| Critical mass |
The smallest mass of fissionable material that will support a self-sustaining
chain reaction. |
| Critical organ |
That part of the body that is most susceptible to radiation damage under the
specific conditions under consideration. |
| Criticality |
A term used in reactor physics to describe the state when the number of
neutrons released by fission is exactly balanced by the neutrons being
absorbed (by the fuel and poisons) and escaping the reactor core. A reactor is
said to be "critical" when it achieves a self-sustaining nuclear chain reaction,
as when the reactor is operating. |
| Crud |
A colloquial term for corrosion and wear products (rust particles, etc.) that
become radioactive (i.e., activated) when exposed to radiation. The term is
actually an acronym for Chalk River Unidentified Deposits, the Canadian
plant at which the activated deposits were first discovered. |
| Cumulative dose |
The total dose resulting from repeated exposures of ionizing radiation to an
occupationally exposed worker to the same portion of the body, or to the
whole body, over a period of time (see 10 CFR 20.1003). |
| Curie (Ci) |
The basic unit used to describe the intensity of radioactivity in a sample of
material. The curie is equal to 37 billion (3.7X1010) disintegrations per
second, which is approximately the activity of 1 gram of radium. A curie is
also a quantity of any radionuclide that decays at a rate of 37 billion
disintegrations per second. It is named for Marie and Pierre Curie, who
discovered radium in 1898. |
| Daughter products |
Isotopes that are formed by the radioactive decay of some other isotope. In the
case of radium-226, for example, there are 10 successive daughter products,
ending in the stable isotope, lead-206. |
| Decay heat |
The heat produced by the decay of radioactive fission products after a reactor
has been shut down. |
| Decay, radioactive |
The decrease in the amount of any radioactive material with the passage of
time due to the spontaneous emission from the atomic nuclei of either alpha or
beta particles, often accompanied by gamma radiation. |
| Declared pregnant woman |
A woman who is also an occupational radiation worker and has voluntarily
informed her employer, in writing, of her pregnancy and the estimated date of
conception (see 10 CFR 20.1003, 20.1208). |
| Decommission |
The process of closing down a facility followed by reducing residual
radioactivity to a level that permits the release of the property for unrestricted
use (see 10 CFR 20.1003). |
| Decon |
A method of decommissioning in which the equipment, structures, and
portions of a facility and site containing radioactive contaminants are removed
and safety buried in a low-level radioactive waste landfill or decontaminated
to a level that permits the property to be released for unrestricted use shortly
after cessation of operations. |
| Decontamination |
The reduction or removal of contaminating radioactive material from a
structure, area, object, or person. Decontamination may be accomplished by:
(1) treating the surface to remove or decrease the contamination, (2) letting the
material stand so that the radioactivity is decreased as a result of natural
radioactive decay, or (3) covering the contamination to shield or attenuate the
radiation emitted (see 10 CFR 20.1003 and §20.1402). |
| Defense-in-depth |
A design and operational philosophy with regard to nuclear facilities that calls
for multiple layers of protection to prevent and mitigate accidents. It includes
the use of controls, multiple physical barriers to prevent release of radiation,
redundant and diverse key safety functions, and emergency response
measures. |
| Departure from nucleate boiling (DNB) |
The point at which the heat transfer from a fuel rod rapidly decreases due to
the insulating effect of a steam blanket that forms on the rod surface when the
temperature continues to increase. |
| Departure From Nuclear Boiling Ratio (DNBR) |
The ratio of the heat flux to cause departure from nucleate boiling to the actual
local heat flux or a fuel rod. |
| Depleted uranium |
Uranium having a percentage of uranium-235 smaller than the 0.7 percent
found in natural uranium. It is obtained from spent (used) fuel elements or as
byproduct tails, or residues, from uranium isotope separation. |
| Derived air concentration (DAC) |
The concentration of radioactive material in air and the time of exposure to
that radionuclide, in hours. An NRC licensee may take 2,000 hours to
represent one ALI, equivalent to a committed effective dose equivalent of 5
rems (0.05 sievert). |
| Design-basis accident |
A postulated accident that a nuclear facility must be designed and built to
withstand without loss to the systems, structures, and components necessary to
assure public health and safety. |
| Design-basis phenomena |
Earthquakes, tornadoes, hurricanes, floods, etc., that a nuclear facility must be
designed and built to withstand without loss of systems, structures, and
components necessary to assure public health and safety. |
| Detector |
A material or device that is sensitive to radiation and can produce a response
signal suitable for measurement or analysis. A radiation detection instrument. |
| Deterministic effect |
The health effects, the severity of which varies with the dose and for which a
threshold is believed to exist. Radiation-induced cataract formation is an
example of a deterministic effect (also called a non-stochastic effect) (see 10
CFR 20.1003). |
| Deuterium |
An isotope of hydrogen with one proton and one neutron in the nucleus. |
| Deuteron |
The nucleus of deuterium. It contains one proton and one neutron. See also
heavy water. |
| Differential pressure (dp or P) |
The difference in pressure between two points of a system, such as between
the inlet and outlet of a pump. |
| Doppler coefficient |
Another name used for the fuel temperature coefficient of reactivity. |
| Dose |
The absorbed dose, given in rads (or the international system of units, grays),
that represents the energy absorbed from the radiation in a gram of any
material. Furthermore, the biological dose or dose equivalent, given in rem or
sieverts, is a measure of the biological damage to living tissue from the
radiation exposure. |
| Dose, absorbed |
The amount of energy deposited in any substance by ionizing radiation per
unit mass of the substance. It is expressed numerically in rads or grays. |
| Dose equivalent |
The product of absorbed dose in tissue multiplied by a quality factor, and then
sometimes multiplied by other necessary modifying factors at the location of
interest. It is expressed numerically in rems or sieverts (see 10 CFR 20.1003). |
| Dosimeter |
A small portable instrument (such as a film badge, thermoluminescent or
pocket dosimeter) for measuring and recording the total accumulated
personnel dose of ionizing radiation. |
| Dosimetry |
The theory and application of the principles and techniques involved in the
measurement and recording of ionizing radiation doses. |
| Dose rate |
The ionizing radiation dose delivered per unit time. For example, rem or
sieverts per hour. |
| Drywell |
The containment structure enclosing a boiling water reactor vessel and its
recirculation system. The drywell provides both a pressure suppression system
and a fission product barrier under accident conditions. |
| Earthquake, operating basis |
An earthquake that could be expected to affect the reactor plant site, but for
which the plant power production equipment is designed to remain functional
without undue risk to public health and safety. |
| Effective halflife |
The time required for the amount of a radioactive element deposited in a
living organism to be diminished 50% as a result of the combined action of
radioactive decay and biological elimination. |
| Efficiency, plant |
The percentage of the total energy content of a power plant's fuel that is
converted into electricity. The remaining energy is lost to the environment as
heat. |
| Electrical generator |
An electromagnetic device that converts mechanical (rotational) energy into
electrical energy. Most large electrical generators are driven by steam or water
turbine systems. |
| Electromagnetic radiation |
A traveling wave motion resulting from changing electric or magnetic fields.
Familiar electromagnetic radiation range from x-rays (and gamma rays) of
short wavelength, through the ultraviolet, visible, and infrared regions, to
radar and radio waves of relatively long wave length. |
| Electron |
An elementary particle with a negative charge and a mass 1/1837 that of the
proton. Electrons surround the positively charged nucleus and determine the
chemical properties of the atom. |
| Element |
One of the 103 known chemical substances that cannot be broken down
further without changing its chemical properties. Some examples include,
hydrogen, nitrogen, gold, lead, and uranium. See the periodic table of
elements. |
| Emergency classifications |
Response by an offsite organization is required to protect local citizens near
the site. A request for assistance from offsite emergency response
organizations may be required. |
| Emergency core cooling systems (ECCS) |
Reactor system components (pumps, valves, heat exchangers, tanks, and
piping) that are specifically designed to remove residual heat from the reactor
fuel rods should the normal core cooling system (reactor coolant system) fail. |
| Emergency feedwater |
Another name that may be used for auxiliary feedwater. |
| Entomb |
A method of decommissioning in which radioactive contaminants are encased
in a structurally long-lived material, such as concrete. The entombment
structure is appropriately maintained, and continued surveillance is carried out
until the radioactivity decays to a level permitting decommissioning and
ultimate unrestricted release of the property. |
| Exclusion area |
That area surrounding the reactor, in which the reactor licensee has the
authority to determine all activities, including exclusion or removal of
personnel and property from the area. |
| Excursion |
A sudden, very rapid rise in the power level of a reactor caused by
supercriticality. Excursions are usually quickly suppressed by the negative
temperature coefficient, the fuel temperature coefficient or the void coefficient
(depending upon reactor design), or by rapid insertion of control rods. |
| Exposure |
Being exposed to ionizing radiation or to radioactive material. |
| External radiation |
Exposure to ionizing radiation when the radiation source is located outside the
body. |
| Extremities |
The hands, forearms, elbows, feet, knee, leg below the knee, and ankles
(permissible radiation exposures in these regions are generally greater than in
the whole body because they contain less blood forming organs and have
smaller volumes for energy absorption) (see 10 CFR 20.1003). |
| Fast fission |
Fission of a heavy atom (such as uranium-238) when it absorbs a high energy
(fast) neutron. Most fissionable materials need thermal (slow) neutrons in
order to fission. |
| Fast neutron |
A neutron with kinetic energy greater than its surroundings when released
during fission. |
| Feedwater |
Water supplied to the reactor pressure vessel (in a BWR) or the steam
generator (in a PWR) that removes heat from the reactor fuel rods by boiling
and becoming steam. The steam becomes the driving force for the plant
turbine generator. |
| Fertile material |
A material, which is not itself fissile(fissionable by thermal neutrons), that can
be converted into a fissile material by irradiation in a reactor. There are two
basic fertile materials, uranium-238 and thorium-232. When these fertile
materials capture neutrons, they are converted into fissile plutonium-239 and
uranium-233, respectively. |
| Film badge |
Photographic film used for measurement of ionizing radiation exposure for
personnel monitoring purposes. The film badge may contain two or three films
of differing sensitivities, and it may also contain a filter that shields part of the
film from certain types of radiation. |
| Fissile material |
Although sometimes used as a synonym for fissionable material, this term has
acquired a more restricted meaning. Namely, any material fissionable by
thermal (slow) neutrons. The three primary fissile materials are uranium-233,
uranium-235, and plutonium-239. |
| Fission (fissioning) |
The splitting of a nucleus into at least two other nuclei and the release of a
relatively large amount of energy. Two or three neutrons are usually released
during this type of transformation. |
| Fission gases |
Those fission products that exist in the gaseous state. In nuclear power
reactors, this includes primarily the noble gases, such as krypton and xenon. |
| Fission products |
The nuclei (fission fragments) formed by the fission of heavy elements, plus
the nuclide formed by the fission fragments' radioactive decay. |
| Fissionable material |
Commonly used as a synonym for fissile material, the meaning of this term
has been extended to include material that can be fissioned by fast neutrons,
such as uranium-238. |
| Flux |
A term applied to the amount of some type of particle (neutrons, alpha
radiation, etc.) or energy (photons, heat, etc.) crossing a unit area per unit
time. The unit of flux is the number of particles, energy, etc., per square
centimeter per second. |
| Fuel assembly |
A cluster of fuel rods (or plates). Also called a fuel element. Many fuel
assemblies make up a reactor core. |
| Fuel cycle |
The series of steps involved in supplying fuel for nuclear power reactors. It
can include mining, milling, isotopic enrichment, fabrication of fuel elements,
use in a reactor, chemical reprocessing to recover the fissionable material
remaining in the spent fuel, reenrichment of the fuel material, refabrication
into new fuel elements, and waste disposal. |
| Fuel reprocessing |
The processing of reactor fuel to separate the unused fissionable material from
waste material. |
| Fuel rod |
A long, slender tube that holds fissionable material (fuel) for nuclear reactor
use. Fuel rods are assembled into bundles called fuel elements or fuel
assemblies, which are loaded individually into the reactor core. |
| Fuel temperature coefficient of reactivity |
The change in reactivity per degree change in the fuel temperature. The
physical property of fuel pellet material (uranium-238) that causes the uranium
to absorb more neutrons away from the fission process as fuel pellet
temperature increases. This acts to stabilize power reactor operations. This
coefficient is also known as the doppler coefficient. |
| Fusion reaction |
A reaction in which at least one heavier, more stable nucleus is produced from
two lighter, less stable nuclei. Reactions of this type are responsible for
enormous release of energy, as in the energy of stars, for example. |
| Gap |
The space inside a reactor fuel rod that exists between the fuel pellet and the
fuel rod cladding. |
| Gamma radiation |
High-energy, short wavelength, electromagnetic radiation emitted from the
nucleus. Gamma radiation frequently accompanies alpha and beta emissions
and always accompanies fission. Gamma rays are very penetrating and are best
stopped or shielded by dense materials, such as lead or depleted uranium.
Gamma rays are similar to x-rays. |
| Gas-cooled reactor |
A nuclear reactor in which a gas is the coolant. |
| Gases |
Normally, formless fluids that completely fill the space, and take the shape of,
their container. |
| Gaseous Diffusion Plant |
A facility where uranium hexafluoride gas is filtered, uranium-235 is
separated from uranium-238, increasing the percentage of uranium-235 from 1
to about 3 percent. The process requires enormous amounts of electric power. |
| Geiger-Mueller counter |
A radiation detection and measuring instrument. It consists of a gas-filled tube
containing electrodes, between which there is an electrical voltage, but no
current flowing. When ionizing radiation passes through the tube, a short,
intense pulse of current passes from the negative electrode to the positive
electrode and is measured or counted. The number of pulses per second
measures the intensity of the radiation field. It was named for Hans Geiger and
W. Mueller, who invented it in the 1920s. It is sometimes called simply a
Geiger counter or a G-M counter, and is the most commonly used portable
radiation instrument. |
| Generation (gross) |
The total amount of electric energy produced by a generating station as
measured at the generator terminals. |
| Generation (net) |
The gross amount of electric energy produced less the electric energy
consumed at a generating station for station use. |
| Generation (net) |
The gross amount of electric energy produced less the electric energy
consumed at a generating station for station use. |
| Graphite |
A form of carbon, similar to the lead used in pencils, used as a moderator in
some nuclear reactors. |
| Gigawatt |
One billion (109) watts. |
| Gigawatthour |
One billion (109) watt-hours. |
| Gray (Gy) |
The new international system (SI) unit of radiation dose expressed in terms of
absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose
and replaces the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad. |
| Halflife |
The time in which one half of the atoms of a particular radioactive substance
disintegrates into another nuclear form. Measured halflives vary from
millionths of a second to billions of years. Also called physical or radiological
halflife |
| Halflife, biological |
The time required for the body to eliminate one half of the material taken in by
natural biological means. |
| Halflife, effective |
The time required for a radionuclide contained in a biological system, such as
a human or an animal, to reduce its activity by one-half as a combined result
of radioactive decay and biological elimination. |
| Half-thickness |
The thickness of any given absorber that will reduce the intensity of its
original beam of ionizing radiation to one-half of its initial value. |
| Head, reactor vessel |
The removable top section of a reactor pressure vessel. It is bolted in place
during power operation and removed during refueling to permit access of fuel
handling equipment to the core. |
| Health physics |
The science concerned with the recognition, evaluation, and control of health
hazards which may arise from the use and application of ionizing radiation. |
| Heat exchanger |
Any device that transfers heat from one fluid (liquid or gas) to another fluid or
to the environment. |
| Heat sink |
Anything that absorbs heat. It is usually part of the environment, such as the
air, a river, or a lake. |
| Heatup |
The rise in temperature of the reactor fuel rods resulting from an increase in
the rate of fission in the core. |
| Heavy water (D20) |
Water containing significantly more than the natural proportions (one in
6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms.
Heavy water is used as a moderator in some reactors because it slows down
neutrons effectively and also has a low probability of absorption of neutrons. |
| Heavy water moderated reactor |
A reactor that uses heavy water as its moderator. Heavy water is an excellent
moderator and thus permits the use of unenriched uranium as a fuel. |
| High-enriched uranium |
Uranium enriched to 20 percent or greater in the isotope uranium-235. |
| High-level waste |
High-level radioactive waste (HLW) means (1) irradiated (spent) reactor fuel;
(2) liquid waste resulting from the operation of the first cycle solvent
extraction system, and the concentrated wastes from subsequent extraction
cycles, in a facility for reprocessing irradiated reactor fuel; and (3) solids into
which such liquid wastes have been converted. HLW is primarily in the form
of spent fuel discharged from commercial nuclear power reactors. It also
includes some reprocessed HLW from defense activities, and a small quantity
of reprocessed commercial HLW (see 10 CFR Part 60). |
| High Radiation Area |
Any area with dose rates greater than 100 millirems (1 millisievert) in one
hour 30 centimeters from the source or from any surface through which the
ionizing radiation penetrates. Areas at licensee facilities must be posted as
"high radiation areas" and access into these areas is maintained under strict
control. |
| Hot |
A colloquial term meaning highly radioactive. |
| Hot spot |
The region in a radiation/contamination area in which the level of
radiation/contamination is significantly greater than in neighboring regions in
the area. |
| Kilo- |
A prefix that multiplies a basic unit by 1,000 or 103. |
| Kilovolt |
The unit of electrical potential equal to 1,000 volts. |
| Kinetic energy |
The energy that a body possesses by virtue of its mass and velocity. Also
called the energy of motion. |
| Lethal dose |
(Lethal dose 50/30) The dose of radiation expected to cause death to an
exposed population within 30 days to 50 percent (LD 50/30) of those exposed.
Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5 sieverts)
received over a very short period of time. |
| Light water |
Ordinary water (H20) as distinguished from heavy water (D20). |
| Light water reactor |
A term used to describe reactors using ordinary water as coolant, including
boiling water reactors (BWRs) and pressurized water reactors (PWRs), the
most common types used in the United States. |
| Limiting condition for operation |
The section of Technical Specifications that identifies the lowest functional
capability or performance level of equipment required for safe operation of the
facility. |
| Limiting safety system settings |
Settings for automatic protective devices related to those variables having
significant safety functions. Where a limiting safety system setting is specified
for a variable on which a safety limit has been placed, the setting will assure
that automatic protective action will correct the abnormal situation before a
safety limit is exceeded. |
| Linear heat generation rate |
The heat generation rate per unit length of fuel rod, commonly expressed in
kilowatts per foot of fuel rod (kw/ft). |
| Loop |
In a pressurized water reactor, the coolant flow path through piping from the
reactor pressure vessel to the steam generator, to the reactor coolant pump,
and back to the reactor pressure vessel. Large PWRs may have as many as
four separate loops. |
| Loss of coolant accident (LOCA) |
Those postulated accidents that result in a loss of reactor coolant at a rate in
excess of the capability of the reactor makeup system from breaks in the
reactor coolant pressure boundary, up to and including a break equivalent in
size to the double-ended rupture of the largest pipe of the reactor coolant
system. |
| Low-level waste |
Low-level radioactive waste (LLW) is a general term for a wide range of
wastes. Industries, hospitals and medical, educational, or research institutions;
private or government laboratories; and nuclear fuel cycle facilities (e.g.,
nuclear power reactors and fuel fabrication plants) using radioactive materials
generate low-level wastes as part of their normal operations. These wastes are
generated in many physical and chemical forms and levels of contamination
(see 10 CFR Part 61). |
| Low population zone (LPZ) |
An area of low population density often required around a nuclear installation
before being built. The number and density of residents is of concern in
emergency planning so that certain protective measures (such as notification
and instructions to residents) can be accomplished in a timely manner. |
| Mass-energy equation |
The equation developed by Albert Einstein, which is usually given as E =
mc2, showing that, when the energy of a body changes by an amount E (no
matter what form the energy takes), the mass, m, of the body will change by
an amount equal to E/c2. The factor c2, the square of the speed of light in a
vacuum (3x108 meters/second), may be regarded as the conversion factor
relating units of mass and energy. The equation predicted the possibility of
releasing enormous amounts of energy by the conversion of mass to energy. It
is also called the Einstein Equation. |
| Mass number |
The number of nucleons (neutrons and protons) in the nucleus of an atom.
Also known as the atomic weight of an atom. |
| Maximum dependable capacity (gross) |
Dependable main-unit gross generating capacity, winter or summer, whichever
is smaller. The dependable capacity varies because the unit efficiency varies
during the year due to temperature variations in cooling water. It is the gross
electrical output as measured at the output terminals of the turbine generator
during the most restrictive seasonal conditions (usually summer). |
| Maximum dependable capacity (net) |
Gross maximum dependable generating capacity less the normal station
service loads. |
| Mega- |
A prefix that multiplies a basic unit by 1,000,000 or 106. |
| Megacurie |
One million (106) curies. |
| Megawatt (MW) |
One million (106) watts. |
| Megawatt hour (MWh) |
One million (106) watt-hours. |
| Metric ton |
Approximately 2,200 pounds in the English system of measurements (Note: in
the international system of measurements, 1 metric ton = 1000 kg.) |
| Micro- |
A prefix that divides a basic unit into one million parts (10-6). |
| Microcurie |
One millionth (10-6) of a curie. |
| Mill tailings |
Naturally radioactive residue from the processing of uranium ore into
yellowcake in a mill. Although the milling process recovers about 93 percent
of the uranium, the residues, or tailings, contain several naturally-occurring
radioactive elements, including uranium, thorium, radium, polonium, and
radon. |
| Milli- |
A prefix that divides a basic unit by 1,000. |
| Millirem |
One thousandth of a rem. (1 mrem = 10-3 rem) |
| Milliroentgen |
One thousandth of a roentgen, R. (1 mR = 10-3R) |
| Mixed oxide fuel |
A mixture of uranium oxide and plutonium oxide used to fuel a reactor. Often
it is abbreviated as "MOX." Conventional fuel is made of pure uranium oxide. |
| Moderator |
A material, such as ordinary water, heavy water, or graphite, that is used in a
reactor to slow down high-velocity neutrons, thus increasing the likelihood of
fission. |
| Moderator temperature coefficient of reactivity |
The change in reactivity per degree change in moderator temperature due to
the property of the reactor moderator to slow down fewer neutrons as its
temperature increases. This acts to stabilize power reactor operations. |
| Molecule |
A group of atoms held together by chemical forces. A molecule is the smallest
unit of a compound that can exist by itself and retain all of its chemical
properties. |
| Monitoring |
Periodic or continuous determination of the amount of ionizing radiation or
radioactive contamination present in an occupied region, as a safety measure,
for the purpose of health protection. |
| Nano- |
A prefix that divides a basic unit by one billion (10-9). |
| Nanocurie |
One billionth (10-9) of a curie. |
| Natural circulation |
The circulation of the coolant in the reactor coolant system without the use of
the reactor coolant pumps. The circulation is due to the natural convection
resulting from the different densities of relative cold and heated portions of the
system. |
| Natural uranium |
Uranium as found in nature. It contains 0.7 percent uranium-235, 99.3 percent
uranium-238, and a trace of uranium-234. |
| Net summer capability |
The steady hourly output that generating equipment is expected to supply to
system load exclusive of auxiliary power, as demonstrated by tests at the time
of summer peak demand. |
| Neutron |
An uncharged elementary particle with a mass slightly greater than that of the
proton, and found in the nucleus of every atom heavier than hydrogen. |
| Neutron capture |
The process in which an atomic nucleus absorbs or captures a neutron. |
| Neutron chain reaction |
A process in which some of the neutrons released in one fission event cause
other fissions to occur. There are three types of chain reactions: (1)
Nonsustaining chain reaction--An average of less than one fission is produced
by the neutrons released by each previous fission (reactor subcriticality). (2)
Sustaining chain reaction--An average of exactly one fission is produced by
the neutrons released by each previous fission (reactor criticality). (3)
Multiplying chain reaction--An average of more than one fission is produced
by the neutrons released by precious fission (reactor supercriticality). |
| Neutron flux |
| Neutron generation |
The release, thermalization, and absorption of fission neutrons by a fissile
material and the fission of that material producing a second generation of
neutrons. In a typical nuclear power reactor system, there are about 40,000
generations of neutrons every second. |
| Neutron leakage |
Neutrons that escape from the vicinity of the fissionable material in a reactor
core. Neutrons that leak out of the fuel region are no longer available to cause
fission and must be absorbed by shielding placed around the reactor pressure
vessel for that purpose. |
| Neutron source |
A radioactive material (decays by neutron emission) that can be inserted into a
reactor to ensure that a sufficient quantity of neutrons are available to register
on neutron detection equipment for power level indication. |
| Neutron, thermal |
A neutron that has (by collision with other particles) reached an energy state
equal to that of its surroundings, typically on the order of 0.025 eV (electron
volts). |
| Noble gas |
A gaseous chemical element that does not readily enter into chemical
combination with other elements. An inert gas. Examples are helium, argon,
krypton, xenon and radon. |
| Nonpower reactor |
Reactors used for research, training, and test purposes, and for the production
of radioisotopes for medical and industrial uses. |
| Non-stochastic effect |
The health effects, the severity of which vary with the dose and for which a
threshold is believed to exist. Radiation-induced cataract formation is an
example of a deterministic effect (also called a deterministic effect) (see 10
CFR 20.1003). |
| Non-vital plant systems |
Systems at a nuclear facility that may or may not be necessary for the
operation of the facility (i.e., power production), but that would have little or
no effect on public health and safety should they fail. These systems are not
safety related. |
| Nozzle |
As used in PWRs and BWRs, the interface for fluid (inlet and outlet) between
reactor plant components (pressure vessel, coolant pumps, steam generators,
etc.) and their associated piping systems. |
| Nuclear energy |
The energy liberated by a nuclear reaction (fission or fusion) or by radioactive
decay. |
| Nuclear force |
A powerful short-ranged attractive force that holds together the particles
inside an atomic nucleus. |
| Nuclear power plant |
An electrical generating facility using a nuclear reactor as its power (heat)
source. |
| Nuclear steam supply system |
The reactor and the reactor coolant pumps (and steam generators for a
pressurized water reactor) and associated piping in a nuclear power plant used
to generate the steam needed to drive the turbine generator unit. |
| Nuclear waste |
see High-level waste and Low-level waste. |
| Nucleon |
Common name for a constituent particle of the atomic nucleus. At present,
applied to protons and neutrons, but may include any other particles found to
exist in the nucleus. |
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| Nucleus |
The small, central, positively charged region of an atom that carries the atom's
nuclei. Except for the nucleus of ordinary (light) hydrogen, which has a single
proton, all atomic nuclei contain both protons and neutrons. The number of
protons determines the total positive charge, or atomic number. This is the
same for all the atomic nuclei of a given chemical element. The total number
of neutrons and protons is called the mass number. |
| Nuclide |
A general term referring to all known isotopes, both stable (279) and unstable
(about 5,000), of the chemical elements. |
| Operable |
A system, subsystem, train, component, or device is operable or has
operability when it is capable of performing its specified functions, and when
all necessary attendant instrumentation, controls, electrical power, cooling or
seal water, lubrication or other auxiliary equipment that are required for the
system, subsystem, train, component or device to perform its functions are
also capable of performing their related support functions. |
| Operational mode |
An operational mode corresponds to any one inclusive combination of core
reactivity condition, power level, and average reactor coolant temperature. |
| Parent |
A radionuclide that upon radioactive decay or disintegration yields a specific
nuclide (the daughter). |
| Parts per million (ppm) |
Parts (molecules) of a substance contained in a million parts of another
substance (or water). |
| Pellet, fuel |
As used in pressurized water reactors and boiling water reactors, a pellet is a
small cylinder approximately 3/8-inch in diameter and 5/8-inch in length,
consisting of uranium fuel in a ceramic form--uranium dioxide, UO2. Typical
fuel pellet enrichments in nuclear power reactors range from 2.0 percent to 3.5
percent uranium-235. |
| Performance-based Regulation |
Required results or outcome of performance rather than a prescriptive process,
technique, or procedure. |
| Periodic Table |
An arrangement of chemical elements in order of increasing atomic number.
Elements of similar properties are placed one under the other, yielding groups
or families of elements. Within each group, there is a variation of chemical
and physical properties, but in general, there is a similarity of chemical
behavior within each group. See an online periodic table. |
| Personnel monitoring |
The use of portable survey meters to determine the amount of radioactive
contamination on an individual, or the use of dosimetry to determine an
individual's occupational radiation dose. |
| Photon |
A quantum (or packet) of energy emitted in the form of electromagnetic
radiation. Gamma rays and x-rays are examples of photons. |
| Pico- |
A prefix that divides a basic unit by one trillion (10-12). |
| Picocurie |
One trillionth (10-12) of a curie. |
| Pig |
A colloquial term describing a container (usually lead or depleted uranium)
used to ship or store radioactive materials. The thick walls of this shielding
device protect the person handling the container from radiation. Large
containers used for spent fuel storage are commonly called casks. |
| Pile |
A colloquial term describing the first nuclear reactors. It is called a pile
because the earliest reactors were "piles" of graphite and uranium blocks. |
| Planned special exposure |
An infrequent exposure to radiation, separate from and in addition to, the
annual dose limits (see 10 CFR 20.1003 and §20.1206). |
| Plutonium (Pu) |
A heavy, radioactive, manmade metallic element with atomic number 94. Its
most important isotope is fissile plutonium-239, which is produced by neutron
irradiation of uranium-238. It exists in only trace amounts in nature. |
| Pocket dosimeter |
A small ionization detection instrument that indicates ionizing radiation
exposure directly. An auxiliary charging device is usually necessary. |
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| Poison, neutron |
In reactor physics, a material other than fissionable material, in the vicinity of
the reactor core that will absorb neutrons. The addition of poisons, such as
control rods or boron, into the reactor is said to be an addition of negative
reactivity. |
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| Pool reactor |
A reactor in which the fuel elements are suspended in a pool of water that
serves as the reflector, moderator, and coolant. Popularly called a "swimming
pool reactor," it is used for research and training, not for electrical generation. |
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| Positron |
Particle equal in mass, but opposite in charge, to the electron (a positive
electron). |
| Power coefficient of reactivity |
The change in reactivity per percent change in power. The power coefficient is
the summation of the moderator temperature coefficient of reactivity, the fuel
temperature coefficient of reactivity, and the void coefficient of reactivity. |
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| Power defect |
The total amount of reactivity added due to a given change in power. It can
also be expressed as the integrated power coefficient over the range of the
power change. |
| Power reactor |
A reactor designed to produce heat for electric generation, as distinguished
from reactors used for research, for producing radiation or fissionable
materials, or for reactor component testing. |
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| Pressure vessel |
A strong-walled container housing the core of most types of power reactors. It
usually also contains the moderator, neutron reflector, thermal shield, and
control rods. |
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| Pressurized water reactor (PWR) |
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A power reactor in which heat is transferred from the core to an exchanger by
high temperature water kept under high pressure in the primary system. Steam
is generated in a secondary circuit. Many reactors producing electric power are
pressurized water reactors. |
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| Pressurizer |
A tank or vessel that acts as a head tank (or surge volume) to control the
pressure in a pressurized water reactor. |
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| Primary system |
A term that may be used for referring to the reactor coolant system. |
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| 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. |
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| Proton |
An elementary nuclear particle with a positive electric charge located in the
nucleus of an atom. |
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| Quality factor |
The factor by which the absorbed dose (rad or gray) is to be multiplied to
obtain a quantity that expresses, on a common scale for all ionizing radiation,
the biological damage (rem or sievert) to an exposed individual. It is used
because some types of radiation, such as alpha particles, are more biologically
damaging internally than other types. |
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| Quantum theory |
The concept that energy is radiated intermittently in units of definite
magnitude, called quanta, and absorbed in a like manner. |
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| Rad |
The special unit for radiation absorbed dose, which is the amount of energy
from any type of ionizing radiation (e.g., alpha, beta, gamma, neutrons, etc.)
deposited in any medium (e.g., water, tissue, air). A dose of one rad means the
absorption of 100 ergs (a small but measurable amount of energy) per gram of
absorbing tissue (100 rad = 1 gray). |
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| Radiation, nuclear |
Particles (alpha, beta, neutrons) or photons(gamma) emitted from the nucleus
of unstable radioactive atoms as a result of radioactive decay. |
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| 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. |
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| Radiation detection instrument |
A device that detects and displays the characteristics of ionizing radiation. |
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| Radiation shielding |
Reduction of radiation by interposing a shield of absorbing material between
any radioactive source and a person, work area, or radiation-sensitive device. |
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| Radiation sickness (syndrome) |
The complex of symptoms characterizing the disease known as radiation
injury, resulting from excessive exposure (greater than 200 rads or 2 gray) of
the whole body (or large part) to ionizing radiation. The earliest of these
symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed
by loss of hair (epilation), hemorrhage, inflammation of the mouth and throat,
and general loss of energy. In severe cases, where the radiation exposure has
been approximately 1,000 rad (10 gray) or more, death may occur within two
to four weeks. Those who survive 6 weeks after the receipt of a single large
dose of radiation to the whole body may generally be expected to recover. |
| Radiation source |
Usually a sealed source of radiation used in teletherapy and industrial
radiography, as a power source for batteries (as in use in space craft), or in
various types of industrial gauges. Machines, such as accelerators and
radioisotope generators, and natural radionuclides may be considered sources. |
| Radiation standards |
Exposure standards, permissible concentrations, rules for safe handling,
regulations for transportation, regulations for industrial control of radiation,
and control of radioactive material by legislative means. |
| Radiation warning symbol |
An officially prescribed symbol (a magenta or black trefoil) on a yellow
background that must be displayed where certain quantities of radioactive
materials are present or where certain doses of radiation could be received. |
| Radioactive contamination |
Deposition of radioactive material in any place where it may harm persons or
equipment. |
| Radioactive series |
A succession of nuclides, each of which transforms by radioactive
disintegration into the next until a stable nuclide results. The first member is
called the parent, the intermediate members are called daughters, and the final
stable member is called the end product. |
| Radioactivity |
The spontaneous emission of radiation, generally alpha or beta particles, often
accompanied by gamma rays, from the nucleus of an unstable isotope. Also,
the rate at which radioactive material emits radiation. Measured in units of
becquerels or disintegrations per second. |
| Radiography |
The making of a shadow image on photographic film by the action of ionizing
radiation. |
| Radioisotope |
An unstable isotope of an element that decays or disintegrates spontaneously,
emitting radiation. Approximately 5,000 natural and artificial radioisotopes
have been identified. |
| Radiological sabotage |
Any deliberate act directed against a nuclear facility licensed by the NRC or
against a component of such a facility which could endanger the public health
and safety by exposure to radiation. |
| Radiological survey |
The evaluation of the radiation hazards accompanying the production, use, or
existence of radioactive materials under a specific set of conditions. Such
evaluation customarily includes a physical survey of the disposition of
materials and equipment, measurements or estimates of the levels of radiation
that may be involved, and a sufficient knowledge of processes affecting these
materials to predict hazards resulting from expected or possible changes in
materials or equipment. |
| Radiology |
That branch of medicine dealing with the diagnostic and therapeutic
applications of radiant energy, including x-rays and radioisotopes. |
| Radionuclide |
A radioisotope. |
| Radiosensitivity |
The relative susceptibility of cells, tissues, organs, organisms, or other
substances to the injurious action of radiation. |
| Radium (Ra) |
A radioactive metallic element with atomic number 88. As found in nature,
the most common isotope has a mass number of 226. It occurs in minute
quantities associated with uranium in pitchblende, camotite, and other
minerals. |
| Radon (Rn) |
A radioactive element that is one of the heaviest gases known. Its atomic
number is 86. It is a daughter of radium. |
| Reaction |
Any process involving a chemical or nuclear change. |
| Reactivity |
A term expressing the departure of a reactor system from criticality. A positive
reactivity addition indicates a move toward supercriticality (power increase).
A negative reactivity addition indicates a move toward subcriticality (power
decrease). |
| Reactor coolant system |
The system used to remove energy from the reactor core and transfer that
energy either directly or indirectly to the steam turbine. |
| Reactor, nuclear |
A device in which nuclear fission may be sustained and controlled in a
self-supporting nuclear reaction. The varieties are many, but all incorporate
certain features, including fissionable material or fuel, a moderating material
(unless the reactor is operated on fast neutrons), a reflector to conserve
escaping neutrons, provisions of removal of heat, measuring and controlling
instruments, and protective devices. The reactor is the heart of a nuclear power
plant. |
| Recycling |
| Reference man |
A person with the anatomical and physiological characteristics of an average
individual which is used in calculations assessing internal dose (also may be
called "Standard Man"). |
| Reflector |
A layer of material immediately surrounding a reactor core that scatters back
(or reflects) into the core many neutrons that would otherwise escape. The
returned neutrons can then cause more fissions and improve the neutron
economy of the reactor. Common reflector materials are graphite, beryllium,
water, and natural uranium. |
| REM |
(Roentgen Equivalent Man) - a standard unit that measures the effects of
ionizing radiation on humans. |
| Restricted area |
Any area to which access is controlled for the protection of individuals from
exposure to radiation and radioactive materials. |
| Risk-informed regulation |
Incorporating an assessment of safety significance or relative risk in NRC
regulatory actions. Making sure that the regulatory burden imposed by
individual regulations or processes is commensurate with the importance of
that regulation or process to protecting public health and safety and the
environment. |
| Roentgen (R) |
A unit of exposure to ionizing radiation. It is the amount of gamma or x-rays
required to produce ions resulting in a charge of 0.000258 coulombs/kilogram
of air under standard conditions. Named after Wilhelm Roentgen, the German
scientist who discovered x-rays in 1895. |
| Rubblization |
A decommissioning technique involving demolition and burial of formerly
operating nuclear facilities. All equipment from buildings is removed and the
surfaces are decontaminated. Above-grade structures are demolished into
rubble and buried in the structure's foundation below ground. The site surface
is then covered, regraded and landscaped for unrestricted use. |
| Safeguards |
The protection of special nuclear material(SNM) to prevent its theft, loss, or
sabotage. |
| Safe shutdown earthquake |
A design-basis earthquake. |
| Safety injection |
The rapid insertion of a chemically soluble neutron poison (such as boric acid)
into the reactor coolant system to ensure reactor shutdown. |
| Safety limit |
A limit placed upon important process variables which are found to be
necessary to reasonably protect the integrity of the physical barriers which
guard against the uncontrolled release of radioactivity. |
| Safety related |
The managerial controls, administrative documents, operating procedures,
systems, structures, and components that have been designed to mitigate the
consequences of postulated accidents that could cause undue risk to public
health and safety. |
| SAFSTOR |
A method of decommissioning in which the nuclear facility is placed and
maintained in such condition that the nuclear facility can be safely stored and
subsequently decontaminated to levels that permit release for unrestricted use. |
| Scattered radiation |
Radiation that, during its passage through a substance, has been changed in
direction. It may also have been modified by a decrease in energy. It is one
form of secondary radiation. |
| Scintillation detector |
The combination of phosphor, photomultiplier tube and associated electronic
circuits for counting light emissions produced in the phosphor by ionizing
radiation. |
| Scram |
The term used to mean the sudden shutting down of a nuclear reactor, usually
by rapid insertion of control rods, either automatically or manually by the
reactor operator. May also be called a reactor trip. It is actually an acronym for
"safety control rod axe man," the man assigned to insert the emergency rod on
the first reactor (the Chicago pile) in the U.S. |
| Sealed source |
Any special nuclear material or byproduct encased in a capsule designed to
prevent leakage or escape of the material. |
| Secondary radiation |
Radiation originating as the result of absorption of other radiation in matter. It
may be either electromagnetic or particulate in nature. |
| Secondary system |
The steam generator tubes, steam turbine, condenser, and associated pipes,
pumps, and heaters used to convert the heat energy of the reactor coolant
system into mechanical energy for electrical generation. Most commonly used
in reference to pressurized water reactors. |
| Seismic category I |
A term used to define structures, systems, and components that are designed
and built to withstand the maximum potential earthquake stresses for the
particular region where a nuclear plant is sited. |
| Shielding |
Any material or obstruction that absorbs radiation and thus tends to protect
personnel or materials from the effects of ionizing radiation. |
| Shutdown |
A decrease in the rate of fission (and heat production) in a reactor (usually by
the insertion of control rods into the core). |
| Shutdown margin |
The instantaneous amount of reactivity by which the reactor is subcritical or
would be subcritical from its present condition assuming all full-length rod
cluster assemblies (shutdown and control) are fully inserted except for the
single rod cluster assembly of highest reactivity worth which is assumed to be
fully withdrawn. |
| Sievert (Sv) |
The new international system (SI) unit for dose equivalent equal to 1
Joule/kilogram. 1 sievert = 100 rem. |
| Somatic effects of radiation |
Effects of radiation limited to the exposed individual, as distinguished from
genetic effects, which may also affect subsequent unexposed generations. |
| Source material |
Uranium or thorium, or any combination thereof, in any physical or chemical
form or ores which contain by weight one-twentieth of one percent (0.05%) or
more of: (1) uranium, (2) thorium or (3) any combination thereof. Source
material does not include special nuclear material. |
| Special nuclear material |
Includes plutonium, uranium-233, or uranium enriched in the isotopes
uranium-233 or uranium-235. |
| Spent (depleted) fuel |
Nuclear reactor fuel that has been used to the extent that it can no longer
effectively sustain a chain reaction. |
| Spent fuel pool |
An underwater storage and cooling facility for fuel elements that have been
removed from a reactor. |
| Spent nuclear fuel |
Fuel that has been removed from a nuclear reactor because it can no longer
sustain power production for economic or other reasons. |
| Stable isotope |
An isotope that does not undergo radioactive decay. |
| Standard Technical Specifications |
NRC staff guidance on model technical specifications for an operating license.
(See also Technical Specifications.) |
| Startup |
An increase in the rate of fission (and heat production) in a reactor (usually by
the removal of control rods from the core). |
| Stay time |
The period during which personnel may remain in a restricted area in a reactor
before accumulating some permissible occupational dose. |
| Steam generator |
The heat exchanger used in some reactor designs to transfer heat from the
primary (reactor coolant) system to the secondary (steam) system. This design
permits heat exchange with little or no contamination of the secondary system
equipment. |
| Stochastic effects |
Effects that occur by chance, generally occurring without a threshold level of
dose, whose probability is proportional to the dose and whose severity is
independent of the dose. In the context of radiation protection, the main
stochastic effects are cancer and genetic effects. |
| Subcriticality |
The condition of a nuclear reactor system when the rate of production of
fission neutrons is lower than the rate of production in the previous generation
due to increased neutron leakage and poisons. |
| Supercritical reactor |
A reactor in which the power level is increasing with time. |
| Supercriticality |
The condition for increasing the level of operation of a reactor. The rate of
fission neutron production exceeds all neutron losses, and the overall neutron
population increases. |
| Supercritical reactor |
A reactor in which the power level is increasing with time. |
| Superheating |
The heating of a vapor, particularly steam, to a temperature much higher than
the boiling point at the existing pressure. This is done in some power plants to
improve efficiency and to reduce water damage to the turbine. |
| Survey meter |
Any portable radiation detection instrument especially adapted for inspecting
an area or individual to establish the existence and amount of radioactive
material present. |
| Technical Specifications |
Part of an NRC license authorizing the operation of a nuclear production or
utilization facility. A Technical Specification establishes requirements for
items such as safety limits, limiting safety system settings, limiting control
settings, limiting conditions for operation, surveillance requirements, design
features, and administrative controls. (See also Standard Technical
Specifications.) |
| Terrestrial radiation |
The portion of the natural background radiation that is emitted by naturally
occurring radioactive materials, such as uranium, thorium, and radon in the
earth. |
| Thermal breeder reactor |
A breeder reactor in which the fission chain reaction is sustained by thermal
neutrons. |
| Thermalization |
The process undergone by high-energy (fast) neutrons as they lose energy by
collision. |
| Thermal power |
The total core heat transfer rate to the reactor coolant. |
| Thermal reactor |
A reactor in which the fission chain reaction is sustained primarily by thermal
neutrons. Most current reactors are thermal reactors. |
| Thermal shield |
A layer, or layers, of high-density material located within a reactor pressure
vessel or between the vessel and the biological shield to reduce radiation
heating in the vessel and the biological shield. |
| Thermoluminescent dosimeter |
A small device used to measure radiation by measuring the amount of visible
light emitted from a crystal in the detector when exposed to ionizing radiation. |
| Thermonuclear |
An adjective referring to the process in which very high temperatures are used
to bring about the fusion of light nuclei, such as those of the hydrogen
isotopes deuterium and tritium, with the accompanying liberation of energy. |
| Transient |
A change in the reactor coolant system temperature and/or pressure due to a
change in power output of the reactor. Transients can be caused by adding or
removing neutron poisons, by increasing or decreasing electrical load on the
turbine generator, or by accident conditions. |
| Trip, reactor |
A term that is used by pressurized water reactors for a reactor scram. |
| Tritium |
A radioactive isotope of hydrogen (one proton, two neutrons). Because it is
chemically identical to natural hydrogen, tritium can easily be taken into the
body by any ingestion path. It decays by beta emission. It has a radioactive
halflife of about 12.5 years. |
| Turbine |
A rotary engine made with a series of curved vanes on a rotating shaft, usually
turned by water or steam. Turbines are considered the most economical means
to turn large electrical generators. |
| Turbine generator (TG) |
A steam (or water) turbine directly coupled to an electrical generator. The two
devices are often referred to as one unit. |
| Ultraviolet |
Electromagnetic radiation of a wavelength between the shortest visible violet
and low energy x-rays. |
| Unrestricted area |
The area outside the owner-controlled portion of a nuclear facility (usually the
site boundary). An area in which a person could not be exposed to radiation
levels in excess of 2 millirems in any one hour from external sources (see 10
CFR 20.1003). |
| Unstable isotope |
A radioactive isotope. |
| Uranium |
A radioactive element with the atomic number 92 and, as found in natural
ores, an atomic weight of approximately 238. The two principal natural
isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile,
and uranium-238 (99.3 percent of natural uranium), which is fissionable by
fast neutrons and is fertile. Natural uranium also includes a minute amount of
uranium-234. |
| Uranium fuel fabrication facility |
A facility that (1) manufactures reactor fuel containing uranium for any of the
following (i) preparation of fuel materials; (ii) formation of fuel materials into
desired shapes; (iii) application of protective cladding; (iv) recovery of scrap
material; and (v) storage associated with such operations; or (2) conducts
research and development activities. |
| Uranium hexafluoride production facility |
A facility that receives natural uranium in the form of ore concentrate;
enriches it, either by gaseous diffusion or gas centrifuge methods; and
converts it into uranium hexafluoride (UF6). |
| Vapor |
The gaseous form of substances that are normally in liquid or solid form. |
| Void |
An area of lower density in a moderating system (such as steam bubbles in
water) that allows more neutron leakage than does the more dense material
around it. |
| Very high radiation area |
An area in which radiation levels exceed 500 rad (5 gray) in one hour at 1
meter from the source or from any surface that the radiation penetrates (see 10
CFR 20.1003). |
| Viability assessment |
A Department of Energy decision making process to judge the prospects for
geologic disposal of high-level radioactive wastes at Yucca Mountain based
on (1) specific design work on the critical elements of the repository and waste
package, (2) a total system performance assessment that will describe the
probable behavior of the repository, (3) a plan and cost estimate for the work
required to complete a license application, and (4) an estimate of the costs to
construct and operate the repository (see 10 CFR Part 60). |
| Void coefficient of reactivity |
The change in reactivity per percent change in void content due to an increase
in the neutron leakage as the density of the moderator decreases with an
increasing void content. |
| Waste, radioactive |
Solid, liquid, and gaseous materials from nuclear operations that are
radioactive or become radioactive and for which there is no further use.
Wastes are generally classified as high-level (having radioactivity
concentrations of hundreds of thousands of curies per gallon or foot),
low-level (in the range of 1 microcurie per gallon or foot), or intermediate
level (between these extremes) (see 10 CFR Parts 60 and 61). |
| Watt |
An electrical unit of power. 1 watt = 1 Joule/second. |
| Watt-hour |
An electrical energy unit of measure equal to 1 watt of power supplied to, or
taken from, an electrical circuit steadily for 1 hour. |
| Weighting factor (Weighting factor, WT) |
Multipliers of the equivalent dose to an organ or tissue used for radiation
protection purposes to account for different sensitivities of different organs
and tissues to the induction of stochastic effects of radiation (see 10 CFR
20.1003). |
| Well-logging |
A technique used in oil and gas exploration to help predict the commercial
viability of new or existing wells. It involves lowering a well-logging tool,
including a sealed source of radioactive material, into a well on a wire. This
device sends data on the well's underground characteristics to the surface,
where it is plotted on a chart. |
| Wheeling Service |
The movement of electricity from one system to another over transmission
facilities of intervening systems. Wheeling service contracts can be
established between two or more systems. |
| Whole-body counter |
A device used to identify and measure the radioactive material in the body of
human beings and animals. It uses heavy shielding to keep out naturally
existing background radiation and ultrasensitive radiation detectors and
electronic counting equipment. |
| Whole-body exposure |
An exposure of the body to radiation, in which the entire body, rather than an
isolated part, is irradiated. Where a radioisotope is uniformly distributed
throughout the body tissues, rather than being concentrated in certain parts, the
irradiation can be considered as whole-body exposure. |
| Wipe sample |
A sample made for the purpose of determining the presence of removable
radioactive contamination on a surface. It is done by wiping, with slight
pressure, a piece of soft filter paper over a representative type of surface area.
It is also known as a "swipe or smear" sample. |
| X-rays |
Penetrating electromagnetic radiation (photon) having a wavelength that is
much shorter than that of visible light. These rays are usually produced by
excitation of the electron field around certain nuclei. In nuclear reactions, it is
customary to refer to photons originating in the nucleus as x-rays. |
| Yellowcake |
A solid uranium-oxygen compound (U3O8) that takes its name from its color
and texture. It is a product of the uranium milling process and is the feed
material used for fuel enrichment and fuel pellet fabrication. |
