Nuclear and Radiation Glossary
This glossary provides definitions for commonly-used technical terms found throughout our web pages.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
A
Absorbed dose: The energy deposited by ionizing radiation to a suitably small volume of matter divided by the mass of that volume. Unit: gray; symbol Gy.
Activity: The rate at which nuclear disintegrations occur in a radioactive material. Used as a measure of the amount of a radionuclide present. Unit: becquerel; symbol Bq. 1 Bq = 1 disintegration per second.
Acute dose: Exposure received within a short period of time (hours or days). The adjective 'acute' relates only to the duration of exposure, and does not imply anything about the magnitude of the doses involved.
ALARA: An optimization tool in radiation protection used to keep individual, workplace and public dose limits As Low As Reasonably Achievable (ALARA), social and economic factors being taken into account. ALARA is not a dose limit; it is a practice that aims to keep dose levels as far as possible below regulatory limits.
Alpha particles: Positively charged particles consisting of two protons and two neutrons that are emitted by the nuclei of radioactive (unstable) elements as they decay. Alpha particles are relatively large and can be stopped by skin or a sheet of paper. An alpha particle is a helium nucleus.
Atom: Unit of matter consisting of a single nucleus surrounded by a number of electrons equal to the number of protons in the nucleus. The atom is the smallest portion of an element that can combine chemically with other atoms. All atoms other than hydrogen-1 have neutrons.
Atomic mass: The mass of an isotope of an element expressed in atomic mass units, which are defined as one-twelfth the mass of an atom of Carbon-12. An atomic mass of 1 is equivalent to about 1.66 x 10-27kg.
Atomic number: The number of protons in the nucleus of an atom. Symbol Z.
B
Becquerel: The SI unit of radioactivity, equal to one transformation (decay) per second. Supersedes the non-SI unit curie (Ci). 1 Bq = 27 pCi (2.7 × 10–11 Ci) and 1 Ci = 3.7 × 1010 Bq.
Beta particles: High-energy negatively charged electrons or positively charged positrons that are ejected by radioactive (unstable) elements as they decay. A beta particle is identical in mass and charge to an electron. Beta particles are relatively small and can be stopped by a sheet of aluminum or plastic a few millimetres thick.
Bioassay: Any procedure used to determine the nature, activity, location or retention of radionuclides in the body by direct measurement or by analysis of material excreted or otherwise removed from the body.
Boiling water reactor (BWR): A common type of light-water reactor (LWR), where water is allowed to boil in the core, generating steam directly in the reactor vessel.**
C
Cataract: A condition that occurs when the normally clear lens inside the eye becomes cloudy or dark and causes blurred vision that is not correctable by ordinary glasses. The most important factor in cataract formation is increasing age, but there are additional factors, including smoking, diabetes, excessive exposure to sunlight and radiation.
Cell: The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living.
Cosmic rays: High-energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions.
Chronic dose: Exposure persisting in time (months or years). The adjective ‘chronic’ relates only to the duration of exposure, and does not imply anything about the magnitude of the doses involved.
Commission Hearing: The Commission Tribunal makes decisions on the licensing of nuclear facilities. At these hearings, interested parties and members of the public can be heard before the Commission. For more information about participating at CNSC public hearings, please read CNSC document INFO-0789, What you need to know to attend or participate at public hearings.
Common spent fuel pool: In addition to pools in each of Japan’s Fukushima Daiichi plant's BWR reactor buildings, there is another facility -- the common use spent fuel pool -- where spent fuel is stored after cooling at least 18 months in the reactor buildings. This fuel is much cooler than the assemblies stored in the reactor buildings.
Comprehensive Study: A type of environmental assessment that is typically conducted for large and complex projects with the potential for adverse environmental effects and public concern. Comprehensive studies require public participation opportunities and a decision from the Federal Minister of the Environment.
Comprehensive Study Report (CSR): A report that summarizes the results of a comprehensive study, and provides conclusions and recommendations on this study. The comprehensive study report is prepared and provided to the Federal Minister of the Environment and to the Canadian Environmental Assessment Agency. The Federal Minister of the Environment will then make a decision based on the comprehensive study report.
Containment vessel: Encloses radioactive substances in the nuclear reactor and prevents their release to the environment.
Core: The central part of a nuclear reactor containing the fuel elements and any moderator. **
Critical mass: The smallest mass of fissile material that will support a self-sustaining chain reaction under specified conditions.**
Criticality: Condition of being able to sustain a nuclear chain reaction.**
D
Decay (Radioactive): The transformation of a radioactive nuclide into a different nuclide by the spontaneous emission of radiation such as alpha, beta, or gamma rays, or by electron capture. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life.
Deterministic effects: Changes in cells and tissues that are certain to occur after an acute dose of radiation (in excess of a threshold value of at least 1000 mSv), below which the radiation effect is not detected. The severity of health effects – such as skin reddening, burns, and hair loss – increase with the radiation dose received.
DNA: Deoxyribonucleic acid (DNA) is the molecular compound in the nucleus of a cell that forms the blueprint for the structure and function of the cell.
Dose: The energy absorbed by tissue from ionizing radiation. One gray is one joule per kg, but is adjusted for the effect of different kinds of radiation, so the sievert is the unit of dose equivalent used in setting exposure standards.**
Dosimeter: A device for measuring a dose of radiation that is worn or carried by an individual.
Dosimetry: A scientific subspecialty in radiation protection and medical physics that focuses on calculating the internal and external doses from ionizing radiation.
Drywell: Part of a boiling water reactor (BWR) containment system. If an accidental leak occurs in a BWR reactor, the reactor coolant will turn to steam in the drywell, quickly pressurizing it. Vent pipes from the drywell direct the steam below the water level maintained in the wetwell (also known as a torus or suppression pool), condensing the steam into liquid and limiting the pressure in the system. BWR drywells are enclosed by a secondary containment building.
E
EA Guidelines: A document which contains requirements and guidance for the applicant in carrying out an environmental assessment and related studies.
Effective Dose: A measure of dose designed to reflect the amount of radiation detriment. It is obtained by multiplying the equivalent dose of each tissue or organ by an appropriate tissue weighting factor and summing the products. Unit: sievert (Sv).
Electron capture: A radioactive decay process in which an orbital electron is captured by and merges with the nucleus. After the process, the mass number is unchanged, but the atomic number is decreased by one because a proton is converted to a neutron.
Electron: A stable elementary particle having a negative electric charge of 1.6 x 10-19 coloumbs and a mass of 9.1 x 10-31 kg.
Element: Specific types of atoms that have the same number of protons in their nucleus and therefore have the same atomic number. The number of neutrons may differ.
Energy: A physical quantity that describes the amount of work that can be performed by a given force, subject to a conservation law. Different forms of energy include kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy.
Environmental Assessment (EA): A planning tool used to identify the possible environmental effects of a proposed project and to determine if the adverse effects can be mitigated.
Environmental Impact Statement (EIS): A document that provides a detailed review of the environmental assessment and related studies.
Equivalent Dose: A measure of the dose to a tissue or organ designed to reflect the amount of harm caused to the tissue or organ. Obtained by multiplying the absorbed dose by a “radiation weighting” factor to allow for the biological effectiveness of the various types of radiation in causing harm to tissue. Unit: sievert (Sv).
F
Federal Authority (FA): A federal body (e.g., Federal Minister of the Crown, a department or agency) that may have expertise or a responsibility relevant to a proposed project.
Fission: See Nuclear fission. The division of a heavy nucleus into two (or, rarely, more) parts with masses of equal order of magnitude; usually accompanied by the emission of neutrons and gamma radiation.
G
Gamma rays: Penetrating electromagnetic radiation emitted by an atomic nucleus during radioactive decay; a wave form of ionizing radiation.
Gray (Gy): The SI* unit of absorbed radiation dose, one joule per kilogram of tissue.**
H
Half-life: Time required for a given radionuclide's activity to decrease by half through radioactive decay. Symbol t1/2. A shorter life means a more radioactive substance.
Heavy water: Water containing an elevated concentration of molecules with deuterium (heavy hydrogen) atoms. **
I
Ion: An atom, molecule, or fragment of a molecule that has acquired an electrical charge through the loss or capture of electrons.
Ionizing radiation: Radiation capable of producing ion pairs in biological materials, for the purpose of radiation protection. Examples are alpha particles, gamma rays, X-rays and neutrons.
Isotopes: Various forms of atoms of the same chemical element, which are distinguished by the number of neutrons in the nucleus. The number of protons remains the same, but the number of neutrons differs. For example, uranium has 16 different isotopes.
L
Licence Application Documentation: Information provided by the applicant during the licensing process to demonstrate how regulatory requirements will be met. Some examples of these requirements are the Nuclear Safety and Control Act, the Uranium Mines and Mills Regulations and other regulations.
Licence to Prepare Site and Construct (LPSC): A document authorizing the siting and construction of a nuclear facility, granted by the Commission Tribunal.
Licensing Process (licensing):The steps an applicant follows to obtain authorization from the CNSC. The process may include the completion of an environmental assessment in addition to the submission of the licence application documentation (programs, procedures and facility/system designs), which are reviewed and assessed by CNSC staff prior to a Commission Hearing.
Light water: Ordinary water (H20) as distinct from heavy water.**
Light water reactor (LWR): A common nuclear reactor cooled and usually moderated by ordinary water. It is a generic designation including BWR and PWR types.**
M
Malignant: Cancerous tumors which tend to grow rapidly are known as malignant tumors. These can invade and destroy nearby normal tissues, and spread throughout the body.
Mass: See atomic mass.
Microsievert: One one-millionth of a sievert (see sievert). Symbol: µSv.
Millisievert: One one-thousandth of a sievert (see sievert). Symbol: mSv.
Mineralized Zone: a volume of rock with higher than normal concentration of a mineral of interest (like uranium).
Molecule: A group of atoms chemically bonded to each other.
Mutation: A chemical change in the DNA in the nucleus of a cell. Mutations in sperm or egg cells or their precursors may lead to inherited effects in children or later generations. Mutations in body cells may lead to effects such as cancer.
N
Natural radiation: Natural background radiation is constantly present in the environment and emitted from a variety of sources. These sources include cosmic rays, terrestrial sources (radioactive elements in the soil), ambient air (radon), and internal sources (food and drink). The annual global per caput effective dose due to natural radiation sources is 2.4 mSv (UNSCEAR 2000).
Neutron: An elementary particle found in the nucleus of atoms having no electrical charge, with a mass of about 1.6 x 10-27 kg.
Neutron capture: A type of nuclear reaction in which an atomic nucleus absorbs a free neutron and they merge to form a heavier nucleus. Neutrons can react with an atomic nucleus in several ways each ending with a different product.
Non-ionizing radiation: Radiation that is not ionizing radiation; that is, it does not possess sufficient energy to produce ions. Examples are ultraviolet (UV), visible light, infrared, and radio waves.
Nuclear Facility: is defined in section 2 of the Nuclear Safety and Control Act
Nuclear fission: The division of a heavy nucleus into two (or, rarely, more) parts with masses of equal order of magnitude; usually accompanied by the emission of neutrons and gamma radiation.
Nuclear reactor: A device in which a nuclear fission chain reaction occurs under controlled conditions so that the heat can be harnessed or the neutron beams utilized. All commercial reactors are thermal reactors, using a moderator to slow down the neutrons.**
Nucleus (of an atom): The positively charged central portion of an atom that contains protons and neutrons.
Nuclide: A species of atom characterized by the number of protons and neutrons and the energy state of the nucleus.
O
Optimization: The process of determining what level of protection and safety makes exposures and the probability and magnitude of potential exposures, as low as reasonably achievable, economic and social factors being taken into account.
Ore: A type of rock with a grade of mineral that can be mined for a profit.
P
Participant Funding Program: A program to provide funding to members of the public and Aboriginal groups to participate in parts of the licensing process (including the environmental assessment).
Photon: A quantum (smallest possible amount) of electromagnetic radiation.
Positron: A stable elementary particle having a positive electric charge of 1.6 x 10-19 coulombs and a mass of 9.1 x 10-31 kg (i.e. similar to an electron, but positively charged).
Positron emission: In those instances where the neutron-to-proton ratio is too low and alpha emission is not energetically possible, the nucleus may, under certain conditions, attain stability by emitting a positron.
Proton: A stable elementary particle found in the nucleus of atoms with a positive electric charge of 1.6 x 10-19 kg.
R
Radiation: Energy travelling through space in the form of waves or particles. Ionizing radiation (e.g. alpha particles, beta particles, gamma rays, x-rays, and neutrons) has the ability to remove electrons from the matter it encounters. The term radiation, as used on this Web site, implies ionizing radiation.
Radiation weighting factor: A number by which the absorbed dose in a tissue or organ is multiplied to reflect the relative biological effectiveness of the radiation in inducing stochastic effects at low doses, the result being the equivalent dose.
Radioactive: Exhibiting radioactivity; emitting or relating to the emission of ionizing radiation or particles such as alpha and beta particles, neutrons or gamma rays.
Radioisotope: An isotope that undergoes spontaneous decay and emits radiation.
Radionuclide: A radioactive nuclide.
Radon: A chemical element with symbol Rn and atomic number 86. Radon is a colorless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium. It is one of the heaviest substances that remains a gas under normal conditions and is a health hazard.
Radon decay products: A term used to refer collectively to the immediate products of the radon decay chain. These include Polonium-218, Lead-214, Bismuth-214, and Polonium-214. They have an average combined half-life of about 30 minutes. Also called radon progeny and radon daughters.
Reactor pressure vessel: In a BWR, the reactor’s cooling water is maintained at about 75 atm (7.6 MPa, 1000–1100 psi) so that it boils in the core at about 285°C (550°F). The reactor pressure vessel is designed to contain this high-pressure radioactive steam and direct it towards turbines that are turned to generate electricity.
Relative biological effectiveness: A relative measure of the effectiveness of different radiation types at inducing a specified health effect. It is expressed as the inverse ratio of the absorbed doses of two different radiation types that would produce the same degree of a defined biological end point.
Responsible Authority: A federal authority that is required under the Canadian Environmental Assessment Act to ensure that an environmental assessment of the project is completed.
S
Sievert: The SI unit of absorbed radiation dose in living organisms modified by radiation type and tissue weighting factors. The sievert is the unit of dose measuring the “equivalent dose” and “effective dose”. It replaces the classical radiation unit the rem. Multiples of sievert (Symbol Sv) used in practice include millisievert (mSv) and microsievert (μSv).
Spent fuel: Used fuel assemblies removed from a reactor after several years use and treated as waste. Often it is another term for used fuel.**
Stochastic effects: A term used to group radiation-induced health effects (such as cancer or inheritable diseases) which have a statistical risk. For these diseases, the probability of their occurrence increases proportionally to the radiation dose received: the higher the dose, the higher the probability of occurrence. However, at no time, even for high doses, is it certain that cancer or genetic damage will result.
Suppression pool: Part of a boiling water reactor (BWR) containment system. If an accidental leak occurs in a BWR reactor, the suppression pool (also known as a torus or wetwell), condenses steam from the drywell into liquid and limits the pressure in the system. BWR suppression pool are enclosed by a secondary containment building.
T
Tritium: Tritium occurs both naturally and as a by-product in nuclear reactors. It is a radioactive isotope of hydrogen having two neutrons and one proton in its nucleus; hydrogen, by comparison, has only one proton. Tritium decays by emitting an electron (beta radiation) and has a half-life of 12.33 years.
U
Used fuel: Fuel assemblies removed from a reactor after several years' use.**
V
Void coefficient of reactivity: The change in reactivity due to boiling of coolant or moderator in the actual core. A negative void coefficient means that the reactivity is diminished, and the balance of the chain reaction is affected so as to reduce the rate of fission and hence reduce the temperature - a natural negative feedback.**
W
Wetwell: Part of a boiling water reactor (BWR) containment system. If an accidental leak occurs in a BWR reactor, the wetwell (also known as a torus or suppression pool), condenses steam from the drywell into liquid and limits the pressure in the system. BWR wetwells are enclosed by a secondary containment building.
X
X-ray: Ionizing electromagnetic radiation emitted by an atom when it has been bombarded with electrons. X-rays differ from gamma rays in that they are emitted from the orbiting electrons, not the nucleus, and they have a much wider energy range, or spectra.
** Source: World Nuclear Association
