Actinide: One of the 15 transuranic elements that have an atomic number between 89 and 103. Named after actinium, the actinide with the lowest atomic number.

Activity: The rate of decay in a radioactive material. Expressed in becquerels.

ALARA: Acronym for “as low as [is] reasonably achievable,” when referring to personal or environmental exposure to radiation. The ALARA principle applies to processes and protection measures for handling radioactive materials; licensing these activities takes into account the economics of what protections are feasible, as well as advances in technology and societal considerations.

Alpha particle: A positively charged particle emitted from the nucleus of an atom during radioactive decay. Alpha particles are helium nuclei, containing two protons and two neutrons. They have low penetrating power, and can easily be stopped by a sheet of paper.

Atom: A particle of matter that cannot be broken up by chemical means. Atoms have nuclei consisting of positively charged protons and uncharged neutrons. The positive charges of the protons are balanced by an equal number of negatively charged electrons in motion around the nucleus. An atom that loses or gains electrons is called an ion.

Atomic energy: Another term for nuclear energy.

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


Background radiation: The naturally occurring ionizing radiation that every person is exposed to, arising from natural decay of radioactive elements (mainly in the Earth’s crust), from radon in the air, and from cosmic radiation that penetrates the Earth’s atmosphere.

Base load: The portion of electricity demand that is continuous, and does not vary over a 24-hour period. Approximately equivalent to the minimum daily load.

Becquerel (Bq): The unit of intrinsic radioactivity in a material. One Bq indicates one radioactive decay per second. Because this is a very small amount, radioactivity is usually measured in GBq or TBq. Another measure of radioactivity, the curie, is equivalent to 37 billion Bq.

Beta particle: A particle emitted from an atom during radioactive decay. Most beta particles are electrons but they may also be positrons. Beta particles are more penetrating than alpha particles but can be stopped by thin sheets of metal or plastic; large amounts of beta radiation can cause skin burns.

Boiling water reactor (BWR): A common type of light water reactor, where water is allowed to boil in the reactor core. The resulting steam then drives turbines, which activate generators to produce electrical power.

Breed: To create fissile nuclei, usually through neutron capture. This process can lead to radioactive decay.


Calandria: In a pressurized heavy water reactor, a cylindrical vessel that contains the heavy water moderator. The calandria is penetrated from end to end by hundreds of pressure tubes containing the fuel bundles and coolant.

CANDU: Acronym for “CANadian Deuterium Uranium,” a reactor type that uses heavy water as moderator and coolant. These are the most common type of pressurized heavy water reactors.

Carbon (C): A common element that has an atomic number of 6. Carbon forms organic compounds in combinations with other elements such as hydrogen or oxygen. It occurs in a pure state as diamond and graphite, and in an impure state as charcoal.

Carbon dioxide (CO2): A colourless, odorless incombustible gas, naturally present in the atmosphere and emitted through human activities. Carbon dioxide is the primary greenhouse gas emitted during the burning of fossil fuels such as coal, gas, or oil.

Chain reaction: A reaction that stimulates its own repetition. In a nuclear chain reaction, the fission of one atomic nucleus (of uranium-235, for example) releases neutrons, which can be absorbed by other fissionable nuclei that can also fission, releasing still more neutrons. If more neutrons are released than are lost or absorbed by other materials, the chain reaction is said to achieve criticality.

Control rod: In a reactor core, a device to absorb neutrons so that the nuclear chain reaction may be slowed or stopped by inserting them further, or accelerated by withdrawing them.

Control room: The area in a nuclear power plant from which most of the plant’s power production and emergency safety equipment can be operated, by remote control.

Condenser: A large heat exchanger designed to keep exhaust steam from a turbine below the boiling point, so that it can be returned to the heat source as water.

Conversion: The process of transforming uranium trioxide (UO3) into uranium dioxide (UO2) for use in heavy water reactors, or into uranium hexafluoride (UF6) for enrichment, before it is used in light water reactors.

Coolant: The liquid or gas circulated through a nuclear reactor to transfer heat from the reactor core to steam generators or directly to turbines. Coolants include light waterheavy water, air, carbon dioxide, helium, liquid sodium, and a sodium-potassium alloy.

Cosmic radiation: A natural form of ionizing radiation, made up of both particles and cosmic rays, originating from the sun and stars. Cosmic radiation is one component of natural background radiation.

Critical mass: The smallest mass of fissile material that will support a self-sustaining nuclear chain reaction.

Criticality: The condition of being able to sustain a nuclear chain reaction. This is the normal operating condition of a nuclear reactor.

Curie (Ci): A unit used to measure the intensity of radioactivity in a sample of material. 1 Ci is equal to 37 billion decays per second, or 37 billion becquerels.


Decay: Also referred to as “disintegration,” the spontaneous transformation of one radioisotope into one or more different isotopes, releasing radiation. Decay also refers to the exponential decrease in radioactivity of a material as nuclear disintegration takes place and more stable nuclei are formed.

Decay chain: A sequence of isotopes formed through a series of decays of an original radioisotope. A decay chain ends with a stable, less-energetic isotope.

Decommissioning: The process of permanently retiring a nuclear facility from service, including decontamination to reduce residual radioactivity, and the subsequent actions of safely storing radioactive materials, dismantling the facility, and preparing the site (if possible) for unrestricted use.

Defence in depth: An approach to designing and operating nuclear facilities to prevent and mitigate accidents. Defence in depth involves using multiple and redundant layers of defence, including use of access controls, physical barriers, redundant and diverse key safety functions, and emergency response measures, to ensure that the failure of any single layer will not allow an accident to occur.

Depleted uranium: Uranium that contains less than the natural level of uranium-235 (0.7% by mass). As a by-product of enrichment in the nuclear fuel cycle, it generally contains 0.25 to 0.30% U-235, the rest being U-238. Depleted uranium can be blended with highly enriched uranium (HEU) to make fuel for nuclear reactors.

Deuterium: A stable isotope of hydrogen (also called “heavy hydrogen”), containing one proton and one neutron in the nucleus. Normal hydrogen, which contains one proton and no neutrons, is 6500 times more common than deuterium in nature.

Disintegration: Another term for decay.

Dose: The energy absorbed by tissue from ionizing radiation. Radiation doses can be measured in gray (Gy) units, which are equivalent to one joule per kilogram; however, a practical measurement of doses adjusts this value for the effects of different kinds of radiation, and thus the sievert (Sv) is the unit used in setting exposure standards.

Dosimeter: A small portable instrument, carried by personnel working in nuclear facilities, used to measure and record the total accumulated personal dose of ionizing radiation.


Electrical generator: An electromagnetic device that converts mechanical energy (such as the rotation of a turbine) into electrical energy. Most large electrical generators are driven by steam or water turbine systems.

Electron: An elementary particle that has a negative charge equal to the positive charge of a proton, and a mass 1/1837 that of a proton. Electrons surround the positively charged nuclei of atoms, and determine their chemical properties.

Element: A chemical substance (such as carbon or thorium) that cannot be divided into simpler substances by chemical means; for each element, there are several isotopes that all have the same number of protons but different numbers of neutrons in their nuclei.

Emergency preparedness: The programs, plans, training, exercises, and resources necessary to preparing emergency personnel to rapidly identify, evaluate, and react to emergencies.

Enriched uranium: Uranium in which the proportion of uranium-235 has been increased above the natural level of 0.7%. Reactor-grade uranium is usually enriched to between 3% and 5% U-235, while weapons-grade uranium is more than 90% U-235.

Enrichment: The process of increasing the proportion of uranium-235 to uranium-238.


Fast breeder reactor (FBR): A reactor type configured to produce more fissile material than it consumes, using fertile material such as depleted uranium in a blanket around the reactor core.

Fast neutron: A neutron that, after being released during fission, has lost very little energy by colliding with other neutrons.

Fertile: An isotope (such as uranium-238 or plutonium-240) that is capable of becoming fissile by capturing neutrons, possibly followed by radioactive decay.

Fissile: An isotope (such as uranium-235, uranium-233, or plutonium-239) capable of capturing slow neutrons and undergoing nuclear fission.

Fission: The splitting of a heavy nucleus into two smaller nuclei, accompanied by the release of energy and usually one or more neutrons. Fission may be spontaneous but usually is the result of a nucleus absorbing a neutron and thus becoming unstable.

Fissionable: An isotope capable of undergoing fission; fissionable isotopes that capture slow neutrons are called fissile.

Fossil fuel: A fuel (such as coal, gas, or oil) that is based on carbon derived originally from living matter. When burned with oxygen, fossil fuels yield energy.

Fuel bundle, fuel assembly, fuel element: Structured collections of long, slender, metal tubes containing uranium fuel pellets, which provide fuel for nuclear reactors.

Fuel pellet: A small cylinder, measuring 10 to 13 mm long and 8 to 13.5 mm wide, consisting of pressed uranium dioxide (UO2) powder. Fuel pellets are stacked in tubes, which are structured to form fuel bundles for nuclear reactors.

Fusion: Fusion is a type of nuclear reaction that occurs when atomic nuclei fuse together. It is essentially the opposite of fission, which involves splitting atoms apart. While fusion generates vast amounts of energy, it has not yet been harnessed for practical use in industry.


Gamma rays: A form of high-energy ionizing radiation that is the product of fission, along with alpha particles and beta particles. However, gamma radiation is not a particle, but a wave, and is more penetrating; it is best stopped or shielded by dense materials, such as lead or depleted uranium.

Gas centrifuge: A method of enrichment that involves spinning cylinders of uranium hexafluoride (UF6) very rapidly, drawing the heavier uranium-238 toward the outside of the centrifuge, while a slightly higher concentration of uranium-235 remains at the centre; this part of the gas is siphoned off and the process is repeated many times, until the desired concentration of U-235 is reached.

Gas-cooled reactor: A reactor type common in the United Kingdom; these reactors use carbon dioxide as a coolant and graphite as a moderator.

Gaseous diffusion: A method of enrichment that involves forcing uranium hexafluoride (UF6) gas through semipermeable membranes. Because the lighter uranium-235 is slightly more likely to pass through a membrane, a large number of membranes can separate some of the heavier uranium-238 out, producing UF6 that has a higher concentration of U-235.

Geiger-Mueller counter (or Geiger counter):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.

Graphite: A form of carbon, most commonly found in pencil lead. Very pure graphite is used as a moderator, principally in gas-cooled reactors, but also in Soviet-designed high-power channel-type reactors.

Gray (Gy): The unit of absorbed radiation dose, equivalent to one joule per kilogram of tissue.

Greenhouse gases (GHGs): Gases in the Earth’s atmosphere that absorb heat radiation from the surface and re-radiate it, warming the globe. Carbon dioxide, methane, and water vapour are the main GHGs.


Half-life: The time required for half of the atoms of a radioisotope to decay and become an isotope of another element. Half-lives range from millionths of a second to billions of years.

Heavy water: Water containing an elevated concentration of molecules with deuterium atoms. Heavy water is used as a moderator in some nuclear reactors because it slows down neutrons effectively and also has a low probability of absorbing neutrons.

Heavy water reactor: A nuclear reactor that uses heavy water as its coolant and/or moderator.

High-level waste (HLW): Highly radioactive material arising from nuclear fission. It can be what is left over from reprocessing used fuel, though some countries regard used fuel itself as HLW. It requires very careful handling, storage and disposal.

Highly enriched uranium (HEU): Uranium that has undergone enrichment so that it contains at least 20% uranium-235 by mass.


In situ recovery (ISR): The recovery by chemical leaching of minerals from porous ore-carrying rock without physical excavation (“in situ” is a Latin term meaning “in position”). Also referred to as in situ leaching or solution mining.

Intermediate-level waste (ILW): A range of materials resulting from the reprocessing and decommissioning of nuclear facilities. It is sufficiently radioactive to require shielding and is disposed of in engineered facilities underground.

Ion: An atom or molecule that is electrically charged because of loss or gain of electrons.

Ionization: The process of adding one or more electrons to, or removing one or more electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, or nuclear radiation can cause ionization.

Ionizing radiation: Radiation that has enough energy to break chemical bonds and displace electrons in atoms, thus creating ions and potentially damaging materials, including living tissue. Ionizing radiation from nuclear processes includes alpha particlesbeta particles, and gamma rays.

Irradiation: Subjecting material to ionizing radiation. Excessive irradiation can damage materials and living tissue, and prolonged irradiation of reactor components can cause them to become radioactive themselves. Irradiation has beneficial applications, such as in cancer treatments or sterilization of medical instruments.

Isotope: A form of an atomic element having a particular number of neutrons, but the same number of protons as other isotopes of the same element. Different isotopes of an element  (such as uranium-235 and uranium-238) therefore have different atomic masses. Some isotopes are unstable and decay to form isotopes of other elements.


Light water: Ordinary water (H20) that has the normal proportion of hydrogen to deuterium atoms, as distinct from heavy water.

Light water graphite reactor (LWGR): A type of reactor used in Russia, with ordinary water as a coolant and graphite as a moderator.

Light water reactor (LWR): A common nuclear reactor that uses light water as a coolant and often as a moderator as well.  Its two main types are boiling water reactors and pressurized water reactors.

Low-enriched uranium (LEU): Uranium enriched so that the concentration of uranium-235 is higher than the natural level of 7% by mass, but lower than 20%.

Low-level waste (LLW): Mildly radioactive material that represents a very low hazard to people and the environment. It is usually disposed of by incineration and burial.


Megawatt (MW): A derived unit of power, equivalent to one million joules per second. Because this unit can describe any kind of energy, electrical power from a nuclear generator is most commonly referred to in megawatt electrical (MWe) units. The thermal (heat) output of a generator can be referred to in megawatt thermal (MWt) units, and is typically around three times the MWe figure.

Milling: The process by which minerals are extracted from ore, usually at the mine site, to produce a mineral concentrate for sale. In the case of uranium mining, the mineral U3O8 is most commonly extracted from uranium ore.

Mining: The process of removing minerals or ores from the earth, either on the surface or below it. The most common forms of mining are: open-pit mining; underground mining; and in situ recovery, which involves chemical leaching of minerals from rock without physical excavation.

Mixed oxide fuel (MOX): A type of reactor fuel that consists of both uranium and plutonium oxides, usually at about 5% plutonium, which is the main fissile component.

Moderator: A material (such as light water, heavy water, or graphite) used in a nuclear reactor to slow fast neutrons down so that they collide with lighter nuclei, which increases the chances that more fissions will occur.


Natural uranium: Uranium in which the natural concentrations of its isotopes (99.3% uranium-238, 0.7% uranium-235, and a trace of uranium-234) has not been changed, for example through enrichment. Natural uranium can be used as fuel in nuclear reactors that use heavy water or graphite as moderators.

Neutron: An uncharged particle found in the nuclei of atoms, along with protons, which have a slightly smaller mass and a positive charge. Fission reactions emit neutrons at various speeds, which can fission other atoms: slow (thermal) neutrons can readily cause fission in nuclei of fissile isotopes; fast neutrons can cause fission in nuclei of fertile isotopes.

Nuclear energy: The energy that is released through a nuclear reaction or radioactive decay. Nuclear reactors use the nuclear reaction known as fission to produce energy, usually in the form of heat, which in turn produces steam that can drive turbines.

Nuclear fuel cycle: The series of steps involved in supplying fuel for nuclear reactors, including mining, milling, refining, enrichment, fuel manufacturing, electricity generation, storage, reprocessing, and disposal.

Nuclear power plant: An electrical generating facility using a nuclear reactor as its heat source, to provide steam to a turbine.

Nuclear reactor: The main component of a nuclear power plant, in which a nuclear chain reaction occurs under controlled conditions, generating heat that can be used to drive turbines, or producing radiation (such as neutron beams) for industrial or scientific purposes.

Nucleus: The small, central, positively charged region of an atom.

Nuclide: Any isotope, whether one of the 279 stable isotopes or the approximately 2700 unstable ones.


Photon: A distinct amount (also called a quantum or packet) of energy emitted in the form of electromagnetic radiation. Photons have some of the characteristics of waves, but also characteristics of particles. Visible light, gamma rays, and x-rays are examples of photons.

Plutonium (Pu): A heavy, radioactive, artificial element that has an atomic number of 94. Plutonium is formed in a nuclear reactor by neutron capture. It has several isotopes, some of which are fissile and some of which undergo spontaneous fission, releasing neutrons. About one third of the energy in a light water reactor comes from the fission of Pu-239, and this is the main isotope of value recovered from reprocessing used fuel.

Positron: Particle equal in mass but opposite in charge to the electron; a positive electron.

Pressurized heavy water reactor (PHWR): A reactor type that uses heavy water as its moderator and coolant. CANDU reactors are the most common type of PHWRs.

Pressure tubes: In a pressurized heavy water reactor, a series of tubes holding the fuel and conveying the coolant through the surrounding moderator.

Pressurized water reactor (PWR): The most common type of light water reactor. PWRs use fission to heat water, keeping the water at very high pressure to prevent it from boiling. In turn, the hot, pressurized water boils water circulating in separate pipes, and the resulting steam drives turbines.

Proton: A nuclear particle with a positive electric charge, located in the nucleus of an atom.


Radiation: The emission and propagation of energy by means of electromagnetic waves or particles.

Radiation therapy: The therapeutic use of ionizing radiation to treat disease in patients. Although most radiotherapy procedures are intended to kill cancerous tissue or reduce the size of a tumor, therapeutic doses may also be used to reduce pain or treat benign conditions.

Radioactivity: The characteristic of some isotopes to undergo spontaneous decay of their nuclei, which causes the emission of electromagnetic waves (such as gamma rays) or particles (such as alpha particles and beta particles). Radioactivity is measured in curies (Ci) or becquerels (Bq).

Radiography: The use of sealed sources of ionizing radiation for nondestructive examination of the structure of materials. When the radiation passes through the material, it produces a shadow image on a sheet of photographic film placed behind the material, which can reveal the material’s internal structure, including flaws and unevenness.

Radioisotope, radionuclide: An unstable isotope of an element that decays spontaneously, causing the emission of radiation.

Radium (Ra): A highly radioactive, metallic element that has an atomic number of 88. Radium is the radioactive decay product of uranium and is often found in uranium ore. It has several radioisotopes. Radium-226 decays to radon-222.

Radon (Rn): A heavy, radioactive gas that has the atomic number of 86. Radon-222 is the radioactive decay product of radium-226, and is emitted by rocks that contain radium (or thorium). Because of its weight, radon can accumulate in the basements of buildings situated near such rocks; radon in the home can be detected by a professional or through store-bought kits.

Reactor core: The central portion of a nuclear reactor, which contains the fuel bundles, moderator, coolant, control rods, and support structures. The reactor core is where fission takes place.

Refining: A series of chemical processes that remove impurities from uranium oxide (U3O8) and convert it to uranium trioxide (UO3).

Renewable resources: Natural energy resources that can be replenished or used continuously. The most commonly harnessed renewable resources include biomass, hydro, geothermal, solar, and wind. In the future, renewable resources such as ocean thermal, wave, and tidal action technologies could become more practical and more widely adopted.

Repository: A permanent disposal place for radioactive wastes.

Reprocessing: The process of recovering residual uranium or by-product plutonium, both of which are still useful sources of energy, from used fuel.


Sealed source: Any radioactive material or by-product encased in a specialized capsule designed to prevent leakage or escape of the material.

Shielding: Any material or obstruction that absorbs ionizing radiation, for the protection of people or materials.

Sievert (Sv): A derived unit of measurement of the biological damage caused by radiation doses, which is measured in grays (Gy): 1 Gy of radiation from beta particles or gamma rays has 1 sievert (Sv) of biological effect; 1 Gy of radiation from alpha particles has a 20-Sv effect; and 1 Gy of neutrons has a 10-Sv effect.

Slow neutron: A neutron produced by fission in a nuclear reactor, and slowed by a moderator.

Small modular reactor: A nuclear reactor designed to be built economically in factory-like conditions (rather than onsite), and with capacities between approximately 10 MWe and 300 MWe.

Source material: Uranium or thorium, or any combination of these elements, in any physical or chemical form, or ores that contain 0.05% or more of these by mass.

Spent fuel: Another term for used fuel.

Stable: The characteristic of an isotope that is incapable of spontaneous radioactive decay.

Steam generator: A component of some types of nuclear reactors, in which very hot water under high pressure enters a heat exchanger and makes steam in a secondary circuit, to drive a turbine.


Tailings: Ground rock remaining after ore minerals (such as uranium oxides) are extracted.

Thermal reactor, thermal breeder reactor: A nuclear reactor in which the nuclear chain reaction is sustained primarily by slow neutrons, and therefore requires a moderator. All commercial reactors are thermal reactors.

Thorium (Th): A radioactive, transuranic element that occurs naturally in the Earth’s crust. Thorium has an atomic number of 90. The most common thorium isotope, thorium-232, is fertile and can be used in some thermal reactors.

Transuranic element: A very heavy element formed artificially by neutron capture and possibly subsequent beta decay. Transuranic elements all have higher atomic numbers than uranium (92) and are all radioactive. Neptunium, plutonium, americium, and curium are the best-known.

Tritium: A radioisotope of hydrogen. Like hydrogen and deuterium, tritium has one proton in its nucleus, but it has two neutrons, giving it greater mass and making it unstable. Because tritium is chemically identical to natural hydrogen, it can easily be absorbed into the body. Tritium decays by emitting beta particles and has a half-life of about 12.5 years.

Turbine: A rotary engine made with a series of curved vanes on a rotating shaft, usually turned by water or steam. Turbines are considered the most economical means to turn large electrical generators.


Unstable: The characteristic of an isotope that undergoes spontaneous radioactive decay.

Uranium (U): A mildly radioactive, heavy, metallic element that has an atomic number of 92. Uranium occurs naturally in ores in the Earth’s crust. Uranium has two fissile isotopes (U-235 and U-233) and two fertile isotopes (U-238 and U-234). Uranium is the basic fuel of atomic energy.

Uranium dioxide (UO2): The chemical form in which uranium is most commonly used as fuel for nuclear reactors.

Uranium hexafluoride (UF6): A compound of uranium that is a gas above 56°C, making it a practical form for enrichment through gaseous diffusion or in gas centrifuges.

Uranium ore: Any ore from which uranium can be extracted.

Uranium oxide concentrate: The mixture of uranium oxides (predominantly U3O8, but also including UO2 and UO3) produced after milling uranium ore from a mine. It is khaki in colour and is sometimes loosely called yellowcake. Uranium is normally sold in this form.

Used fuel: Fuel bundles removed from a nuclear reactor after several years’ use. Also referred to as spent fuel.


Waste: Any material associated with nuclear facilities that remains, or might remain, radioactive for a time after use. These range from used fuel to the mop heads used to clean the facilities. The safety and security measures applied to waste depend on a classification of high-level waste, intermediate-level waste, and low-level waste.


X-ray: Penetrating electromagnetic radiation (photons) having a wavelength that is much shorter than that of visible light, but longer than that of gamma rays. 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 familiar term for uranium oxide concentrate.


Zirconium alloy: A group of metal alloys (including the trademarked group of alloys called Zircaloy) used to contain uranium fuel pellets, which in turn form part of a reactor fuel bundle.