Acronym for As Low as Reasonably Achievable: one of the three basic criteria recommended by the International Commission on Radiological Protection to minimize radiation risks.

Alpha Radiation
A positively charged particle made up of two neutrons and two protons. It is the least penetrating of the three common forms of radiation, and can be stopped by a sheet of paper.

The basic building block of all matter. An atom has a nucleus made up of positively charged protons and neutral neutrons surrounded by orbiting electrons whose negative charge balances that of the protons in the nucleus.

Atomic Number
The number assigned to each element on the basis of the number of protons found in the element's nucleus.

Atomic Weight (Atomic Mass)
Approximately the sum of the number of protons and neutrons found in the nucleus of an atom.

Background Radiation
The naturally occurring nuclear radiation coming from outer space as cosmic radiation, or from naturally occurring radioactive elements such as uranium and radium in the materials of the earth.

Becquerel (Bq)
The unit of radioactive decay equal to 1 disintegration per second. 37 billion becquerels is equal to 1 curie (Ci). There are 30,000 disintegrations per second taking place inside a household smoke detector.

Beta Radiation
An electron (or a particle of equal weight to an electron but with a positive electrical charge) that is emitted from a radionuclide. It is less damaging than the same dose of alpha radiation but more penetrating. Beta radiation can be stopped by a thin sheet of metal or plastic.

Chain Reaction
A reaction that initiates its own repetition. In a fission reaction, free neutrons are produced which fly off and strike other nuclei, causing them to split and send off yet more free neutrons. The fission will continue as long as there are enough free neutrons carrying the right amount of energy.

Most reactors are enclosed in a thick, concrete, domed building, called the containment. In the event of a release of radioactive material into the reactor building, the containment traps the emissions and prevents their escape.

Radioactive material deposited or dispersed into materials or places where it does not belong.

Control Rods
A rod containing neutron-absorbing materials, such as boron or cadmium. Control rods are moved in and out of the core of the reactor to control the rate of the nuclear reaction.

The liquid or gas used to transfer the heat of nuclear fission to a heat exchanger in which steam is produced to drive the electrical generator. The cooled liquid or gas is then returned to the reactor.

The central region of a nuclear reactor, containing the fuel assemblies, coolant and moderator, in which the fission chain reaction takes place.

Critical Mass
The minimum amount of fuel needed in the core of a nuclear reactor in order to start a self-sustaining chain reaction. When a reactor starts up it is said to "go critical".

Curie (Ci)
A unit used to measure the rate of radioactive decay. One curie equals 37 billion disintegrations per second, or approximately the radioactivity of one gram of radium.

Decay (Radioactive)
The change of one radioactive nuclide into a different nuclide by the spontaneous emission of alpha, beta, or gamma radiation, or by electron capture. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life.

The removal of radioactive contaminants by cleaning and washing with water and/or chemicals.

A stable, naturally occurring hydrogen isotope. It is used as a moderator in the form of deuterium oxide or heavy water.

A general term denoting the quantity of radiation or energy absorbed in a specific mass.

Electromagnetic Radiation
Electric or magnetic waves that travel at the speed of light. Examples: light, heat, radio waves, microwaves, gamma radiation, x-rays.

An elementary particle carrying one unit of negative electrical charge. Electrons surround the atom's positively charged nucleus and determine the atom's chemical properties.

The physical process of increasing the concentration of the uranium-235 isotope relative to the predominant uranium-238 isotope in natural uranium.

The break-up of the nucleus of an atom into two major fragments, plus smaller fragments and free neutrons, when the nucleus is struck by a fast-moving free neutron.

Gamma Radiation
Highly penetrating, short wavelength radiation emitted from the nuclei of atoms. It is stopped by an adequate thickness of lead, concrete or other materials.

Geiger Counter
An instrument for detecting and measuring ionizing radiation. It contains a gas-filled tube which discharges electrically when ionizing radiation passes through it.

The time over which the atoms of a particular radioactive nuclide decay to half their original intensity of emitted radiation. The half-life is a characteristic property of each radioactive isotope.

Heavy Water
Heavy water or deuterium oxide (D20) is a natural form of water used to lower the energy of neutrons in a reactor. It is heavier than normal water by about 10 per cent, and occurs in minute quantities (about one part heavy water per 7,000 parts water).

The International Atomic Energy Agency is an agency set up by the United Nations to monitor and promote the peaceful uses of nuclear energy.

An atomic particle, atom or molecule that is electrically charged.

Ionizing Radiation
Any type of radiation that can, directly or indirectly, change the electric charges of atoms or molecules. It is produced when radionuclides decay.

Different forms of atoms of the same element. They have the same number of protons in their nuclei but a different number of neutrons (the same atomic number but different atomic weights). Uranium-238 and uranium-235 are isotopes of uranium. Isotopes may be stable (not spontaneously decaying) or unstable (spontaneously decaying, emitting ionizing radiation).

Kilowatt (KW) is a metric measurement of power, and is equal to 1,000 watts.

The basic unit of electric energy equal to one kilowatt of power supplied to or taken from an electric circuit steadily for one hour. A kilowatt-hour (KWH) is 1,000 watt-hours.

Megawatt (MW) is a unit of power, is equal to one million watts, and refers to the heat output of a reactor. MWe refers to electrical output.

Moderators are used to lower ("moderate") the energies of a portion of the neutrons emitted by fissioning uranium atoms, to increase their probability of hitting another uranium atom to cause further fissioning. Graphite and light water are frequently used as moderators.

Nuclear Energy
Nuclear energy is the energy stored in the bonds of the sub-atomic particles in the nucleus of atoms.

Nuclear Reactor
A device in which a fission chain reaction can be initiated, maintained, and controlled. Its essential components are fissionable fuel, moderator, shielding, control rods, and coolant.

The core of the atom, where most of its mass and all of its positive charge is concentrated. Except for hydrogen, it consists of protons and neutrons.

Any species of atom that exists for a measurable length of time. A nuclide can be distinguished by its atomic weight, atomic number, and energy state.

An electrically neutral particle with negligible mass. It is produced in many nuclear reactions such as in beta decay.

One of the basic particles which make up an atom. A neutron and a proton have about the same weight, but the neutron has no electrical charge.

One of the basic particles which makes up an atom. The proton is found in the nucleus and has a positive electrical charge equivalent to the negative charge of an electron and a mass similar to that of a neutron.

Radioactive Dating
A technique for estimating the age of an object by measuring the concentrations of various radioisotopes in it.

Energy given off by atoms when they are moving or changing state. It can take the form of electromagnetic waves, such as heat, light, X-rays, or gamma rays, or streams of particles such as alpha particles, beta particles, neutrons or protons.

The emission of alpha particles, beta particles, neutrons and gamma or x-radiation from the disintegration of an atomic nucleus.

Atoms of chemical elements may have many isotopes (different forms) with different atomic numbers and different atomic weights. If an isotope is radioactive, it is sometimes referred to as a radioisotope or a radionuclide.

An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.

Scintillation Counter
An instrument that detects and measures gamma radiation by counting the light flashes (scintillations) induced by the radiation.

A protective barrier for reducing or eliminating the transfer of radiation from radioactive materials to the surroundings. Depending on the radiation level, shielding can range from paper, to water, to lead, to concrete.

Sievert (Sv)
A unit that is used for describing the absorption of radiation by the human body.

A radioactive material that produces radiation for experimental or industrial use.

Spent Fuel
Fuel assemblies taken out of a nuclear reactor after a period of useful energy production. Also referred to as used fuel.

A small amount of radioactive isotope introduced into a system in order to follow the behavior of some component of that system.

The heaviest naturally occurring element, with an atomic number of 92.

An X-ray is a form of electromagnetic radiation with a very short wave length. (see radiation)

Zirconium is a metallic element with atomic number of 40. An alloy of zirconium known as Zircaloy is extensively used for the cladding of nuclear fuel elements.