Glossary

Activity

The number of spontaneous nuclear disintegrations occurring per unit of time in a quantity of radioactive material. Activity is measured in Becquerel, or Bq for short (number of disintegrations per second). The old unit was the curie, Ci.

ALARA principle

This is the principle that the exposure of people and the environment to ionising radiation should be ‘as low as reasonably achievable’, taking into account economic and social factors. It is one of the basic principles in radiation protection and is the tenet of the International Commission on Radiological Protection.

Alpha particles 

Positively charged particles emitted during certain types of radioactive decay. An alpha particle consists of two neutrons and two protons and is identical to the nucleus of a helium atom. Alpha radiation is less penetrating than beta or gamma radiation. A sheet of paper is sufficient to absorb alpha radiation.

Atom

The smallest particle of a chemical element, which cannot be broken down further in a chemical reaction. Each atom consists of a nucleus of positively charged protons and electrically neutral neutrons, surrounded by a ‘cloud’ or ‘shell’ of negatively charged electrons that orbit around the nucleus. From an external point of view, the behavior of atoms is electrically neutral, since the number of protons in the nucleus equals the number of electrons in the shell. Atoms are tiny: in an average drop of water there are approximately 6,000 trillion (21 zeroes after the six) atoms.

Background radiation

Naturally occurring ionizing radiation, including cosmic rays and radiation from naturally occurring radioactive materials.

Barrier

Natural or man-made shield to protect against the dispersion of radioactive materials and against ionising radiation. See also multiple barrier principle.

Becquerel (Bq)

The unit used to measure radioactivity. 1 Bq equals one disintegration per second. This unit replaces the curie.

Bentonite

Special type of clay formed from volcanic ash. Used as a backfill material in underground disposal of radioactive waste in deep geological clay layers.

Beta particles

Particles emitted from a nucleus during certain types of radioactive decay. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Beta particles can be stopped, for instance, by an aluminium sheet a few millimetres thick or by 3 metres of air.

Care and maintenance

Actions such as surveillance, inspection, testing and maintenance to ensure that facilities are maintained in a safe state between decommissioning phases.

Cementation

Encapsulation in cement. Method used to encapsulate certain types of radioactive waste.

Clay

Soft or slightly solidified rock that mainly consists of tiny particles (smaller than 2 microns). Clay has the ability to slow down the movement of radionuclides and has low permeability. Furthermore, it is a plastic material with good ‘self-healing power’; in other words, openings that appear in clay (fissures, fractures) tend to close up by themselves over time.

Clearance levels

A set of values, established by the regulatory body (NNR in South Africa), expressed in terms of activity concentrations and/or total activities, at or below which sources of radiation can be released from nuclear regulatory control.

Compaction

Industrial technique used to crush materials in order to reduce their volume.

Containment

All tools and techniques used to protect people and the environment against the dispersion of radionuclides in the biosphere.

Contamination 

Presence of radioactive substances in a material, on the surface of objects or in places where they should not be or where they can have harmful consequences. For humans a distinction is made between external and internal contamination. In the case of internal contamination, radioactive particles are present in the body, for instance by inhalation or by ingestion of radioactively contaminated food or liquids.

Cosmic radiation

Ionising radiation originating in outer space.

Daughter products

Nuclides formed by the radioactive decay of other radionuclides. In the case of radium-226, for example, there are ten successive daughter products, ending in the stable isotope lead-206.

Decay, radioactive

Reduction of radioactivity through the emission of radiation as a result of the transformation of radionuclides into more stable isotopes. Radioactive decay is a natural phenomenon. See also half-life.

Decommissioning

All the administrative and technical procedures that make it possible to remove a nuclear facility from the list of classified facilities. The administrative procedures include drawing up decommissioning plans and obtaining the requisite permits and certificates for release of the facilities and of the site for unrestricted use. The technical procedures include decontamination, dismantling and management of the radioactive waste. The purpose of decommissioning is not to demolish the buildings, but rather to release them from the obligations and controls associated with their particular class.

Decontamination

The removal or reduction of radioactive contamination in or on the surfaces of structures, areas, objects or people. Decontamination can be carried out using mechanical, chemical or electrochemical processes.

Disposal

Possible technical solution for the long-term management of processed radioactive waste. Covers all operations to isolate processed radioactive waste from man and the environment. The object is to protect people and the environment against the potential hazards arising from this waste during the period when its radioactivity is diminishing by natural decay.

Dose, absorbed

Amount of energy that is transferred to a material by ionising radiation per unit of mass of that material. The unit of absorbed dose is the gray (Gy). 1 gray equals 1 joule per kilogram.

Dose, effective

Some tissues and organs are more sensitive to radiation than others. To take this into account, the dose equivalent is weighted by a specific risk factor for each tissue or organ to give the effective dose. The unit used is the millisievert.

Dose equivalent

The quantity obtained by multiplying the absorbed dose by a quality factor, depending on the type of radiation and the biological effect on tissues. The dose equivalent is expressed in sievert (Sv).

Dosimeter

A small portable instrument for measuring and recording the total accumulated personal dose of ionising radiation.

Exposure

Being exposed to radiation from a radioactive source.

Gamma radiation or rays

High-energy electromagnetic radiation with a very short wavelength and no mass, which is emitted from many types of nuclei. Gamma rays are just like ordinary light and X-rays, but have a much higher energy. Gamma rays are very penetrating and can only be effectively absorbed by dense materials such as iron, concrete or lead. The thickness of shielding required can be anything between a few centimetres and a few metres, depending on the energy and intensity of the radiation.

Geiger counter

An instrument for detecting and measuring radiation. It consists of a tube filled with gas, in which an electrical discharge takes place whenever ionising radiation penetrates. The discharges are counted and are an indicator of the intensity of the radiation. It was named after H. Geiger and W. Müller, who invented it in the 1920s.

Half-life 

The time taken for half the amount of a radioactive material to change to a more stable form. Half-lives vary a great deal for different radionuclides, for instance from 1.5x1024 years for tellurium-128 to 2x10-16 seconds for beryllium-8. Iodine-131, for example, has a half-life of 8 days. This means that 1 kg of iodine-131 changes into 0.5 kg of iodine-131 and 0.5 kg of non-radioactive material over a period of 8 days. After 80 days (10 times the half-life), just one gram of the original kilogram of iodine-131 remains.

IAEA

International Atomic Energy Agency, a United Nations agency in Vienna, Austria.

Interim storage 

Temporary storage of processed radioactive waste pending solutions for its long-term management.

Intermediate level waste

Radioactive wastes in which the concentration of or quantity of radionuclides is above clearance levels established by the regulatory body, but with a radionuclide content and thermal power below those of high-level waste. Low and intermediate level waste is often separated into short lived an long lived wastes. Short-lived waste may be disposed of in near surface disposal facilities. Plans call for the disposal of long-lived waste in geological repositories.

Isotopes

Atoms of a chemical element with the same number of protons, but different numbers of neutrons in their nuclei. These atoms therefore have the same atomic number, but a different mass number, and are called isotopes of that element. Thus, carbon-12, carbon-13 and carbon-14 are isotopes of the element carbon. Isotopes of the same element have the same chemical properties, but different physical properties. For example, carbon-12 and carbon-13 are stable, whereas carbon-14 is radioactive.

Long-lived waste

Usually taken to mean radioactive waste containing significant quantities of radionuclides with half-lives of more than 30 years.

Multiple barrier principle 

Principle that involves working with several successive shielding materials in order to protect people and the environment against the potential hazards arising from radioactive waste. These successive barriers may be either natural or man-made.

NORM

Natural Occurring Radioactive Material

Nuclear energy

The energy from nuclei. According to Einstein’s famous formula, E = mc2, the energy contained in nuclei can be liberated in two ways: by radioactive decay or by nuclear fission. In common parlance nuclear energy usually means the enormous amount of energy liberated in a nuclear reactor during nuclear fission.

Nuclear facility

Any site, plant or facility in which radioactive materials are used. There are 2 regisitered sites in South Africa, being Pelindaba and Koeberg.

Nuclear fuel

Fissile (fissionable) material that produces energy by nuclear fission in a nuclear reactor by means of a controlled chain reaction. The energy contained in the nuclei is liberated in the form of heat. Examples of fissile material are uranium-235 and plutonium-239.

Nuclear power plant

An electricity-generating facility that uses nuclear fission in a nuclear reactor as its power (heat) source.

Plutonium (Pu)

A heavy, radioactive, man-made metallic element. Its most important isotope is fissile plutonium-239, which is produced by neutron irradiation of uranium-238 in a nuclear reactor. Radioactivity Physical phenomenon characterised by the disintegration, i.e. the reorganisation, of unstable nuclei, accompanied by the emission of ionising radiation. After one or more disintegrations, the unstable nucleus is transformed into a stable, non-radioactive nucleus.

Radionuclide

A radioactive nuclide or, in other words, a radioactive isotope of a chemical element. Hence, an element with an unstable nucleus that spontaneously decays, emitting ionising radiation. Approximately 2500 different radionuclides have been identified, divided over 112 chemical elements. More than 2200 of these are radioactive.

Regulator - for South Africa - NNR (National Nuclear Regulator)

An authority or a system of authorities designated by the government of a country or state as having legal authority for conducting the licensing process, for issuing licences and thereby for regulating the siting, design, construction, commissioning, operation, closure, closeout, decommissioning and, if required, subsequent institutional control of the nuclear facilities (e.g. neat surface repository) or specific aspects thereof. This authority could be a body (existing or the be established) in the field of nuclear related health and safety, mining safety or environmental protection vested and empowered with such legal authority.

Reprocessing

The processing of irradiated reactor fuel to separate the remaining unused fissile material from waste material (i.e. fission products). This fissile material is then recycled in fresh reactor fuel. The fission products are treated as waste.

Shielding

Any material that absorbs radiation from a radioactive source or reduces its intensity.

Short-lived waste

Usually taken to mean radioactive waste containing radionuclides that have a half-life less than or equal to 30 years, with a restricted concentration of long-lived alpha-emitting radionuclides.

Sievert (Sv)

The unit for dose equivalent, equal to 1 Joule/kilogram. It is an indicator of the harmfulness of a quantity of absorbed radiation energy or of the biological effect of radiation on living tissue.

Spent nuclear fuel

Nuclear fuel that has been permanently removed from a nuclear reactor because it can no longer sustain power production.

Surface or near-surface disposal

Disposal of radioactive waste in an infrastructure on or near the earth's surface.

Uranium

A naturally radioactive element with atomic number 92 (number of protons). Its principal natural isotopes are uranium-235 (0.72% of natural uranium), which is fissile in nuclear reactors, uranium-238 (99.3% of natural uranium) and uranium-234 (0.0056% of natural uranium), which is a daughter product of the radioactive decay of uranium-238.

Waste package

Comprises a quantity of processed radioactive waste and the container in which it is placed: a steel drum for low- and intermediate-level waste and a stainless steel canister for vitrified high-level waste.

Waste, radioactive

Any material for which no use is foreseen and which contains radioactive substances in quantities that cannot be released for radiation protection reasons.