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Nuclear Glossary G - Z

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.


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.


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.


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.


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. 
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.


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.


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.


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.


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.