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Nuclear Glossary : A - F


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.


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.


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.


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.


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


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.


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


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


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.


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.


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.


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).


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


Being exposed to radiation from a radioactive source.