Glossary
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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 ionising 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.
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.
High level waste
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.
Low-level waste
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.
