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

Acceptance
Verifying, with the help of administrative and physical controls, the compliance of the radioactive waste package with the acceptance criteria applicable to it.
See our section "Acceptance".

(Waste) Acceptance criteria (or specifications)
Document formulating the demands relative (of regarding) to a producer of radioactivewaste, to an installation, to a procedure, to a product and to the related documentation.
See our section "Acceptance criteria".

Activation
Action tending to render certain nuclides radioactive, in particular in materials in the structure of nuclear reactors, by bombarding them with neutrons or other particles.

Activity
The number of spontaneous disintegrations of the atom occurring per unit of time in a quantity of radioactive material. Activity is measured in becquerel, or Bq. The old unit was the curie, or Ci.

AFCN
See FANC.

ALARA principle
'As Low As Reasonably Achievable'. This is the principle that the exposure of man and the environment to ionizing 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 (ICRP).

Alpha particle
Positively charged particle emitted by certain radioactive substances during their radioactive disintegration. 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 or the outer layers of human skin are sufficient to absorb alpha radiation. Consequently, it is highly ionizing, i.e. that it easily rips electrons off the atoms of the penetrated material because alpha particles give up all their energy over a short distance.
See our section "Three types of radiation".

Alpha radiation
Radiation emitting alpha particles.
See our section "Three types of radiation".

Approval
See Qualification.

Aquifer (or water-bearing layer)
Permeable and porous geological unit, which contains exploitable quantities of water, for example by pomping.

Assembly (nuclear fuel)
Grouping of a number of fuel elements in a structural entity used for operating a nuclear reactor.

Atom
The smallest particle of a chemical element, which cannot be further broken down into a chemical reaction. Each atom consists of a nucleus of positively charged protons and neutral neutrons, surrounded by a 'cloud' or 'shell' of negatively charged electrons that orbit on one or several orbits round the nucleus. From an external point of view, the behaviour of atoms is electrically neutral, since the number of protons in the nucleus equals the number of electrons in the cloud. Atoms are tiny: in an average drop of water there are approximately 6000 trillion (21 zeroes after the six) atoms.

Atomic number
Number attributed to each element in the periodic classification of the elements. It equals the charge number, which is the number of protons contained in the nucleus of an atom (symbol Z).

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

Barrier
Natural or artificial shield to protect against the dispersion of radioactive materials and against ionizing radiation. See also multiple barrier principle.

Becquerel (Bq)
The unit used to measure radioactivity. 1 Bq equals one disintegration sustained by a radionuclide per second. This unit replaces the curie (Ci).

Belgoprocess
Subsidiary of ONDRAF/NIRAS ensuring the processing, the conditioning and the temporary storage of Belgian radioactive waste, as well as the dismantling of nuclear facilities, Dessel, Belgium.

Beta particle
Particle emitted from a nucleus during a certain type of radioactive disintegration. 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. A beta particle can also result from the disintegration of a neutron or an unstable particle.
See our section "Three types of radiation".

Beta radiation
Radiation emitting beta particles.
See our section "Three types of radiation".

Biosphere
The part of the earth crust, oceans and the atmosphere where living organisms can develop and live.

Bituminisation
Encapsulation in bitumen (tar, asphalt). Method used to confine certain types of radioactive waste, for example sludge arising from the processing of radioactive liquids.

Bq
See becquerel.

BR1
Belgian Reactor 1 : first Belgian nuclear reactor, put into service by the SCK·CEN in 1956.

BR2
Belgian Reactor 2 : testing reactor from the SCK·CEN used for the irradiation of materials with high neutron flux. It was put into service by the SCK·CEN in 1963.

BR3
Belgian Reactor 3 : first European experimental pressurized-water reactor, put into service by the SCK·CEN in 1962. It is being dismantled.

Cementation
Encapsulation in cement or concrete. Method used to confine certain types of radioactive waste, either solid (heterogeneous cementation) or liquid (homogeneous cementation).

CEN·SCK
See SCK·CEN.

Chain reaction
Series of nuclear fissions during which the released neutrons cause new fissions generating in their turn new neutrons causing new fissions etc.

Chemical element
See element.

Ci
See Curie.

CILVA
"Centrale Infrastructuur voor Laagactief Vast Afval", a plant designed for processing solid low-level waste on the site of Belgoprocess.

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

COGEMA
Compagnie générale des matières nucléaires: French company that operates, amongst others, a spent nuclear fuel reprocessing plant, La Hague, France.

Compaction (or supercompaction)
Industrial technique which consists in crushing materials using a press in order to reduce their volume to various degrees.

Conditioned waste
Radioactive waste conditioned into a form that can be handled, transported, stored and disposed of in a safe and economic way.

Conditioning
All the operations that make it possible, using a conditioning matrix, to obtain a solid, compact, chemically neutral and non-dispersing material, which therefore allows the transport and storage of the waste awaiting its final disposal. The conditioning generally comprises the immobilization of the waste by coating it in a matrix or by embedding (injection of a conditioning material in the free spaces), and its placing in a packaging.

Containment
All measures and means used to protect man and the environment against the dispersion of radionuclides in the biosphere.

Contamination (radioactive)
Presence of radioactive substances in a material, on the surface of objects or in places where they should not be present 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 through the inhalation or ingestion of solids, liquids or gas contaminated by radioactive substances. In the case of external contamination, the radioactive substances are in contact with the skin or the external parts of the organism.

Cosmic radiation
Ionizing radiation originating in outer space comprising primary particles with a very high energy (of extra-terrestrial origin) and secondary particles produced by the interaction of primary particles with the high layers of the atmosphere.

Curie (Ci)
Former unit used for measuring radioactivity, officially replaced in 1985 by the becquerel (Bq). 1 curie equals the activity of 1 gramme of radium and equals 37 billion Bq.

Decay
See radioactive decay.

Decommissioning
See our section "Decommissioning".

Decontamination
The removal or reduction of radioactive contamination in or on the surfaces of buildings, sites, objects or living organisms. Decontamination can be carried out using mechanical, chemical or electrochemical processes.

Depleted uranium
Uranium of which its isotope 235 content, the only fissile, is lower than its natural level (0,72 % in mass). It is mainly obtained on the one hand as a co-product resulting from an enrichment operation (around 0,3 % of uranium-235), and on the other hand as a daughter product (1 % of uranium-235) from the processing of spent nuclear fuel after having been used in a nuclear reactor.

Disintegration
See nuclear disintegration.

Dismantling
See our section "Decommissioning".

Disposal
See final disposal.

Disposal system
See final disposal system.

Dose equivalent
The quantity obtained by multiplying the absorbed dose by a coefficient, depending on the type of radiation. The dose equivalent is expressed in sievert (Sv).

Dosimeter
A small portable instrument for measuring and recording the total personal absorbed dose.

Effective dose
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, which is the sum of the weighted equivalent doses given to the various tissues or organs. The unit used is the sievert (Sv).

Electron
Negatively charged elementary particles (unless otherwise specified) that surround the positively charged nucleus. The electrons determine the chemical properties of the atom.

Element
Substance entirely composed of atoms of the same atomic number that cannot be further broken down into a chemical reaction. Currently there are 112 known elements, of which 92 occur naturally and 20 are artificial. Each element has a specific number of protons in its nucleus, referred to as the atomic number Z. Here are a few examples: hydrogen (Z = 1), carbon (Z = 6), gold (Z = 79), lead (Z = 82) and uranium (Z = 92).

Enriched fissile material
Nuclear fuel containing uranium of which one or several of its fissile isotopes has/have been enriched or to which chemically different fissile nuclides have been added.

Enrichment
Process which increases the content of a chemical element in one of its isotopes. In the case of uranium, it allows to increase, using various processes (gas dispersion, ultracentrifugation, selective excitation by laser), the concentration of the isotope 235 in comparison with the isotope 238 predominant in natural uranium.

Enriched uranium
Uranium of which its isotope 235 content, the only fissile, has been increased from its low natural level (0,72 % in mass) to, for example, 3,5 % for fuel destined for a nuclear reactor with water under pressure.

Equivalent dose
See dose equivalent.

Establishment
See nuclear establishment.

(EIG) EURIDICE
"European Underground Research Infrastructure for Disposal of nuclear waste In Clay Environment". EIG EURIDICE is the Economic Interest Grouping (EIG) between ONDRAF/NIRAS and the SCK·CEN, the Belgian nuclear research centre. EIG EURIDICE was created in Mol (Belgium) in December 2000, but was set up back in 1995 under the name EIG PRACLAY.

EUROCHEMIC
Experimental plant for the reprocessing of spent fuel, on the Belgoprocess site.

Exemption
See clearance.

FANC
(Belgian federal agency for nuclear control), Brussels, Belgium.

Far field (or geosphere) of a repository
Composed of the host formation and the aquifers that surround it.

FBFC International
Franco-Belge de Fabrication de Combustibles International, French-Belgian nuclear fuel manufacturer, Dessel, Belgium.

Final disposal
See our section "Long-term management".

Final disposal facility (or repository)
Construction destined to receive radioactive waste in the context of passive long-term management.

Final disposal in deep geological layers
See our section "Long-term management".

Final disposal system
Composed of the final disposal facility and the host formation. The system is situated in an environment itself composed of the aquifers located on each side of the host formation and the biosphere.

Fissile material
See nuclear fuel.

Fission
See nuclear fission.

Fission products
Nuclides produced by fission or daughter products of these nuclides.

Fuel cycle
See nuclear fuel cycle.

Gamma radiation
Electronic radiation composed of photons emitted during the process of nuclear transition or annihilation of the particles. High-energy electromagnetic radiation with a very short wavelength and no mass, which is emitted from many types of nuclei. Gamma radiation is just like ordinary light and X-rays, but has a much higher energy. Gamma radiation is very penetrating and can only be effectively absorbed by dense materials such as iron, concrete, lead or a sufficient thickness of water. 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.
See our section "Three types of radiation".

Geiger-Müller counter
An instrument for detecting and measuring radiation. It consists of a tube filled with gas, in which an electrical discharge takes place whenever ionizing radiation penetrates. The discharges are counted and are an indicator of the intensity of the radiation.

Geosphere
See Far field.

Gray (Gy)
The unit of absorbed dose. Expresses the quantity of energy that is transferred to a material by ionizing radiation per unit of mass of that material. 1 gray equals 1 joule per kilogramme.

Gy
See Gray.

HADES
"High-Activity Disposal Experimental Site". Underground research laboratory in a deep clay layer, the Boom clay, under the site of the SCK·CEN nuclear research centre in Mol (Belgium). The laboratory is managed and operated by EIG EURIDICE. It contributes to the feasibility studies on the disposal of radioactive waste in deep clay layers.

Half-life (radioactive period)
In the case of a unique process of radioactive disintegration, the average time needed for the activity of a radioactive source to be reduced to half of its initial value.

High-level waste
See our section "Classification".

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

ICRP
International Commission on Radiological Protection.

Ion
An atom or fragment of a molecule, molecule or group of molecules, carrying a positive or negative electric charge.

Ionization
The process of adding or removing one or more electrons from atoms or molecules, thereby creating ions. High temperatures, electrical discharges or nuclear radiation can cause ionization. Ionization is also the formation of ions by the splitting of molecules.

Ionizing radiation
Any radiation with sufficient energy to cause ionization in matter. Some examples are X-rays and alpha, beta and gamma radiation.

IRE
"Institut National des Radio-éléments": National Institute of Radioelements in Fleurus, Belgium.

Irradiated fissile material (spent nuclear fuel)
Nuclear fuel pellets of which the radiation level has reached the limit and that have been removed from a nuclear reactor after being used because they can no longer sustain power production without undergoing an appropriate treatment.

Irradiation
Exposure of a living organism or matter to ionizing radiation.

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

Long-lived waste
See our section "Classification".

Low-level waste
See our section "Classification".

Mass number
Total number of protons and neutrons in the nucleus of the atom of a nuclide (symbol A).

Matrix (conditioning or immobilization)
Material in which radioactive waste is contained in order to prevent the dispersion of radioactive substances into the environment. Examples include cement mortar, glass, bitumen, polymer.

Medium-level waste
See our section "Classification".

Molecule
A group of two or more atoms held together by strong (e.g. electrical) forces. A molecule is the smallest unit of a compound that can exist by itself and retain all of its chemical properties. Water, for example, consists of molecules of two hydrogen atoms and one oxygen atom.

MOX (Mixed Oxides)
Nuclear fuel based on a mixture of uranium oxides (natural or depleted) and plutonium.

Natural background radiation
Naturally occurring ionizing radiation, including cosmic radiation and radiation from naturally occurring radioactive substances.

Natural ionizing radiation
Ionizing radiation existing in nature in the absence of any nuclear facility or artificial radioactive source. It is caused by cosmic radiation and natural radioisotopes present in the earth crust and in the air.

NEA
Nuclear Energy Agency of the Organisation for Economic Cooperation and Development (OECD), Paris, France.

Near field
Composed of the components of the repository, including the radioactive waste, and the part of the host formation perturbed by the excavation.

Neutron
Elementary electrically neutrally charged particle, which enters with the protons into the composition of the nucleus of the atom. It is the neutron that causes the fission reaction of the fissile nuclei and whose energy is used by the nuclear reactors.

Non-conditioned waste
Radioactive waste that did not undergo conditioning.

Nuclear disintegration
Transformation of the nucleus through the fission in several nuclei or the emission of particles or beta particles. This transformation can be spontaneous or provoked by a nucleus or a particle.

Nuclear energy
The energy from nuclei. According to Einstein's famous formula, E = mc2, the energy contained in nuclei can be released in two ways: by radioactive disintegration or by nuclear fission. Nuclear energy usually means the enormous amount of energy released by a nuclear reactor during nuclear fission.

Nuclear facility
All of the objects, equipment, devices or buildings composing within an establishment a technical unit where one or more practices or professional activities are practised using ionizing radiation or radioactive substances.

Nuclear fission
The splitting of a nucleus into at least two other nuclei and the release of a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation, and can go on to cause further fission reactions in other nuclei, thus sustaining a chain reaction. It is also accompanied by the emission of gamma photons.

Nuclear fuel
Fissile 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 fuel cycle
The necessary steps to be able to use uranium as nuclear fuel for the production of electricity. These steps include the extraction and preparation of the uranium ore, the enrichment of the uranium, the production of fissile material pellets and fuel assemblies and their use in a reactor, the possible chemical reprocessing aimed at recovering the uranium remaining in the spent fuel as well as the obtained plutonium, and the potential production of new fuel assemblies.

Nuclear power plant
An electricity-generating facility that uses nuclear fission in a nuclear reactor to produce heat.

Nuclear transformation
Transformation of a radionuclide in another, for example alpha or beta disintegration.

Nucleus
The central, positively charged region of an atom. With the exception of the nucleus of ordinary hydrogen, which has a single proton, all the atomic nuclei contain positively charged protons and neutral neutrons.

Nuclide
A general term meaning any isotope X, stable (279) or unstable (approx. 5000), of the chemical elements characterised by its mass number A and its atomic number Z.

Oxidation
Reaction in which an atom or an ion loses electrons.

Package
See wastepackage.

PAMELA
Experimental plant (on the Belgoprocess site) for the vitrification of spent fuel reprocessed by EUROCHEMIC.

Period
See Half-live.

Plutonium (Pu)
A heavy, radioactive, artificial metallic element. Its most important isotope is fissile plutonium-239, which is produced by irradiation of uranium-238 with neutron in a nuclear reactor.

Positron
Elementary particle, anti-particle of the electron of the same mass and opposite charge.

PRACLAY
"Preliminary Demonstration Test for Clay Disposal", preliminary demonstration test in view of the final disposal of high-level radioactive waste in clay. Research programme of ONDRAF/NIRAS and the SCK·CEN.

Primary package
Composed of the primary packaging and the radioactive waste.

Primary packaging
First covering of the waste, including the possible internal shielding.

Processing
All of the mechanic, physical or chemical operations aiming to modify the characteristics of the waste. The object of processing is to render the waste proper for conditioning.
See our section "Processing".

Proton
Elementary stable nuclear particle with a positive electric charge. Each chemical element has a different number of protons in the nucleus. This number is called the atomic number.

Pu
See Plutonium.

Rad
Obsolete unit of absorbed radiation dose, officially replaced by the gray in 1985. 100 rad equals 1 gray.

Radioactive decay
Reduction of radioactivity over time through the emission of radiation as a result of the gradual transformation of radioactive elements into more stable elements.

Radioactive disintegration
Spontaneous nuclear transformation of a nucleus due to its radioactivity. This phenomenon causes radioactive decay.

Radioactive period
See half-life.

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

Radioactivity
Physical phenomenon characterised by the disintegration, i.e. the reorganization, of unstable atomic nuclei, accompanied by the emission of ionizing radiation. After one or more disintegrations, the unstable nucleus is transformed into a stable, non-radioactive nucleus.
See our section "Radioactivity".

Radioelement
A chemical element that is radioactive. The same radioelement can either have a natural or an artificial origin.

Radioisotope
See radionuclide (= radioactive isotope).

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

Release
The removal of all further regulatory radiological control by the competent authorities (Federal Agency for Nuclear Control) of substances, materials, facilities or sites for the reason that the associated risk has become sufficiently low. Implicit in the concept of release is the understanding that once said substances, materials, facilities or sites have been released, they are no longer subject to any restriction or radiological control. Consequently, radioactive materials that has been released, can be treated as residues or ordinary effluents, and substances, materials, facilities or sites that have been released for re-use or recycling can be sold or transferred to any person, company or association that is then free to use them for any purpose.

Release criteria
Criteria to be met before a low radioactive material can be released, that is to say, when the associated risk is deemed to be sufficiently low. The official criteria are laid down in the Royal Decree of 20 July 2001 (published in the Belgian official journal of Laws on 30 August 2001) regulating the protection of the population, workers and the environment from the danger of ionising radiation.

Rem
Obsolete unit for dose equivalent, officially replaced by the sievert (Sv) in 1985. 100 rem equals 1 sievert.

Repository
See final disposal facility.

Reprocessing
The processing of spent nuclear fuel from a reactor to restore the fissile material and separate fission products. The fissile material is processed in fresh nuclear fuel. The fission products are treated as waste.
See our section "Processing".

Retrieval
Possibility, during a certain time, to safely retrieve the waste disposed of with identical or comparable means to those that were used to put them in place. Retrieval is therefore one of the consequences of flexibility.

Rod (nuclear fuel)
Name given to an airtight tube containing the uranium and plutonium pellets used as nuclear fuel in a pressurized water reactor.

SAFIR
"Safety Assessment and Feasibility Interim Report", a preliminary report on the safety and feasibility of deep underground disposal. The SAFIR 1 report assesses the work carried out between 1974 and 1988 on the possible disposal of radioactive waste in the Boom clay layer in Belgium. The SAFIR 2 report assesses the work carried out between 1990-2000.

SCK·CEN
Belgian nuclear research centre, Mol, Belgium.

Shield
Shielding ensuring the biological and thermal protection around a nuclear reactor.

Shielding
Device used to ensure the protection against a radioactive source or to reduce its intensity.

Short-lived waste
See our section "Classification".

Sievert (Sv)
The unit used in radioprotection to determine the dose equivalent or the effective dose, as the case may be. It is equal to 1 joule/kilogramme and is an indicator of the harmfulness of a quantity of absorbed radiation energy or of the biological effect of radiation on a living organism.

Specifications
See acceptance criteria.

Spent (nuclear) fuel
See irradiated fissile material.

Storage
The holding of spent fuel or of radioactive waste in a facility that provides for its containment, with the intention of retrieval (awaiting solutions for its long-term management). Storage is by definition an interim measure.
See our section "Storage".

Supercompaction
See compaction.

Surface disposal
See our section "Long-term management".

Sv
See sievert.

SYNATOM
Société Belge des Combustibles Nucléaires.

Transformation
See nuclear transformation.

TransNuBel
Company taking care of the transport of radioactive waste in Belgium, Dessel.

TRANSRAD
Company taking care of the transport of radioactive waste in Belgium, Fleurus.

Uranium
A natural radioactive element with atomic number 92 (number of protons). Its principal natural isotopes are uranium-235 (0.72% of natural uranium), which is fissile, uranium-238 (99.3% of natural uranium) and uranium-234 (0.0056% of natural uranium). All three are alpha emitters.

Vitrification
Technique used to process and contain radioactive liquid high-level waste. The liquid waste is melted with glass beads (e.g. borosilicate) at a temperature of about 1100°C to obtain a homogeneous vitrified product. This molten mixture is then poured into sealed stainless steel canisters where it will solidify after cooling down.
See our section "Processing".

Vitrified radioactive waste
Processed radioactive waste using the vitrification technique.

Waste package
Composed of the primary packaging and its contents.

Water-bearing layer
See aquifer.