Words in boldface and marked with an asterisk are defined separately.

Chemical element with a nucleus containing more than 88 protons. In order, the actinides are actinium, thorium, protactinium, uranium and the transuranic elements*. Neptunium, americium and curium are often referred to as minor actinides.


Stands for "As Low As Reasonably Achievable". Maximum threshold of release or pollution to be observed, striking a balance between technical caution and cost-effectiveness.

ALPHA (radiation) ((See "RADIOACTIVITY")



Basic component of the chemical elements that form matter. It consists of a nucleus composed of positively charged or neutral particles (protons and neutrons), orbited by negatively charged particles (electrons).

A wall or enclosure which prevents the radioactivity from escaping.

Unit of measure of nuclear activity (1Bq = 1 atomic nucleus disintegration per second). The becquerel is a very small unit. The average human body has an activity of 8 000 Bq.

(radiation) (See "RADIOACTIVITY")

Energy source of vegetal (firewood, agricultural residues, etc.) or animal (dung) origin.

A Breeder is a nuclear reactor where neutrons absorbed in fertile* nuclei produce more fissile* nuclei than the latter disappear by fission.

Enrichment* process in which a gaseous mixture of isotopes* is spun at very high speed, using the centrifugal force to modify the composition of the mixture.

The metallic envelope of the fuel, which constitutes the first barrier*.

Solid mineral fuel made mostly of carbon. Coal is produced by the underground decay of very old forests.


During the construction of a facility designed to house radioactive materials, a series of containment barriers is put up between the materials inside and the environment outside the facility during construction. This creates separate areas called "containment areas".

Presence of an undesirable level of radioactive substances (dust or liquid) at the surface of or inside any medium. Contamination in humans can be external (on the skin) or internal (via the respiratory or digestive tracts).

A substance which, in a nuclear reactor*, impedes the chain reaction* by absorbing neutrons.

Areas where access and occupancy times are restricted for radiological protection reasons.

A liquid or a gas cooling the reactor core and transferring the heat to the turbine system.


The zone inside the nuclear reactor where the fission chain reaction occurs. The core is made of fuel assemblies*.


A medium containing a fissile nuclear material becomes critical when neutrons are produced (by the fission* of this material) at the same rate as they disappear (through absorption and leakage to the outside).

Heat produced by the radioactive decay of the fission products present in the nuclear fuel after the fission chain reaction is stopped.

Decommissioning is a process covering all the steps following the definitive shutdown of a nuclear facility or uranium mine, from the shutdown to the final elimination of radioactivity* on the site, including the dismantlement and decontamination* of all non recyclable pieces of equipment.

Decontamination is a physical, chemical or mechanical operation designed to eliminate or reduce the presence of radioactive or chemical materials deposited on or in a facility, open space, equipment, or personnel.

Successive defense lines to prevent – or eventually mitigate – technical or human failures which could lead to accidents.

DISPOSAL of nuclear waste (see STORAGE)
Final management of radioactive waste by depositing the properly conditionned waste package in a location especially designed to guarantee their containment* without time limitation.

Measurement characterizing the exposure of individuals submitted to radiation*. The term dose is often mistakenly used instead of dose equivalent.
- Absorbed dose: quantity of energy absorbed by matter (living or inert) exposed to radiation. It is expressed in grays (Gy).
- Dose equivalent: in living organisms, an absorbed dose has different effects depending on the type of radiation (X, alpha*, beta* and gamma*). To take these differences into account, a dose multiplying factor is used (known as the "quality factor") to compute a "dose equivalent".
- Effective dose: sum of weighted dose equivalents deposited on the different tissues and organs by internal and external irradiation. The unit of measure for effective dose is the sievert (Sv).
- Lethal dose: fatal dose of nuclear or chemical origin.
- Maximum permissible dose: dose that must not be exceeded for a given period of time.

Gray (Gy): unit of measurement for the absorbed dose (1 gray = 1 J/kg).
Sievert (Sv): unit of measure for the dose equivalent, i.e. the fraction of energy contributed by ionizing radiation and received per kilo of living matter. On the basis of the measured energy dose received (measured in grays), the dose equivalent is calculated by applying different factors according to the type of radiation received and the organ concerned.

For example, the mean annual dose from exposure to natural background radiation (soil, cosmos, etc.) of the population in France is 2.4 mSv/person.

Energy associated with the motion of electrons within a “conductor”.

Fundamental particle with a small mass and a negative electrical charge. The electron is one of the constituents of atoms*.

In the field of Physics, energy is the property of a system able to produce heat or mechanical work.

Particule fondamentale de masse très petite portant une charge électrique négative. L’électron est l’un des constituants des atomes*.

Assemblage solidaire de crayons remplis de pastilles d'uranium ou de MOX (dans le ces des réacteurs à eau ordinaire ou à eau lourde). Suivant les types de centrales, le cœur du réacteur contient entre 100 et 200 assemblages de combustible.

Au sens de la Physique, l’énergie est la propriété d’un système capable de produire de la chaleur ou du travail mécanique.

Before it is used to manufacture fuel elements*, natural uranium is enriched with U235 (the proportion of U235 is then 3% to 5%) Uranium enriched in U235 is obtained from natural uranium using an isotope separation process. The physical or chemical processes used to produce enriched uranium also produce at the same time uranium that has a lower proportion of U235 than natural uranium: this is known as depleted uranium.

Process used to increase the abundance of fissile isotopes* in an element. Naturally occurring uranium* is composed of 0.7% 235U (fissile) and 99.3% 238U (non-fissile). To make it suitable for use in a pressurized water reactor*, the proportion of 235U is increased to about 3 to 4%.

All the natural and societal conditions within which Mankind develops and which constitutes the frame of its life on Earth.

Exposure of an organism to a source of radiation* characterized by the dose* received.
- External exposure: exposure from a radiation source located outside the organism.
- Internal exposure: exposure from a radiation source located inside the organism.

A nucleus which, after absorption of a neutron an a series of radioactive decays produces a fissile* nucleus.

A nucleaus which undergoes fission easily.

Splitting of a heavy nucleus – generally upon impact with a neutron – into two smaller nuclei (fission products), accompanied by the emission of neutrons and radiation*, and the release of a considerable amount of heat. The energy thus released as heat is the underlying principle of nuclear energy.

Fragments of heavy nuclei produced by nuclear fission* (splitting of 235U or 239Pu nuclei) or subsequent radioactive decay of nuclides formed during this process. All fission fragments and their daughter products are called "fission products". In reprocessing* plants, they are separated by extraction with a solvent after dissolving the fuel in nitric acid, then concentrated by evaporation and stored before being conditioned* as a vitrified product placed in a stainless steel canister.

All the industrial operations undergone by nuclear fuel*. These operations include: extraction, processing uranium ore, conversion, uranium enrichment*, fuel manufacturing*, reprocessing* spent fuels, and waste* management. The fuel cycle is "closed" if it includes the reprocessing of spent fuel and recycling of fissile materials resulting from reprocessing. The term "once-through" cycle means that the fuel is disposed of in a permanent storage site after its use in the reactor.

Assembly of rods joined together and filled with uranium or MOX pellets. Depending on the type of plant, the reactor core contains from 100 to 200 fuel assemblies.

Metal tube (about 4 m in length and 1 cm in diameter) filled with pellets (about 300) of nuclear fuel*.

GAMMA (radiation) (See"RADIOACTIVITY")

Process for separating molecular species, based on the difference in the velocity of these molecules (due to their different mass and dimensions) as they pass through a semi-permeable membrane. This is how uranium* hexafluorides, 235UF6 and 238UF6, are separated for the enrichment* of nuclear fuel with 235U.

GRAY (Gy) (See "DOSE")

INES (International Nuclear Event Scale) is an international scale used to define the severity of an event occurring in a nuclear facility. Like the scales used in the field of seismology or avalanches, for example, this scale serves as a tool for informing the media and the general public. Events are classified in increasing order of severity from level 0 to level 7. As an example, the Chernobyl accident was the only level 7 event recorded to date.

Exposure to radiation* and, by extension, its effects.

Elements whose atoms* have the same number of electrons and protons but a different number of neutrons. For example, uranium has three isotopes: 234U (92 protons, 92 electrons, 142 neutrons), 235U (92 protons, 92 electrons, 143 neutrons), 238U (92 protons, 92 electrons, 146 neutrons). A given chemical element can therefore have several isotopes with a differing number of neutrons. All the isotopes of a given element have the same chemical properties, but different physical properties (mass in particular).

Ratio of the number of atoms of a given isotope* of an element to the total number of atoms* of this element contained in a material. It is expressed as a percentage.

The laws of Kirchhoff describe the way according to which an electrical current is shared among the various branches of a mesh: tThe sum of the intensities entering a node must be equal to the sum of the intensities leaving it, and the voltage between two nodes must be independent from the road taken by the current.

Process for extracting certain compounds contained in a powdery, permeable or porous medium, using a suitable solvent that flows naturally through the material to be processed. This method can be applied directly to highly fragmented soil (in-situ leaching), or to material that has been extracted, broken up and placed on a suitable area (heap leaching). This process is used to extract metal elements, including uranium*. The same process is involved when rainwater runs through a mass of waste and extracts certain components from it.

Light water means "ordinary" water (H2O), as opposed to heavy water (D2O) which is a combination of oxygen and deuterium (heavy hydrogen atom). It is used both to cool the fuel* and to slow down neutrons to provoke fission* in certain reactors*.

LIGNITE (or Brown Coal)
Poor quality coal with a high ashes and water content.

Material with light nuclei able to slow neutrons down after a fission*.

("Mixed Oxides"): mixture of uranium* and plutonium* oxides used to make certain nuclear fuels*.

Mostly methane CH4.


Nuclide that releases energy when it is consumed by fission* inside a reactor*. By extension, any product containing fissile materials that supplies energy in a reactor core by sustaining the chain reaction.

In the nuclear industry, nuclear safety covers all the measures taken at every stage of the design, construction, operation and final shutdown of a facility to ensure operational safety, prevent incidents, and limit their impact.

Natural mineral oil used mainly as a fuel.

Ionized gas. State of high temperature matter in which ions and electrons are dissociated.

Chemical element with the atomic number 94 and conventional symbol Pu. Plutonium-239, a fissile isotope*, is produced in nuclear reactors* from uranium-238.

Electromagnetic waves (such as radio waves, light waves, UV or X rays, cosmic rays, etc.), and particles (electrons, protons, and neutrons) or groups of these particles. The energy of these waves or particles is respectively proportional to their frequency and velocity. These types of radiation act on irradiated objects by stripping electrons from their atoms, leaving ionized atoms (electrically charged) in their wake, whence the generic name of IONIZING RADIATION.

Natural decrease in the nuclear activity of radioactive substance through spontaneous disintegration.


Time required for half the atoms* contained in a sample of radioactive substance to disintegrate naturally. The radioactivity* of the substance has therefore been halved. The half-life varies with the characteristics of each radionuclide:
- 8 days for iodine-131,
- 30 years for Caesium 137,
- 4.5 billion years for uranium-238.
No external physical action is capable of modifying the half-life of a radionuclide*.

Non-reusable by-products of the nuclear industry, research and medicine. They are classified according to their radioactivity and the half-life of the radionuclides present in them.

Emission by a chemical element of electromagnetic waves and/or particles caused by a change in its nucleus. Emission can be spontaneous (natural radioactivity of certain unstable atoms) or induced (artificial radioactivity). Radioactivity has several forms:
- Emission of "alpha* particles" (combination of 2 protons and 2 neutrons), called "alpha radiation".
- The particles making up alpha radiation are helium-4 nuclei that are highly ionizing but not very penetrating. A single sheet of paper stops them.
- Emission of electrons, known as "beta* radiation".
- The particles making up beta radiation are electrons with a negative or positive charge. They can be stopped by a few meters of air or a single sheet of aluminum foil.
- Emission of electromagnetic waves, called "gamma radiation".
- Electromagnetic radiation like light and X rays. Thick, compact materials (concrete, lead) are required to stop it.

All these different types of radiation are grouped under the general heading of "ionizing radiation". The radioactivity of an isolated quantity of an element gradually decreases over time as the unstable nuclei disappear. The half-life is the time required for the radioactivity of a radioactive substance to be halved.

Any radioactive substance. There are only a few naturally occurring radionuclides, including some heavy elements (thorium, uranium*, radium, etc.) and some light elements (carbon-14, potassium-40, etc.). The others, more than 1,500 in all, are created artificially in the laboratory for medical purposes or for use in nuclear reactors* as fission products.

Natural radioactive gas contained in the soil. It reaches the atmosphere through natural cracks and cavities in the soil and builds up in caves, cellars, homes, etc when ventilation is inadequate.

Device in which controlled nuclear reactions are carried out. The heat released by these reactions is harnessed to form the water vapor used to operate a turbine driving an electric generator. Models vary according to the type of fuel, the moderator used to control the reaction, and the coolant used to remove the heat to be recovered. The model currently used by EDF uses slightly enriched uranium fuel, and pressurized light water as moderator and coolant (PWR).

- Boiling Water Reactor (BWR): nuclear reactor in which boiling pressurized water is used to extract the heat from the reactor.
- Pressurized Water Reactor (PWR): nuclear reactor moderated and cooled by light water maintained in the liquid state in the core through appropriate pressurization under normal operating conditions.

Processing spent fuel to extract fissile and fertile materials (uranium* and plutonium*) for recycling purposes and to condition the different types of waste* in a form suitable for disposal. Fission products and transuranic elements* are vitrified.

Non-reusable material remaining after a physical or chemical operation. In the field of reprocessing, the term has a more specific meaning and covers all waste that has undergone conditioning*.

Packing specially designed to ensure total containment of certain radioactive materials (spent fuel, vitrified waste, etc.) during their transportation, and to withstand any accidents.


Pool in which spent fuel* is stored after being removed from the reactor* to allow the assemblies to lose most of their radioactivity* through radioactive decay. The water protects the personnel from the radiation* emitted by the spent fuel.

STIRLING (motor, cycle)

The Stirling motor, or hot air motor, operates on a conventional 4 steps cycle (heating, decompression, cooling, compression), but the heat source is external and there is neither admission nor escape of gas.

of nuclear waste (see DISPOSAL)
Interim deposition of radioactive waste under surveillance.


A way of social life which allows satisfying the needs of a human generation without compromising the satisfaction of the needs of future generations.

Chemical elements in which the nucleus contains more protons than that of uranium*. The first transuranic elements are, in increasing order, neptunium, plutonium*, americium and curium.


Chemical element with the atomic number 92 and conventional symbol U, with three natural isotopes: 234U, 235U (0.71%) and 238U (99.28%). 235U is the only naturally occurring fissile nuclide, which is why it is used as a source of energy.

The uranium contained in nuclear fuels* must be enriched with fissile 235U. Enrichment* is achieved by gaseous diffusion* or using the centrifugal process* (ultracentrifugation). For this purpose, the uranium is first converted into a gas called uranium hexafluoride.


Process used to solidify concentrated solutions of fission products and transuranic elements separated during spent fuel reprocessing by mixing them with a glass matrix at high temperature.


A device or facility which gathers the wind energy and transforms it into electricity.

Uranium concentrate with 80% U.