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ASN - French nuclear safety authority

The independent administrative authority (created by Act no. 2006-686 of 13 June 2006 on transparency and security in the nuclear field – known as the “TSN” law) responsible for monitoring civilian nuclear activity in France. Working on behalf of the State, the ASN regulates nuclear safety and radiation protection in France, to protect workers, patients, the public and the environment from the risks involved in nuclear activities. It contributes to informing the general public.

IRSN - The French Institute for Radiation Protection and Nuclear Safety

The IRSN is France’s public service expert in the field of research on and the expert assessment of nuclear and radiological hazards.

Ionising radiation

The radioactive elements present in the environment emit alpha, beta and gamma radiation as they decay. Gamma rays are electromagnetic waves; alpha and beta radiation are particles (a helium nucleus and an electron, respectively). This type of radiation produces ionisation in the matter through which it passes; it is therefore potentially dangerous for living organisms.

Radioactive half-life

This is the time needed for the activity (number of decays per second) of a radioactive source to decrease by half.


Nuclear fission is the process by which the nucleus of a heavy atom (a nucleus containing many nucleons, such as uranium and plutonium nuclei) splits into 2 or more rarely 3 lighter nuclides, either after colliding with a neutron or spontaneously. This nuclear reaction also results in the emission of neutrons and gamma rays, and a substantial release of energy (about 200 MeV - compared to the energy generated by chemical reactions, which is measured in eV).

Fission products

Fission products are chemical elements produced by the fission of a fissile element. They are formed on the basis of a statistical distribution and include the isotopes of volatile and gaseous chemical elements (xenon, krypton, iodine ...) and of solid elements (caesium, strontium, ruthenium ...), which are usually radioactive. The most radioactive fission products have very short half-lives and disappear quickly. Other less radioactive fission products have lifetimes ranging from several months to several hundred years. Caesium 137 is one of the main medium-lived fission products, with a half-life of 30 years.


The unit of measurement of radioactivity: the number of radioactive atoms that disintegrate (decay) per unit of time (1 Bq = 1 disintegration per second). Multiples of Bq are generally used: 1MBq = 1 million Bq, 1GBq = 1 billion Bq, 1 TBq = 1000 billion Bq.

External exposure

The exposure to radiation from an external source causes external irradiation. It occurs when the source of radiation is outside the body (radioactive substances in cloud form or deposited on the ground, industrial or medical sources, etc.).

Internal exposure

Internal exposure occurs when the source of radiation is inside the body. It results in internal irradiation. The source may have entered the body by inhalation, ingestion or a skin injury, and then spreads throughout the body. This is called internal contamination. The contamination continues until the radioactive substances have disappeared from the body (a more or less rapid process) by natural elimination, radioactive decay, or medical treatment.

Safety review

These are performed every ten years in all French nuclear installations.

They involve a complete review of the safety studies on certain topics (selected by the operator and approved by the ASN) in the light changes in legislation and developments in know-how. The results of the review may require the operator to undertake major work to improve the safety of the installation.

Ground acceleration in seismology

g: The movement of the ground (“ground motion”) in the event of an earthquake is characterised in particular by its displacement, its velocity, its acceleration, its frequency and its duration.

To quantify ground acceleration in seismology, the unit of acceleration under the force of gravity, known as the “g”, is used. 1 g corresponds to 9.81m/sec2.