Nuclear activities are defined by the Public Health Code as "activities involving a risk of human exposure to ionising radiation, emanating either from an artificial source - whether a substance or device - or from a natural source when natural radionuclides are or have been processed for their fissile or fertile radioactive properties, as well as interventions designed to prevent or mitigate a radiological risk following an accident or contamination of the environment". These nuclear activities include those conducted in basic nuclear installations (BNIs) and during transport of radioactive materials, as well as in all industrial and research facilities and hospitals where ionising radiation is used.

The common goal of nuclear safety and radiation protection is to protect people and property against hazards, detrimental effects or troubles of whatsoever nature, arising from the operation of nuclear or radiological facilities, the transport, use and transformation of radioactive or fissile substances, and exposure to natural radiation.

Nuclear safety is defined as encompassing all technical and organisational provisions relating to the design, construction, operation, shutdown and dismantling of facilities comprising a source of ionising radiation, as well as those relating to the transport of radioactive materials, and intended to prevent accidents and mitigate any consequences thereof.

Radiation protection is defined as the set of prevention and monitoring rules, procedures and means aimed at preventing or minimising the harmful effects of ionising radiation on persons directly or indirectly exposed, including through environmental contamination.

Responsibility for supervising the safety of nuclear installations and radioactive substance transports lies with the ministers for the Environment and Industry, while responsibility for supervising radiation protection lies with the Minister for Health and the Minister for Labour.

Decree 2002-255 of 22 February 2002, which created the Directorate General for Nuclear Safety and Radiation Protection, also gave this Directorate - under the authority of the ministers for Health, Industry and the Environment - responsibility for defining and implementing nuclear safety and radiation protection policy. The DGSNR together with the regional offices for which it organises and supervises activities in its area of competence, is referred to as the "Nuclear Safety Authority" (ASN).

Whether it consists of charged particles, for example an electron (beta radiation) or a helium nucleus (alpha radiation), or of electromagnetic radiation photons (X rays or gamma rays), ionising radiation interacts with the atoms and molecules making up the cells of living matter and alters them chemically. Of the resulting lesions, the most important concern the DNA of the cells and are not fundamentally different from those caused by certain toxic chemical substances.

When not repaired by the cells themselves, these lesions can lead to cell death and the appearance of health effects once tissues are no longer able to carry out their functions. These effects, called "deterministic effects", have been known for a long time, as the first effects were observed with the discovery of X rays by Roentgen. They become apparent once the quantity of radiation absorbed exceeds a certain dose level, depending on the type of tissue exposed; the effects increase proportionally to the dose of radiation received by the tissue.