ASN Report 2020

The electricity generating reactors are at the heart of the nuclear industry in France. Many other installations described in other chapters of this report produce the fuel intended for the Nuclear Power Plants (NPPs) or reprocess it, dispose of the waste from the NPPs or study physical phenomena related to the operation or safety of these reactors. The French reactors are technically very similar and thus form a standardised fleet operated by EDF. Although this uniformity means that the licensee and the French Nuclear Safety Authority (ASN) have extensive experience of their operation, it also means that there is a higher risk if a generic design, manufacturing or maintenance flaw is detected on one of these installations, as it could then affect all the reactors. ASN thus requires a high degree of reactivity and rigour on the part of EDF when analysing the generic nature of these flaws and their consequences for the protection of people and the environment, as well as when processing them. ASN exercises extremely stringent oversight of safety, of environmental protection and radiation protection measures in the NPPs and continuously adapts it, in particular in the light of experience feedback from the design, manufacture, operation and maintenance of NPP reactor components. To monitor the safety of the reactors in operation, under construction or being planned, ASN mobilises nearly 200 staff on a daily basis in the Nuclear Power Plant Department (DCN), the Nuclear Pressure Equipment Department (DEP) and its regional divisions, and can draw on nearly 200 experts from the French Institute for Radiation Protection and Nuclear Safety (IRSN). ASN develops an integrated approach to the oversight of the facilities. It intervenes at all stages in the life of the NPP reactors, from design up to decommissioning and delicensing. Through its expanded scope of intervention it examines the fields of nuclear safety, environmental protection, radiation protection, occupational safety and the application of labour laws, at all stages. For each of these fields, it monitors all aspects, whether technical, organisational, or human. This approach requires that it take account of the interactions between these fields and that it define its monitoring actions accordingly. The resulting integrated overview enables ASN to fine-tune its assessment of the state of nuclear safety, radiation protection, environmental protection and worker protection within the NPPs. The EDF Nuclear Power Plants 10 1. General information about Nuclear Power Plants 1.1  General presentation of a Pressurised Water Reactor By transferring heat from a hot source to a heat sink, an electricity generating thermal power plant produces mechanical energy that it converts into electricity. Conventional thermal power plants use the heat given off by the combustion of fossil fuels (fuel oil, coal, gas). NPPs use that given off by the fission of uranium or plutonium atoms. The heat produced in a Pressurised Water Reactor (PWR) leads to the creation of steam, which does not come into contact with the nuclear fuel. The steam is then expanded in a turbine which drives a generator producing a three-phase electric current with a voltage raised to 400,000 volts (V) by a transformer. After expansion, the steam passes through a condenser where it is cooled on contact with tubes circulating cold water from the sea, a water course (river) or an atmospheric cooling circuit. The condensed water is reused in the steam production cycle. Each reactor comprises a nuclear island, a conventional island, water intake and discharge structures and possibly a cooling tower. The nuclear island mainly comprises the reactor vessel, the reactor coolant system, the Steam Generators (SG) and the systems ensuring reactor operation and safety: the chemical and volumetric control, residual heat removal, safety injection, containment spray, SG feedwater supply, electrical, Instrumentation and Control (I&C) and reactor protection systems. These elements are also associated with systems providing support functions: monitoring and processing of primary effluents, water supply, ventilation and air conditioning, back-up electricity supply (diesel electricity generating sets). The nuclear island also comprises systems for the evacuation of steam to the conventional island, as well as the building housing the fresh and spent fuel storage and Cooling Pool (BK). When mixed with boric acid, the water in this pool helps absorb the neutrons emitted by the nuclei of the fissile elements in the spent fuel, to avoid sustaining nuclear fission, to cool the spent fuel and to provide the workers with radiological protection. The conventional island notably comprises the turbine, the generator and the condenser. Some components of these items take part in reactor safety. The secondary systems are partly in the nuclear island and partly in the conventional island. 1.2  The core, fuel and its management The reactor core consists of fuel assemblies made up of “rods” comprising “pellets” of uranium oxide and depleted uranium oxide and plutonium oxide (for Mixed OXydes fuels – MOX), contained in closed metal tubes, called “cladding”. When fission occurs, 286 ASN Report on the state of nuclear safety and radiation protection in France in 2020