1.2 Safety principles The design of the nuclear reactors is based on safety principles aimed at ensuring the safety functions: ∙ control of core reactivity, that is control of the nuclear chain reactions; ∙ removal of the thermal power produced by the radioactive substances and nuclear reactions; ∙ containment of radioactive substances. The aim is to prevent the dispersal of radioactive substances into the environment and to protect people and the environment from ionising radiation. The design of nuclear facilities is based on the principle of “Defence-in-Depth”, which leads to the implementation of successive defence levels (intrinsic characteristics, material provisions and procedures), intended to prevent incidents and accidents, and then, if the preventive measures fail, to mitigate their consequences. Radioactive substances are contained by three successive containment barriers between these substances and the outside environment: ∙ the cladding around the fuel rods retains the radioactive products contained in the fuel pellets; ∙ the primary system, which constitutes a second envelope capable of retaining the dispersal of radioactive products contained in the fuel if the cladding fails; ∙ the containment, which is the concrete building housing the primary system. In the event of an accident, it is designed to contain the radioactive products released by a failure of the primary system. 1.3 The core, fuel and its management The reactor core consists of fuel assemblies made up of “rods” comprising “pellets” of uranium oxide or depleted uranium oxide and plutonium oxide (for Mixed OXide –MOX fuels), contained in closed metal tubes, called “cladding”. When fission occurs, the uranium or plutonium nuclei, said to be “fissile”, emit neutrons which in turn trigger other fissions: this is the chain reaction. The nuclear fissions give off a large amount of energy in the form of heat. The water in the reactor coolant system, which enters the lower part of the core at a temperature of about 285°C, heats up as it rises along the fuel rods and comes out through the top at a temperature of close to 320°C. VVP EAS RCV NUCLEAR ISLAND Separator Superheater Condenser Generator Secondary system Steam Generator Reactor Vessel Control room Primary system RRA Fuel pool ARE TEP Turbine LP LP HP Turbine Turbine RIS RRI SEC WATERCOURSE Reactor coolant pump Pressurizer PTR Reheater ARE: Feedwater Flow Control System ASG: Steam Generators Auxiliary Feedwater System EAS: Reactor building Containment Spray System PTR: Reactor Cavity and Spent Fuel Pit Cooling and Treatment System RCV: Chemical and Volume Control System RIS: Safety Injection System : Exchanger RRA: Residual Heat Removal System RRI: Component Cooling System SEC: Essential Service Water System (ESWS) TEP: Boron Recycle System LP or HP Turbine: Low-Pressure or High-Pressure Turbine VVP: Main Steam Systems : Pump CONVENTIONAL ISLAND PRESSURISED WATER REACTOR OPERATING PRINCIPLE ASN Report on the state of nuclear safety and radiation protection in France in 2021 281 10 – THE EDF NUCLEAR POWER PLANTS 08 07 13 04 10 06 12 14 03 09 05 11 02 AP 01
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