LES ACTIVITÉS NUCLÉAIRES : RAYONNEMENTS IONISANTS ET RISQUES POUR LA SANTÉ ET L’ENVIRONNEMENT ∙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. When the reactor is in operation, radioactive substances are contained by the positioning of three containment barriers between these substances and the outside environment: ∙the cladding around the fuel pellets retains the radioactive products contained in them; ∙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. At the beginning of an operating cycle, the core has a considerable energy reserve. This gradually decreases during the cycle, as the fissile nuclei are consumed. The chain reaction and thus the power of the reactor is controlled by: ∙the insertion of “control rod clusters”, containing neutron-absorbing elements, into the core to varying extents. This enables the reactor’s reactivity to be controlled and its power adjusted to the required production of electricity. Gravity dropping of the control rods is used for emergency shutdown of the reactor; ∙adjustment of the concentration of boron (neutron absorbing element) in the reactor coolant system water, which also helps compensate for the gradual depletion of the fissile elements in the fuel during the cycle; ∙the presence of neutron-absorbing elements in the fuel rods which, at the beginning of the cycle, compensate the excess core reactivity after partial renewal of the fuel. At the end of the cycle, the reactor core is unloaded so that some of the fuel can be replaced. VVP EAS RCV NUCLEAR ISLAND Separator Superheater Condenser Generator Secondary system Steam generator Reactor vessel Control room Primary system RRA Fuel pool ARE ASG TEP LP HP Turbine LP Turbine Turbine RIS RRI SEC WATER COURSE Reactor coolant pump Pressuriser 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 : Heat exchanger RRA: Residual Heat Removal System RRI: Component Cooling System SEC: Essential Service Water System TEP: Boron Recycle System LP or HP turbineP: Low-Pressure or High-Pressure Turbine VVP: Main Steam Systems : Pump CONVENTIONAL ISLAND Pressurised water reactor operating principle ASNR Report on the state of nuclear safety and radiation protection in France in 2025 283 01 05 02 06 03 07 04 08 09 10 11 13 12 A / Z
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