ASN Report 2018

The nuclear island also comprises the systems removing steam to the conventional island as well as the building housing the new 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 equipment includes the turbine, the AC generator and the condenser. Some components of this equipment contribute to reactor safety. The secondary systems belong partly to the nuclear island and partly to the conventional island. 1.2  ̶  Core, fuel and fuel management The reactor’s core consists of fuel assemblies in the form of “rods” comprising “pellets” of uranium oxide or oxides of depleted uranium and plutonium (known as MOX fuel) contained in closed metal tubes, referred to as the “cladding”. As a result of fission, the uranium or plutonium nuclei, referred to as “fissile”, emit neutrons which, in turn, produce further fissions: this is known as the chain reaction. These nuclear fissions release a large amount of energy in the form of heat. The primary system water enters the core from below at a temperature of about 285°C, heats up as it flows up along the fuel rods and exits through the top at a temperature close to 320°C. At the beginning of the operating cycle, the core has a considerable energy reserve. This gradually falls during the cycle, as the fissile nuclei are consumed. The chain reaction, and hence reactor power, is controlled by: ∙ ∙ inserting control rod cluster assemblies, which contain elements that absorb neutrons, to varying depths in the core. This enables the reactor’s reactivity to be controlled and its power adjusted to the required production of electricity. Dropping the control rod assemblies under the effects of gravity enables the reactor to be shut down in an emergency; ∙ ∙ adjusting the boron concentration (which absorbs neutrons) in the primary system water during the cycle as the fissile material in the fuel gradually becomes depleted; ∙ ∙ 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 for renewal of part of the fuel. The principle of pressurised water reactor operation CONVENTIONAL ISLAND WP EAS RCV NUCLEAR ISLAND Moisture separator- Superheater Condenser Generator Secondary system Steam generator Vessel Control room Primary system RRA Fuel pool ARE TEP LP Turbine LP Turbine Turbine HP RIS RRI SEC RIVER Primary Pressurizer pump ASG PTR Heater ARE: Feedwater Flow Control System ASG: Auxiliary Feedwater System EAS: Containment Spray System PTR: Reactor Cavity and Spent Fuel Pit Cooling and Treatment System RCV: Chemical and Volume Control System RIS: Safety Injection System 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 VVP: Main Steam System ASN report on the state of nuclear safety and radiation protection in France in 2018  277 10 – EDF NUCLEAR POWER PLANTS 10