ASN Report 2020

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 during the cycle according to the gradual depletion of the fissile elements in the fuel; ∙ 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. EDF uses two types of nuclear fuel in the PWRs: ∙ uranium oxide (UO 2 ) based fuels enriched with uranium-235 to a maximum of 4.5% by mass. These fuels are fabricated in several French and foreign plants, by Framatome and Westinghouse; ∙ fuels consisting of a mixture of depleted MOX. MOX fuel is produced by Orano’s Melox plant. The maximum authorised plutonium content is currently set at 9.08% (average per fuel assembly) giving an energy performance equivalent to UO 2 fuel enriched to 3.7% uranium‑235. This fuel can be used in the twenty-four 900 Megawatts electric (MWe) reactors, for which the Creation Authorisation Decrees (DAC) authorise the use of plutonium fuel. EDF has standardised how the fuel is used in its reactors, referred to as “fuel management”. Fuel management, which concerns similar reactors, is more particularly characterised by: ∙ the nature of the fuel and its initial fissile material content; ∙ the maximum burnup of the fuel when removed from the reactor, characterising the quantity of energy extracted per ton of material, expressed in gigawatt days per tonne (GWd/t); ∙ the duration of a reactor operating cycle; VVP EAS RCV NUCLEAR ISLAND Separator Superheater Condenser Generator Secondary system Steam Generator Reactor Vessel Control room Primary RRA system Fuel pool ARE TEP LP Turbine LP Turbine HP Turbine RIS RRI SEC WATERCOURSE Reactor coolant pump Pressurizer ASG 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 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 CONVENTIONAL ISLAND Pressurised Water Reactor operating principle ASN Report on the state of nuclear safety and radiation protection in France in 2020 287 10 – THE EDF NUCLEAR POWER PLANTS 10