ASN Report 2017

333 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants 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: ཛྷ ཛྷ 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; ཛྷ ཛྷ 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; ཛྷ ཛྷ the presence of neutron-absorbing elements in the fuel rods which, at the beginning of the cycle, compensates 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. EDF uses two types of fuels in its Pressurised Water Reactors (PWR): ཛྷ ཛྷ uranium oxide based fuels (UO 2 ) with uranium-235 enrichment to a maximum of 4.5%. These fuels are fabricated in several plants in France and abroad by Areva NP and Westinghouse; ཛྷ ཛྷ fuels consisting of a Mixture of depleted uranium oxides and plutonium (MOX). The MOX fuel is produced by the Areva NC Melox plant. The maximum authorised plutonium content is currently limited to 9.08% (average per fuel assembly) and provides an energy performance equivalent to UO 2 fuel enriched to 3.7% with uranium-235. This fuel can be used in the twenty-eight 900 MWe reactors for which the Creation Authorisation Decrees (DAC) provide for the use of MOX fuel. THE PRINCIPLE of pressurised water reactor operation CONVENTIONAL ISLAND NUCLEAR ISLAND Moisture separator- Superheater Generator Condenser Secondary system Steam generator Pressurizer Control room Primary pump Vessel Primary system Fuel pool Heater LP Turbine LP Turbine HP Turbine VVP EAS ASG ARE TEP RIS RRI RRA RCV PTR RIVER SEC LP or HP Turbine: low pressure or high pressure VVP: Main Steam System ASG: Auxiliary Feedwater System ARE: Feedwater Flow Control System PTR: Reactor Cavity and Spent Fuel Pit Cooling and Treatment System EAS: Containment Spray System RRA: Residual Heat Removal System RIS: Safety Injection System TEP: Boron Recycle System RCV: Chemical and Volume Control System RRI: Component Cooling System SEC: Essential Service Water System (ESWS)