ASN Report 2017

334 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants 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: ཛྷ ཛྷ the nature of the fuel used and its initial fissile content; ཛྷ ཛྷ the maximum degree of fuel depletion at removal from the reactor, characterising the quantity of energy extracted per tonne of material (expressed in gigawatt days per tonne - GWd/t); ཛྷ ཛྷ the duration of a reactor operating cycle; ཛྷ ཛྷ the number of new fuel assemblies loaded at each reactor refuelling outage (generally 1/3 or 1/4 of the total number of assemblies); ཛྷ ཛྷ the reactor operating mode (at constant power or by varying the power to match demand), which determines the loads to which the fuel is subjected. 1.3 Primary system and secondary systems The primary system and the secondary systems transport the energy given off by the core in the form of heat to a turbo- generator set which produces electricity. The primary system consists of cooling loops (three loops for a 900 MWe reactor and four for a 1,300 MWe, 1,450 MWe or 1,650 MWe type EPR reactor). The role of the primary system is to extract the heat given off in the core by circulating pressurised water, referred to as the primary coolant or reactor coolant. Each loop, connected to the reactor vessel containing the core, comprises a circulating pump (known as the primary or reactor coolant pump) and a Steam Generator (SG). The primary water, heated to more than 300°C, is kept at a pressure of 155 bar by the pressuriser, to prevent it from boiling. The entire primary system is located inside the containment. The water in the primary system transfers its heat to the water in the secondary systems in the steam generators. The steam generators are heat exchangers that contain 3,500 to 5,600 tubes, depending on the model, through which the reactor coolant circulates. These tubes are immersed in the water of the secondary system and boil it, without ever coming into contact with the primary water. Each secondary system principally consists of a closed loop through which water runs in liquid form in one part and as steam in the other part. The steam produced in the steam generators is partly expanded in a high-pressure turbine and then passes through moisture separators before final expansion in the low-pressure turbines, from which it is then routed to the condenser. The condensed water is then heated by reheaters and sent back to the steam generators by the condensate extraction pumps and the feedwater pumps. 1.4 The secondary system cooling system The function of the secondary system cooling system is to condense the steam exiting the turbine. This is achieved by a condenser comprising a heat exchanger containing thousands of tubes through which cold water from outside (sea or river) circulates. When the steam comes into contact with the tubes it condenses and can be returned in liquid form to the steam generators (see point 1.3). The cooling system water that is heated in the condenser is then discharged to the natural environment (open circuit) or, when the river flow is too low or heating too great in relation to the sensitivity of the environment, it is cooled in a cooling tower (TAR) (closed or semi-closed circuit). The cooling systems are environments favourable to the development of pathogenic micro-organisms. Replacing brass by titanium or stainless steel in the construction of riverside reactor condensers, in order to reduce metal discharges into the natural environment, requires the use of disinfectants, mainly by means of biocidal treatment. Cooling towers can contribute to the atmospheric dispersal of legionella bacteria, whose proliferation can be prevented by reinforced treatment of the structures (descaling, implementation of biocidal treatment, etc.) and monitoring. A STEAM GENERATOR and a main primary system of a 1,300 MWe reactor Steam discharge Moisture separators Feedwater ring Bundle wrapper Tube bundle Tube support platev Channel head Primary pumps Core instrumentation Control rod drive mechanisms Steam generator Reactor core Reactor vessel Vessel head Pressurizer