ASN Report 2020

∙ the number of new fuel assemblies loaded following each reactor refuelling outage (generally one third or one quarter of the total number of assemblies). 1.3  The primary system and the secondary systems The primary system and the secondary systems transport the energy given off by the core in the form of heat to a turbine generator set which produces electricity. The reactor coolant system comprises cooling loops, of which there are three for a 900 MWe reactor and four for the 1,300MWe, 1,450MWe or 1,650MWe Evolutionary Power Reactor (EPR) type reactors. The role of the reactor coolant system is to extract the heat given off by the core by means of circulating pressurised “primary water” or “reactor coolant”. Each loop, connected to the reactor vessel containing the core, comprises a circulating pump, called the “reactor coolant pump” and a SG. The reactor coolant, heated to more than 300°C, is maintained at a pressure of 155 bar by the pressuriser, to avoid boiling. The primary system is entirely situated within the containment. The primary system coolant transfers its heat to the water of the secondary systems in the SGs. The SGs are heat exchangers which, depending on the model, contain from 3,500 to 5,600 tubes through which the reactor coolant circulates. These tubes are immersed in the secondary system water, which thus boils without coming into contact with the reactor coolant. Each secondary system primarily consists of a closed loop through which water passes, in the form of liquid in one part and in the form of steam in the other. The steam produced in the SGs is partially expanded in a high-pressure turbine and then passes through moisture separator-reheaters before entering the low- pressure turbines for final expansion, from which it passes to the condenser. Once condensed, the water is then sent to the SGs by the extraction pumps, followed by the feedwater pumps after passing through the reheaters. 1.4  The secondary system cooling system The function of the secondary system cooling system is to con­ dense the steam exiting the turbine. To do this, it has a con­ denser comprising a heat exchanger containing thousands of tubes through which cold water from outside (sea or river) cir­ culates. On contact with these tubes, the steam condenses and can be returned in liquid form to the SGs (see point 1.3). The water in the cooling system heats up in the condenser and is then either discharged into the environment (once-through circuit) or, if the river discharge is too low or the heating too great for the sensitivity of the environment, is cooled in a Cooling Tower (TAR) – semi-closed or closed circuit. The cooling systems are environments favourable to the devel­ opment of pathogenic micro-organisms. Replacing brass by tita­ nium 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. The copper contained in brass has bac­ tericidal properties that titanium and stainless steels do not. Air cooling towers can contribute to the atmospheric dispersal of legionella bacteria, whose proliferation can be prevented by stricter maintenance of the works (descaling, implementation of biocidal treatment, etc.) and monitoring. 1.5  The containment The PWR containment performs two functions: ∙ the containment of radioactive substances liable to be dispersed in the event of an accident; to do this, the containments were designed to withstand the temperatures and pressures that would result from the most severe loss of coolant accident (double-ended circumferential rupture of a reactor coolant system pipe) and to ensure satisfactory leaktightness in these conditions; ∙ reactor protection against external hazards. Moisture separators Feedwater ring Bundle wrapper Tube bundle Tube support plate Channel head Primary pumps Core instrumentation Control rod drive mechanisms Steam Generator Reactor core Reactor pressure vessel Reactor vessel head Pressurizer Steam discharge A Steam Generator and a main primary system for a 1,300 MWe reactor 288 ASN Report on the state of nuclear safety and radiation protection in France in 2020 10 – THE EDF NUCLEAR POWER PLANTS