1.5 The secondary system cooling system The function of the secondary system cooling system is to condense the steam exiting the turbine. To do this, it has a condenser comprising a heat exchanger containing thousands of tubes through which cold water from outside (sea or river) circulates. On contact with these tubes, the steam condenses and can be returned in liquid form to the SGs (see point 1.4). 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 (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. The copper contained in brass has bactericidal 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. 1.6 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 a primary or secondary system rupture and to ensure satisfactory leaktightness in these conditions; ∙ reactor protection against external hazards. There are three containment model designs: ∙ Those of the 900 MWe reactors comprise a single pre-stressed concrete wall (concrete comprising steel tendons tensioned to compress the structure in order to increase its tensile strength). This wall provides mechanical pressure resistance and ensures the integrity of the structure in the event of an external hazard. Tightness is provided by a metal liner covering the entire internal face of the concrete wall. ∙ Those of the 1,300 and 1,450 MWe reactors are made of two walls: the inner pre-stressed concrete wall and the outer reinforced concrete wall. Leaktightness is provided by the inner wall and by a Ventilation System (EDE) which, between the two walls, collects and filters residual leaks from the inner wall before discharge. Resistance to external hazards is primarily provided by the outer wall. ∙ That of the Flamanville EPR consists of two concrete walls and a metal liner covering the entire internal face of the inner wall. 1.7 The main auxiliary and safeguard systems In normal operating conditions, at power, or in reactor outage states, the auxiliary systems control nuclear reactions, remove heat from the primary system and residual heat from the fuel and provide containment of radioactive substances. They mainly comprise the reactor’s Chemical and Volumetric Control System (RCV) and the reactor’s Residual heat Removal System (RRA). The role of the safeguard systems is to control and limit the consequences of incidents and accidents. This chiefly concerns the following systems: ∙ the Safety Injection System (SIS), the role of which is to inject water into the primary system in the event of it leaking; ∙ the reactor building Containment Spray System (EAS), the role of which is to reduce the temperature and thus the pressure in the containment, in the event of a major primary system leak; ∙ the SGs Auxiliary feedwater System (ASG), which supplies water to the SGs if the normal feedwater system is lost, thus enabling heat to be removed from the primary system. This system is also used in normal operation during reactor outage or restart phases. After the Fukushima Daiichi NPP accident (Japan), the decision was taken to install a diversified water source, called the “ultimate water source”, which can be used in extreme situations to supply the SGs with water when the water reserves in the ASG system are empty and the various resupply solutions are no longer available. Pre-stressed concrete wall Pre-stressed concrete wall Reinforced concrete wall Containment of 900 MWe reactors Containment of 1,300 / 1,450 MWe reactors Containment of 1,650 MWe reactors (EPR) Reinforced concrete wall Annulus Reinforced concrete wall Annulus Pre-stressed concrete wall Metal sealing liner REACTOR CONTAINMENTS ASN Report on the state of nuclear safety and radiation protection in France in 2022 289 • 10 • The EDF Nuclear Power Plants 10 01 07 08 13 AP 04 06 12 14 03 09 05 11 02
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