ASN Report 2018

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 is 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. The copper contained in brass has bactericidal properties that titanium and stainless steels do not. 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. 1.5  ̶  Reactor containment building The PWR containment building has two functions: ∙ ∙ confine radioactive products liable to be dispersed in the event of an accident. The containments are therefore designed to withstand the pressures and temperatures that could result from the most severe reactor loss of coolant accident (double-ended circumferential rupture of a pipe in the primary system) and offer sufficient leaktightness in such conditions; ∙ ∙ protect the reactor against external hazards. There are three containment model designs: ∙ ∙ Those of the 900 MWe reactors comprise a single pre- stressed concrete wall (concrete comprising tensioned steel tendons 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. ∙ ∙ The 1,300 MWe and 1,450 MWe reactor containments consist of two walls: an inner wall made of pre-stressed concrete and an outer wall made of reinforced concrete. Leaktightness is provided by the inner wall and the ventilation system (EDE) which, in the annulus between the two walls, collects and filters residual leaks from the inner wall before they can be released. Resistance to external hazards is primarily provided by the outer wall. ∙ ∙ The Flamanville EPR reactor containment consists of two concrete walls and a metal liner covering the entire internal face of the inner wall. 1.6  ̶  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 in outage states, as well as providing containment of radioactive substances. This chiefly involves the Chemical and Volume Control System (RCV) and the Residual Heat Removal System (RRA). The purpose of the safeguard systems is to control incidents and accidents and mitigate their consequences. This chiefly concerns the following systems: ∙ ∙ the Safety Injection System (RIS), the role of which is to inject water into the primary system in the event of its 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 leak from the primary system; ∙ ∙ the Steam Generators 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. Reactor containments Pre-stressed concrete wall Pre-stressed concrete wall Metal sealing liner 900 MWe reactor containments 1,300 MWe/ 1,450 MWe reactor containments 1,650 MWe reactor containments Reinforced concrete wall Annulus Reinforced concrete wall Annulus Pre-stressed concrete wall Metal sealing liner ASN report on the state of nuclear safety and radiation protection in France in 2018  279 10 – EDF NUCLEAR POWER PLANTS 10