ASN Report 2020

There are three containment model designs: ∙ Those of the 900MWe 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 MWe and 1,450MWe reactors are made of two walls: the inner prestressed concrete wall and the outer reinforced concrete wall. Leaktightness is provided by the inner wall and the 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. 1.6  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 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 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. 1.7  The other systems important for safety The other main systems or circuits important for safety and required for reactor operation are: ∙ the Component Cooling System (RRI) which cools a certain number of nuclear equipment items. This system functions in a closed loop between the auxiliary and safeguard systems on the one hand and the systems carrying water from the river or sea (heatsink) on the other; ∙ the Essential Service water System (SEC) which cools the RRI system with water from the river or sea (heatsink). This is a backup system comprising two redundant lines. In certain situations, each of its lines is capable of removing heat from the reactor to the heatsink; ∙ the Reactor Cavity and Spent Fuel Pit Cooling and Treatment System (PTR), which in particular removes residual heat from the fuel elements stored in the fuel building pool; ∙ the ventilation systems, which contain radioactive materials by creating negative pressure in the rooms and by filtering discharges; ∙ the fire-fighting water systems; ∙ the I&C system, which processes the information received from all the sensors in the NPP. It uses transmission networks and sends orders to the actuators from the control room, through the programmable logic controllers or operator actions. Its main role with regard to reactor safety is to monitor reactivity, control the removal of residual heat to the heatsink and take part in the containment of radioactive substances; ∙ the electrical systems, which comprise sources and electricity distribution. The French nuclear power reactors have two external electrical sources: the step-down transformer and the auxiliary transformer. These two external sources are supplemented by two internal electrical sources: the backup diesel generators. In the event of total loss of these external and internal sources, each reactor has another electricity generating set comprising a turbine generator and each NPP has an ultimate backup source, the nature of which varies according to the plant in question. Finally, following the Fukushima Daiichi NPP accident (Japan), these resources were supplemented by an “ultimate back-up” diesel generator set for each reactor. 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 2020 289 10 – THE EDF NUCLEAR POWER PLANTS 10