The residual heat removal system (RRA) functions during normal reactor outages to remove the heat from the primary system and the after-power from the fuel and then to keep the primary system water at a low temperature as long as there is fuel in the core. Once the chain reaction stops, the reactor core in fact continues to produce heat for a certain time. This after-power therefore has to be removed to avoid damaging the fuel. The RRA system is also used to drain the reactor cavity after refuelling.

The chemical and volume control system (RCV) is used during nuclear steam supply system (NSSS) operation:

• to adjust the mass of primary system water according to temperature fluctuations;
• to maintain primary system water quality, by reducing the corrosion and fission products content and by injecting chemical products (corrosion inhibitors for example);
• to collect and compensate for normal leakage from the primary pump seals;
• to regulate the boric acid concentration.

The purpose of the safeguard systems is to control incidents and accidents and mitigate their consequences. This primarily concerns the safety injection system (RIS), the reactor building containment spray system (EAS) and the steam generator auxiliary feedwater system (ASG).

The RIS system injects borated water into the reactor core in the event of an accident in order to smother the nuclear reaction and remove the after-power. It comprises passive pressurised accumulators and pumps with varying flow rates and release pressures for different types of accident situations. In the event of an accident, these pumps start by taking in water from a tank of about 2000 m3, the PTR tank. When the tank is empty, they are connected to the reactor building sumps, where the EAS spray water is collected, together with any water that has escaped from the primary system in the event of a leak on this system.

In the event of an accident leading to a pressure and temperature rise in the reactor building, the EAS system sprays water containing additional soda, in order to restore acceptable ambient conditions, protect the integrity of the containment and flush onto the floor any radioactive aerosols in the air.

The ASG system is used to maintain the secondary water level in the steam generators and thereby cool the primary system water in the event of failure of the normal feedwater system (ARE). It is also used in normal operation and during reactor shutdown and restart phases.

• the ventilation systems, which play a vital role in containing radioactive substances by depressurising the premises and filtering all discharges;
• the fire-fighting water systems;
• the reactor cavity and spent fuel pit cooling and treatment system (PTR), used notably to remove after-power from irradiated fuel elements stored in the spent fuel pit;
• the component cooling system (RRI), which cools a number of nuclear equipment items and operates in a closed loop between the auxiliary and safeguard systems and the systems carrying water pumped from the river or the sea;
• the essential service water system (SEC), which uses the heat sink (sea or river) to cool the RRI system.