In case of an accident, the heat removal device should not release steam in the atmosphere during a SGTR (steam generator tube rupture). In case of over pressure transient, the released steam is condensed in a dedicated pool. The steam generator is not considered as the main system for decay heat removal. It acts as a thermal buffer until the safety systems on the primary side are fully operational.

XIX-4.1.2. System on the primary circuit

The primary system is cooled by means of the heat exchangers located in the downcomer (Figure XIX-4). Each exchanger has a dedicated heat sink. So there are sixteen independent loops, called RRP system (Residual heat Removal on Primary circuit). There are two types of heat sinks:

• Four RRP are cooled by immersed heat exchanger in a pool (RRPp);

• Other twelve are cooled by heat exchanger in air-cooling tower (RRPa).

All RRP are able to operate in natural convection in the loop and in the heat sink.

The design of the circuits is very simplified. The RRP loops are designed to resist the primary pressure. Isolating valves are placed on the circuit to avoid the risk of primary water passage outside the containment in the event of heat exchanger tube rupture. A surge tank for the water dilations from the cold shutdown to the full power operating condition carries out the pressure control of RRP circuit.

There is no control valve on the secondary loop, those are placed on the level of the heat sink: thermal valves or air leaves so that the temperature of the RRP loop remains high when the reactor is in power. Therefore, the RRP is ready to passively operate, just by the opening of the air leaves on RRP air coolers or by the opening of the thermal valves on RRP pools.

Forced convection is only required when the chilled cooling is needed for core refuelling. The twelve RRPa are able to cool the primary system down to cold shutdown. They replace the normal reactor heat removal system.