As the reactor has only one steam generator, passive decay heat removal systems are diversified by being included in both the primary and the secondary circuit.

Secondary circuit

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

Primary circuit

The primary coolant system is after cooled by means of heat exchangers located in the downcomer, see Fig. IV-6. Each heat exchanger has a dedicated heat sink. There are 16 independent loops used for this purpose, altogether forming the RRP system (an abbreviated ‘residual heat removal on primary circuit’). There are two types of heat sinks:

• Four RRP loops are cooled by heat exchangers immerged in a pool (RRPp);

• The other twelve RRP loops are cooled by heat exchangers located in an air cooling tower (RRPa).

All RRP loops are designed to operate on natural convection both in the loop and in the heat sink.

The RRP design is very simple. RRP loops are designed to resist the primary pressure. Isolating valves are placed in RRP circuits to minimize the risk of the primary water passing outside the containment in the event of a heat exchanger tube rupture. A surge tank compensating for water dilation from a cold shutdown to a full power operating state carries out pressure control of the RRP circuit.

The control valves are placed on the level of the heat sink: thermal valves or air leaves function so that the temperature of the RRP loop remains high when the reactor is in power [IV-1]. In the case of an accident, the RRPs operate passively by opening the air leaves in the RRP air coolers or by opening the thermal valves in the RRP pools. These valves are automatically opened on a SCRAM signal.

Forced convection is only required when the chilled water cooling is requested for core refuelling. The 12 RRPa are able to cool the primary system down to a cold shutdown state. They replace the normal heat removal system of the reactor.