- 288 - Loss of the cooling systems / heat sink If the core inlet temperature is high, the reactor coolant pumps are stopped, emergency shutdown is initiated and the safeguard cooling systems - which are redundant and designed to earthquake design standards - are started. They comprise the RUC (A and B) systems, which branch from the main primary cooling system (RPP) and are cooled by the RUP (A and B) systems. The RUP (A and B) systems, which function in closed circuit on the pool, are cooled by the RUS (A and B) systems which are cooled in turn by two cooling towers in the safeguard buildings (BAS A and B). The two backup cooling towers installed on either side of the nuclear unit have a dedicated water reserve of 30 m3; once this water reserve has been used up , the low level of power remaining to be removed (less than 700 kW) means that the cooling tower can switch to dry operation. The starting command for the core safeguard cooling system (RUC) and the pools safeguard cooling system (reactor pool and intermediate pool) (RUP) in the reactor building, is accompanied by the starting command for the primary cooling system mixing pump. This pump temporarily ensures sufficient cooling if the two trains of the RUC system fail to start. The licensee indicates that to transfer the residual power from the core system to the reactor pool, it is planned to open the two natural convection valves located upstream and downstream of the core after 20 minutes, and to stop the mixing pump later on. According to the licensee, 1 hour and 20 minutes after reactor shutdown, the natural convection valves alone are sufficient to cool the core. In the ultimate case where the above means are not sufficient to cool the core, the facility has systems for resupplying water to the pools, including one ultimate system for resupplying the BUR pools from outside the nuclear buildings, by an external mobile means and quick-connect coupling (bleed skimmer, BUR pools make-up and filling circuit, REW system). It should be noted that the BAN pools do not have an equivalent ultimate system but the CEA does not consider this situation prejudicial: given the cooling time of the stored fuels, the total residual power of the fuels is 80 kW or less. It nevertheless proposes an improvement in this respect. Conclusion on the measures taken to protect the facilities against the risk of loss of the ultimate cooling system / heat sink The CEA concludes in its CSA report that loss of the heat sink is catered for by design. Management of this situation is based on redundant and independent backup means designed to earthquake design standards. To cope with loss of the backup means, the CEA concludes that improvements can be made to facilitate cooling by external backup means. Measures envisaged to reinforce the robustness of the facilities to loss of the ultimate cooling system / heat sink To cope with ultimate situations, the CEA envisages: creating 2 tappings on the secondary safeguard cooling systems of the core and the reactor building pools (RUS). These tappings will enable external means (tanker truck with stand-alone pump) to supply the RUS system with cold water without using the system pump and cooling tower; extending the capacities of the water make-up circuit (REW system) for resupplying the pools of the BUA. ASN notes the improvements proposed by the licensee to reinforce the robustness of the facility in the event of loss of the heat sink. It will issue requirements for the licensee to create tappings that will allow the supply of water by on-site means and take measures to have an on-site water supply capacity. Simultaneous loss of the cooling systems and the electrical power supplies In the event of simultaneous loss of the core cooling systems (normal and safeguard) and of the off-site and backup electrical power supplies, the mixing pump supplied by the ultimate backup battery (SUS B) ensures forced convection in the core for the time necessary (less than 2 hours) for the core residual power level to
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