- 289 - be compatible with the changeover to natural convection achieved by opening the two natural convection valves situated upstream and downstream of the core. Without water make-up via the skimmer BUR pools filling and water make-up circuit (REW system) from outside the nuclear auxiliary building (external mobile means and quick-connect coupling) as envisaged by the CEA, water evaporation from the pool results in the following time lapses before the fuel elements become exposed: 35 days after reactor shutdown if it was operating at maximum power, 18 days in exceptional maintenance situation (5 days after shutdown of the reactor having operated at maximum power, authorised pool level at -4.5 m), several months in the case of the nuclear auxiliary building spent fuel pools. Measures envisaged to reinforce the robustness of the facility to loss of the main cooling system combined with total loss of the off-site and backup electrical power supplies In addition to the measures envisaged by the CEA further to the study of the loss of both the electrical power supplies and cooling systems, the CEA proposes permanently storing a volume of water in one of the drainage tanks of the Nuclear Unit pools (these tanks are situated in the nuclear auxiliary building, BUA) so that the BUA pools water can be replenished by the pool water make-up system of the water block, connected to the backup power network, after resupplying the train A or B backup electrical panels. Measures to reinforce the robustness of the facilities to the risk on which the CEA has given undertakings under the CSA review and the proposed studies In its CSA report, the CEA highlighted the reactor emergency shutdown function and a number of items of equipment that could guarantee removal of sufficient residual power to prevent core meltdown. The items of equipment are: the mixing pump and the ultimate backup battery SUS B, with an autonomy of two hours; the natural convection valves and the ultimate backup battery SUS A, with an autonomy of six hours. The licensee indicates that these items of equipment were selected because it estimates that by mixing the primary cooling water for 1 hour 20 minutes after reactor shutdown, and opening the natural convection valves 20 minutes after reactor shutdown, the reactor is maintained in a thermohydraulic condition that leaves a substantial time lapse before there is a real meltdown risk. However, no substantiating study was provided for the CSA review. During the review, the CEA undertook to examine in greater depth the possibility of defining a "hard core of severe accident prevention measures" focused on the primary cooling system mixing pump compared with a hard core focused on the safeguard cooling systems. This procedure lies within the framework of the more general request to define a hard core of material and organisational provisions mentioned earlier. Furthermore, with regard to accident situation management, the CEA has undertaken to evaluate the robustness of the instrumentation considered important for monitoring the situation in this respect, that is to say the position of the natural convection components (valves), the core outlet temperature or the reactor pool temperature and the reactor pool level. The ASN considers that for the JHR facility there are no additional demands with respect to the combined situations of loss of the electrical power supplies and the cooling sources compared with the two situations taken individually. Phénix Loss of the electrical power supplies The CEA indicates that in the event of an accident and in the case of the previously examined risks, supplying the following systems with electrical power can enable facility condition monitoring to be maintained: the health chain, which covers the radiological monitoring of the facility; the Ultimate Situation Measurement Acquisition System (SAMU) and its electrical power source.
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