- 318 - Prevention measures for managing a severe accident resulting from loss of cooling. The CEA highlights the following existing severe accident prevention measures: the water block bunkers and the pools that help keep the core under water. Design measures allow balancing of the water levels between the pools and the bunkers in the event of a breach in order to exclude the risk of core exposure; the following means for supplying the reactor building pools with water: o the EPV tanks and the EPL circuit supplied by the normal electrical power network. The EPL circuit transfers the water stored in the EPV tanks to the nuclear unit pools; o the ultimate recirculation system for leaks collected in the water block bunkers (REU) designed to seismic standards and supplied by the normal electrical network. If a breach occurs, it enables the water collected in the water block bunkers to be directed to the pools of the Reactor Unit Building (BUR). This system is powered by the off-site electrical network. The CEA envisages the following complementary measures to increase the robustness of the facility: permanent filling of one EPV tank of the Auxiliary Unit Building (BUA) with a few hundred cubic metres of water: creation on an internal stock of water so that the pools can be resupplied without bringing in water from outside sources; extension of the function of the ultimate recirculation system to the areas adjacent to the water block: collection followed by pumping of the leakage water from the areas adjacent to the reactor pool; design to seismic standards of the section of the EPL system that supplies the reactor pool: increase the robustness of the make-up system; installation of specific sensors to allow monitoring of the water level in each pool from the control room and the fall-back station (in addition to the existing threshold overshoot alarms), unless the equipment already planned for can fulfil this function. Measures for mitigating the consequences of a severe accident The CEA indicates that to mitigate the consequences of a severe accident, the JHR has the reactor containment and associated systems. Containment management is based on total isolation of the reactor containment and the putting into service of a post-accident ventilation system that is independent of the other ventilation systems, situated in the ZRF and equipped with HEPA filters and iodine traps. This ventilation system can collect any leaks in from the containment penetrations and depressurise the containment if necessary. The equipment necessary for containment management is designed to seismic standards and controlled from the control room or, as a last resort, locally. These actions are triggered automatically if there is an increase in activity in the reactor building, or manually by the shift team from the control room. The licensee can also manage the risk of overpressure in the reactor building hall from the control room by opening a decompression line connected to the post-accident ventilation system. This can also be done locally. Analysis of an extreme scenario of total loss of cooling leading to fuel meltdown In its assessment the CEA postulates a severe accident situation of meltdown under water with fast kinetics (less than 15 minutes) with the penalising build-up of failures of all the redundant active backup means and closing of the valves when the coolant pumps discharge as soon as the speed of their flywheel becomes to low. In this case the natural convection valves must be opened manually as soon as possible. With regard to this highly improbable scenario, it must be pointed out that the natural convection valves, the mixing pump and their respective sets of batteries - SUS A and SUS B - are designated by the CEA as key equipment items because they enable cooling and removal of residual power to be ensured in the event of loss of the normal electrical power supply (EDF) and the backup power supply (diesel generator sets of the MEQ network). This extremely penalising scenario enables the kinetics of the hypothetical accident scenario of meltdown under water to be assessed, if it were to happen in spite of the planned prevention measures (ultimate backup pump, natural convection valves, all supplied by the SUS batteries).
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