- 286 - In this context, the CEA envisages the following improvements: study a modification for disabling the system that automatically stops raising of the bridges. This is because if the water level in the pools and channels were to drop, it could be necessary to install gates in a radiological environment that exceeds the threshold activating automatic stopping of raising of the bridges. The CEA specifies that the FLS could carry out the required water make-ups using a remotely controlled robot if necessary. constitute a stock of sand bags that could be used to block a crack in the bottom of a pool or channel. Loss of the main cooling system combined with total loss of the off-site and on-site (electrical power supplies Loss of the off-site electrical power supplies and the on-site emergency supplies deprives the facility of the pumps used to make water transfers, particularly the drainage tank pump and the filling system pump. The emergency water make-up system should however be available, as should the centre's mobile backup means. The CEA indicates from its analysis that core exposure would occur after 43 days, and judges this leaves sufficient time to deploy the external water make-up means. The licensee nevertheless envisages the following improvement measures in addition to those presented earlier: Reflect upon supplementing the procedure to apply if the pool water level drops, in order to integrate the penalising case of combined events (total loss of electrical power supplies, loss of water make-up possibilities, simultaneous leaks in several of the water block rooms). More particularly, if the combined loss of leak-tightness of a channel and loss of electrical supplies is considered, the gates for isolating the break must be handled using a crane powered by a normal diesel generator set (manual electrical connection) or by an emergency mobile generator set, that is complementary to the existing GUS; Provide a backed up electrical power supply from a mobile source for the filling system pump and the pump for transferring water from the drainage tank to the pool (130 m3) in order to keep the fuel elements under water and thereby push back the risk of a cliff-edge effect. Back up the electrical power supply of the backup fan placed at a height of 2 m, and the equipment for configuring the backup ventilation to ensure permanent filtration of the ventilation discharges. The analysis presented by the CEA assumes that in the event of loss of the electrical power supply and/or the heat sink: emergency reactor shutdown is performed; the natural convection valves open; the pools and channels remain watertight; water make-ups are possible. The CEA has already identified these equipment items as essential items, which is satisfactory. Nevertheless, in an accident situation it is necessary to have information on the state of the core and the pools. The CEA has undertaken to assess 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; the core outlet temperature or the reactor pool temperature; the reactor pool level. ASN considers that the improvement proposals presented by the CEA for Osiris are such as to reinforce the robustness of the facility in the event of loss of the electrical power supplies and the cooling systems. ASN also considers that it is necessary to revise the procedure to follow in the event of a drop in the Osiris reactor pool water level.
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