- 285 - Measures envisaged to reinforce robustness of the facilities to the loss of electrical power supplies and proposed studies The CEA indicates that the study of electrical power supply loss reveals good robustness of the facility and a large number of backup means (3 diesel-generator sets at the facility and 2 at the Saclay centre). The CEA nevertheless proposed the following measures to improve the robustness of the facility in its complementary safety assessment: Establish a control procedure to optimise management of the generator sets in the event of prolonged loss of the off-site electrical power supply, in order to extend the autonomy of the emergency electrical supplies (currently limited to 35 hours ), and in particular specify that it is necessary: o to use the GUS in priority, as it consumes less than the other two diesel generator sets; o maintain a minimum reserve of diesel fuel and oil and draw up a formal procedure for topping up a diesel generator with fuel and oil in operation. Carry out a feasibility study for powering the filling system pump via the GUS; Define the actions to take in the event of prolonged loss of the EDF power grid and the two main emergency diesel generator sets when the Osiris reactor is at low level, and study the feasibility of using the GUS to supply the pumps for transferring water from the drainage tank to the Osiris reactor pool; Connect the pool and channel water filling system pump and the hot layer pump (to reinforce biological protection in the Osiris hall) to the electrical network supplied by the GUS; Examine the possibility of powering at least one fan of the backup ventilation system by the GUS or another mobile electrical power source, in order to guarantee permanent filtration of any releases via the ventilation system. Measures envisaged to reinforce the robustness of the facilities to the loss of the ultimate cooling system / heat sink As regards the loss of the main cooling sources (core primary cooling system in the case of Osiris), the CEA has considered an unfavourable initial situation that consists in starting the transient with the Osiris reactor pool at low level. The time allowed to add water, if possible demineralised, is about one month. The numerous possibilities of making water make-ups in the required volumes have led the CEA not to plan for additional measures at this stage. As regards the risk of loss of the main and emergency cooling systems, the CEA indicates that the only unfavourable initial situation in this case is also a low level in the Osiris reactor pool at the start of the transient. It states that as long as the electrical power supplies are available, it is possible to transfer 130 m3 of water from the drainage tank to the reactor pool. The licensee underlines that the Osiris facility contains a large quantity of water in the pools and channels (about 2000 m3), which leaves a comfortable margin (more than 40 days) for deploying external water makeup resources. Moreover, even in the event of loss of sealing of the plug separating the reactor pool from the control mechanisms room which is part of the water block, the time lapse before the core becomes exposed exceeds 16 days. This still leaves sufficient time to deploy the necessary external water make-up resources. It is specified that water make-ups could be made from the fall-back centre situated 300 metres from the facility, by means of an emergency water make-up system (flow of 90 m3/h) connected to the public water supply. If the water make-up systems (normal and emergency) - which are not designed to earthquake design standards - are out of service, the local security organisation (FLS) will have to be called to the site to place a hose in the pool or channel concerned. The means of action available to the FLS are chiefly: the centre's fire-fighting water supply (flow rate of 150 m3/h); pumps that can be connected to the reserve pond, the Villiers pond or the Saclay ponds (flow rate of 120 m3/h).
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