- 186 - In late 2012, EDF shall inform ASN of the progress of the work, particularly with regard to the adequacy of the workforce present on the site. Habitability of the control room The situation considered for evaluating the habitability of the control room of the reactorfleet in the event of a severe accident is a core melt situation initiated by total loss of electrical power supplies, with opening of the containment venting and filtration system (U5) 24 hours after entering the GIAG phase. In the CSA reports for the reactor fleet, EDF states that the existing preliminary studies, based on penalising hypotheses (injection of soda to maintain the alkaline nature of the Reactor Building sumps is not taken into account and the DVC ventilation-filtration of the control room is assumed to be unserviceable), mean that permanent operator presence must be avoided in the control rooms in the period following opening of the U5 system (for 24 hours). Consequently, following the accident that occurred on the Fukushima site in Japan, among the possible measures for mitigating the radiological consequences, EDF envisages installing a system able to guarantee the alkaline nature of the water in the Reactor Building sumps and thus reduce the maximum quantity of organic iodine liable to be released in the event of an accident. EDF also plans to reinforce the electrical back-up of the control room ventilation-filtration system (DVC system) by the Ultimate Backup Diesel Generator (DUS). Pending this modification, the FARN will deploy resources to provide electrical back-up for this equipment. To conclude, ASN considers that everything must be done to ensure that opening of the U5 system on one reactor does not prevent management of all the reactors on the site, considering that these reactors may be damaged to varying degrees at this time and must thus be managed. In this respect, evacuation of the site, if prolonged, means that this requirement cannot be met. EDF undertook to evaluate the dose rates in the control room, in the BDS and on the site by mid-2012, taking account of the impact of the modifications decided on. ASN will issue a requirement on this subject. On the Flamanville EPR, DCL (control room and electrical building conditioning) ventilation guarantees that the control room is habitable. In the case of a LOOP situation, a period of 3 days is available, during which the atmosphere in the control room remains breathable. EDF is studying the provision by the FARN of a mobile electrical power supply source within 3 days. The technical investigation will continue as part of the Flamanville EPR commissioning process. 6.3.9 Conclusions concerning the planned steps to maintain the integrity of the containment in the event of a severe accident The planned steps to maintain the integrity of the reactor fleet containment rely on the U5 venting-filtration system as a last resort. As an earthquake is not considered in the design and during the periodic safety reviews as a plausible initiator of a severe accident, given all the design measures taken on the safety-classified structures, systems and components, the elements of the U5 system, except the containment penetration and the isolation valves, are not therefore seismic-classified. However, EDF states that the metal pre-filter and the piping inside the containment are able to withstand an earthquake. EDF has undertaken to conduct an overall review of U5 system filtration taking account of the following points: the robustness of the current system to hazards; the filter common to a pair of units on the 900 MWe plant series; the impact on the habitability of the control room, the BDS, on site accessibility and the radiological consequences of opening of the U5 system; the feasibility of filtration of iodines and noble gases; the role of the U5 system, taking account of the other foreseeable measures to limit its utilisation or its role.
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