- 190 - EDF also states that oxidisation of the cladding by steam, would lead to the production of hydrogen in sufficiently large quantities to exceed the flammability threshold, but that bearing in mind the means used to prevent uncovering of the fuel assemblies, the risk of hydrogen production by oxidisation of the zirconium cladding is ruled out. EDF therefore proposes completing its thermohydraulic studies of the fuel storage pool before the end of 2012, taking account of the different behaviour of the various areas of the spent fuel pool. In accordance with the hydrogen risk studies, particular steps may need to be taken depending on the result of these studies, such as the installation of passive autocatalytic recombiners in the fuel building. These studies cover both the NPP fleet in service and the EPR. ASN considers these studies to be necessary in order to determine the material and organisational measures that could be taken on the NPPs in operation and on the EPR, such as the installation of passive autocatalytic recombiners in the fuel building. ASN will issue a requirement on this subject. Protection against radiation ASN asked EDF to examine the current situation and the existing and complementary management measures, concerning protection against the level of radiation that could be reached. In the CSA reports, EDF feels that a water height more than 1.5 m above the fuel assemblies is enough to ensure radiation protection compatible with human intervention, but that given the steam generated by the heating of the pool water, this intervention would take place in degraded ambient conditions. EDF however considers that if the water height were to be less than this value, the thickness of the concrete walls would be sufficient to maintain equivalent dose rates at values compatible with human intervention in the adjacent premises, even if the ambient conditions were no longer to allow access to the BK pool area. Nonetheless, the preparatory work for water makeup of the spent fuel pool would be carried out in advance, while the ambient conditions are not yet degraded. The makeup start/stop actions would not subsequently require entry into the spent fuel pool area or adjacent room. For the NPP fleet in operation, outside the fuel building, the radiation from the fuel assemblies induced by skyshine generates dose rates that rise as the water level drops. In the CSA reports, EDF specifies that it is studying this phenomenon (which corresponds to the scattering of gamma radiation by the atmosphere) and gives initial dose rate estimates at 20 metres from the fuel building of about 1 mSv/h. For the EPR, the airplane crash shell covering the fuel building offers a sufficient thickness of concrete (180 cm) to guarantee no dose rates induced by "skyshine" outside the building. ASN considers that the Fukushima accident highlighted the accident management difficulties that could arise when the water inventory in a spent fuel pool is reduced. It thus appears necessary that EDF be able for as long as possible to manage a situation deteriorating in a spent fuel pool. Based on this finding, EDF proposes supplementing the radiological environment studies already performed by developing its analysis of the dose levels liable to be received by the intervention personnel, induced by a reduced water inventory above the fuel assemblies and a two-phase state in the fuel storage pool. ASN considers this approach to be satisfactory and will be drafting a technical requirement on this subject. Mitigation of releases after fuel melt In the CSAs, EDF does not describe the means for mitigation of releases after fuel melt in the spent fuel pool. The fuel building containment was designed to take account of a fuel assembly falling and breaking during handling under water in the spent fuel pool. The elements not retained by the water of the spent fuel pool would be captured by the DVK fuel building ventilation system and filtered by filters and iodine traps.
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