- 183 - In practice, the injection of borated water to the vessel by makeup drawing directly from the PTR tank, this latter if possible being resupplied, is preferred in order to keep the core flooded, while delaying the moment of transition to recirculation. After the CSAs, EDF aims to have the reactor coolant system injection means backed-up by an Ultimate Backup Diesel Generator (DUS). An ASN requirement will concern the composition of the hard-core, of which these systems should be a part. Flooding the corium in the reactor pit Assuming failure of the vessel, the corium pours into the reactor pit. In the CSA reports, EDF states the strategy currently in place on the reactors in operation, which is to inject water: by an input of water subsequent to vessel failure, using reactor cooling system makeup through the breach at the bottom of the vessel, in accordance with severe accident operations. Furthermore, when the reactor pit is initially dry or containing a low water level, the risk of a steam explosion is considered to be low. According to EDF, the conclusions of the MCCI (Molten core concrete interaction) programme run under the aegis of the OECD confirm this ex-vessel reflooding strategy. This international scientific programme dedicated to the ability to cool the corium-concrete mixture, demonstrated on an experimental scale that a corium pool can be stabilised by the injection of water; by flooding of the reactor pit prior to vessel failure, linked to operation of the reactor building containment spray system (EAS) if available before entering the severe accident phase. If the reactor pit is flooded up to the level of the vessel bottom head, this significantly reduces the risk of basemat meltthrough, as the retention of a part of the cooled corium in the vessel and corium contact with the water in the reactor pit reduces the quantity of corium that will contribute to the corium-concrete interaction (CCI). In the CSA reports, EDF states that the current mitigation strategy, which aims to inject water before or after vessel melt-through, should be able to slow down or even prevent basemat melt-through. Complementary corium-concrete interaction tests (tests CCI-7) are planned for 2012 to confirm the possible stabilisation of a corium pool by means of flooding from above. However, ASN considers that transposition to the scale of a reactor is not direct and requires the use of computer codes. It is therefore problematical as things stand to draw complete conclusions on the situation of a reactor. R&D and testing need to be continued in this field. In the CSA report for the Flamanville EPR reactor, EDF states that this reactor will have a corium catcher enabling spreading and cooling of the corium. Passive flooding of the spread corium in the catcher and removal of the residual heat by the EVU system thus ensure long-term protection of the basemat. The detailed design of the EVU system will be studied by ASN as part of the EPR commissioning process. Risk of cliff-edge effects and means of mitigation In the CSA reports, EDF states that the cliff-edge effects liable to compromise corium retention in the vessel are, for the reactor fleet: long-term loss of electrical power supplies; the countermeasure is to restore vessel makeup by a diversified means (generator-driven pump for example); non-restoration of the recirculation function after complete use of the borated water reserves. This takes several days. Limiting the injection flow to that strictly needed for residual heat removal and resupply of the PTR tank with borated water would enable this period to be extended. In a long-duration total loss of electrical power supply situation (situation H3) combined with the loss of water supply to the steam generators (emptying of ASG tank), none of the present injection means would allow flooding of the corium in the vessel and in the reactor pit. As a result of the CSAs, EDF envisages using a generator-driven pump for the reactor fleet, allowing injection of water from the PTR tank to the reactor coolant system. EDF specifies that this will be incorporated into the means available to the FARN. For the reactor fleet, in addition to these preventive measures, examination of countermeasures to the dissemination of radioactive products by the "water route", in other words the potential contamination of the
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