- 182 - Corium in reactor vessel: EDF indicates in the CSA report that as the severe accident management guide (GIAG) prohibits the injection of non-borated water as long as the corium is in the reactor, the re-criticality risk is excluded for the corium-invessel configurations. This point does not prompt any remarks from ASN. Corium in the reactor vessel pit: In the CSA reports, EDF indicates that after reactor vessel melt-through, injection of clarified water could be envisaged after analysis and if recommended by the emergency team. The re-criticality risk is excluded in the short term, as the intense vaporisation of the water on contact with the corium tends to reduce the reactivity (increase in the vacuum level). In the longer term, when the bed of debris can be cooled and there is little or no vaporisation (low vacuum level), the strong presence of neutron absorbing fission products and the incorporation of concrete are factors favouring a substantial reduction in reactivity. Nevertheless EDF and the IRSN do not share the same opinion on the harmlessness of a clarified water injection; borated water makeup points must therefore be provided in the long term. On the Flamanville EPR, as specified in the CSA report for this reactor, measures are taken to guarantee a dry reactor pit and a dry corium spreading area. ASN will examine whether these provisions are sufficient in the framework of EPR commissioning. 6.3.5 Prevention of basemat melt-through The ASN specifications asked EDF to describe the steps taken to manage severe accidents in order to prevent the risk of basemat melt-through in the reactor buildings. Flooding of the corium in the vessel In the CSA reports, EDF states that maintaining the corium in the vessel avoids the ex-vessel corium-concrete interaction phase and thus contributes to the goal of maintaining the integrity of the containment. Stabilisation of the situation in the vessel entails restoring a means of injecting borated water into the reactor coolant system within a sufficiently short period of time to avoid vessel rupture, in other words before core damage is too far advanced to enable it to be cooled in the vessel. The strategies for maintaining the corium in the vessel are based on: borated water makeup in the reactor coolant system; eventual use of the recirculation function to keep the core continuously flooded. EDF states that possibilities for retaining the corium in the vessel are envisaged for the reactor fleet in a severe accident situation, based on existing systems not specifically designed to manage accidents with core melt and depending on their availability. The considerations are as follows: to enable the situation to be stabilised in the vessel, in-vessel injection must be restored before the formation of a significant corium pool in the core and, in any case, before the corium transits to the bottom of the vessel; if water is present in the reactor pit, allowing external cooling of the vessel, water injection into the vessel can allow stabilisation of the situation if it is restored before significant ablation of the vessel walls. It should be recalled that as things currently stand, flooding of the reactor pit is the result of operation of the containment spray system (EAS), when available, by run-off of spray water to the reactor pit.
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