- 178 - Risk of direct heating of the containment: To avoid direct heating of the containment, which would result in rupture of the vessel under pressure, the SA operating procedure on the reactors in service requires depressurisation of the primary system by opening the pressuriser discharge lines immediately from entry into the severe accident (SA) situation. On the EPR, two redundant primary system discharge lines enable the primary system to be depressurised, preventing the risk of reactor vessel rupture at high pressure, which could lead to loss of containment integrity by direct heating of the containment. The operator has one hour after entry into the SA situation to open these lines, which are supplied by the 12 hours batteries. 6.2.3 After reactor vessel melt-through Added to the above-mentioned risks is the risk of basemat melt-through further to rupture of the reactor vessel containing the corium. On the reactor fleet in service, EDF indicates in the CSA reports that restoring water makeup in the reactor vessel and depressurising the primary system - as required by the operating procedure on entry into the SA situation - enable the low-pressure makeups to flow into the primary system and help reflood the core, and - if achieved in required time - stop core meltdown and prevent reactor vessel melt-through. Reflooding the corium in the vessel or injecting water into the reactor pit via the perforated vessel to keep the corium flooded, limit the risk of basemat melt-through, or failing this, delay its occurrence. The severe accident management guide (GIAG) defines the water injection conditions, particularly with respect to the risks of early loss of containment. As the safeguard systems of the damaged plant unit were probably lost on entry into the SA, so-called "ultimate" alignments can be implemented by the emergency teams to flood the corium. For the reactor fleet in service, there is also a risk of ex-vessel vapour explosion. EDF specifies in the CSA reports concerning them that an international research programme is in progress to characterise the conditions of occurrence and the intensity of such phenomena. EDF also indicates that the available studies show the containment to be well able to withstand the loads resulting from a vapour explosion. Its integrity would therefore probably not be compromised in this situation. For the Flamanville EPR, the CSA report indicates that the corium catcher situated in a special compartment on the edge of the reactor pit, is designed to collect, cool and stabilise the corium. Prevention of basemat meltthrough is thus based on a reactor pit and catcher that are both dry when the corium arrives, on the collection and spreading of the corium and on its passive cooling after spreading. In the longer term, the EVU system used in spraying mode enables the residual power to be removed from the corium. 6.3 Maintaining containment integrity after damage to the fuel in the reactor core The ASN specifications required EDF to study the means of preventing and managing: loss of the core cooling function; loss of containment integrity, particularly the reactor containment. The ASN specifications stated that the licensee had to describe the severe accident management measures and facility design elements to protect containment integrity after the occurrence of fuel damage. The ASN specifications also stated that it was necessary to: identify any cliff-edge effects and evaluate the time before they occur; assess the appropriateness of the existing management measures, including the GIAGs, and the possible complementary measures.
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