16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 139 / 295Document ID.No.UKEPR-0002-162 Issue 04Water injection before initial RPV failureHere, cases only need to be considered for which, according to the definition of a “late” re-flood,a non-coolable molten pool has formed in the lower head. In this situation, a second molten poolcan also be present on top of the lower support plate. However, because of the precedingrelocation of the melt into the lower head, either the heavy reflector, or the lower support platecan be assumed to be already penetrated by the melt.This only allows the presence of a shallow melt pool on top of the lower support plate, which canbe quenched and cooled by the added water. The intense steam production during flooding ofthe hot degraded core results in an increase in the pressure in the primary circuit, which islimited by the flow through the open depressurisation valves. Analysis has shown that thisrelease will keep the primary pressure at the time of RPV melt-through below 20 bar.The contact of the molten pool with the lower head will finally lead to RPV failure, which willrelease that part of the molten core above the failure into the pit. The melt release will befollowed by water, leading to the situation of a flooded MCCI pool in the pit.Water injection after initial RPV failureDue to the preceding failure of the RPV lower head, the added water can fill the entire pit regionand cool the RPV from the outside. In addition, because the enclosures, formed by the lowerhead and by the heavy reflector and the lower support plate were previously penetrated by themelt, no deep molten pools can exist inside the RPV. Therefore, it is evident that, under theseconditions, the water addition will quench, cool, and arrest all material still present within theremains of the RPV.The final situation is characterised by some unknown fraction of core material remaining in thevessel in a stable configuration and the rest being incorporated in a flooded ex-vessel MCCIpool. What remains to be shown is that the melt in this MCCI pool will ultimately be transferredinto a coolable and cooled configuration.For this, it is necessary to consider the phenomenology of the MCCI process. As experimentallyconfirmed, e.g. by the MACE experiments [Ref], the heat flux distribution in the molten pool andthe rate of concrete erosion during MCCI do not significantly depend on whether the surface ofthe molten pool is flooded or not. This is because the upward directed heat flux is dominated bythe intensity of convection within the molten pool, which again is mainly driven by the gasrelease at the melt-concrete interface and therefore not influenced by the presence of water atthe upper surface.What will be influenced, though, are the thermodynamic conditions at the surface. The presenceof water and the resulting switch to nucleate boiling heat transfer will drastically reduce thesurface temperature and result in the formation of an oxidic crust, which insulates and isolatesthe melt from the water on top. Underneath this crust, the molten pool will progress towards thegate, and finally relocate into the core catcher, in the same way as in the dry case. Any moltendebris that was quenched in the pit during re-flood will remain there, as long as water isprovided by either the RIS [SIS], or the EVU [CHRS].To summarise: though flooding of the pit may extend the MCCI period and delay gate failure dueto the stabilisation of part of the core material in the RPV or the pit, which reduces the decaypower in the molten pool, late flooding is not able to change the ultimate course of events,leading to gate failure and melt spreading into the core catcher.