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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 140 / 295Document ID.No.UKEPR-0002-162 Issue 042.4.1.2. Assessment of melt retention in the pitTemporary retention is based on the provision of a layer of sacrificial concrete that must bepenetrated by the melt prior to its escape from the pit. This layer is backed-up on the outside bya protective layer (see Sub-section 16.2.2.4 - Figure 1), except at the position of the melt plug,which therefore acts as a predefined weak point. The ultimate purpose of the protective layer isto contain the melt in a predefined volume and to avoid contact between melt and structuralconcrete. The protective layer consists of a zirconia-based refractory material, which is able towithstand the attack of the core melt during the retention period.First contact between melt and protective layer is likely to occur in the region of the top metallayer, where the heat flux to the concrete and the resulting erosion speed are higher than in theoxide region below. The zirconia material is fully stable when in contact with the metallic meltfraction [Ref].The material is also stable against the oxidic fraction, under ongoing MCCI conditions [Ref]. Thisis due to the steady introduction of "cold" concrete decomposition products into the bulk of themolten pool, which leads to the formation of a sub-cooled mixture consisting of:(i) a dispersed solid phase containing the high-melting refractory components zirconia andurania, and(ii) a liquid phase having a high content of low-melting concrete decomposition products.Therefore, the melt is always "saturated" in refractory components, so any significant dissolutionof potentially exposed zirconia is avoided.The provision of a layer of sacrificial concrete of defined thickness has a favourable selfadjustingcharacteristic. This stems from the fact that to ablate a defined amount of concrete themelt must generate a defined integral amount of decay heat. Therefore, a lower initial mass ofreleased melt and/or a lower level of decay heat will result in correspondingly longer retentiontimes, and vice versa. This feedback makes the accumulation process independent of the meltrelease sequence and the time of RPV failure (decay heat level).This independence is further reinforced by the fact that, due to the zirconia layer having a lowthermal conductivity, the RPV, residual core, MCCI pool and sacrificial concrete form a coupledquasi-adiabatic system. The RPV lower head is not only heated from the inside, but also fromthe outside by thermal radiation from the surface of the MCCI pool. Due to the gas-inducedmixing within the MCCI pool, the corresponding radiant heat flux is of the same order as theheat flux into the surrounding concrete. This inherent coupling links the process of concreteablation with the heat-up and ultimate thermal failure of the RPV lower head.The geometrical constraint imposed by the refractory layer pre-determines the total amount ofconcrete ablated during the retention period, which adjusts the final melt properties, includingtemperature and viscosity, into a narrow and predictable range. This is because these propertiesmainly depend on the melt concrete fraction. The admixture of sacrificial concrete leads to anequalisation of the spectrum of possible melt states at the end of the retention process andtherefore makes the spreading process and all subsequent events independent of the inherentuncertainties associated with in-vessel melt pool formation and RPV failure.