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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 135 / 295Document ID.No.UKEPR-0002-162 Issue 04The melt stabilisation process involves the following stages, (see Sub-section 16.2.2.4 –Figure 1):• The failure of the RPV and the subsequent release of the molten core• The temporary retention of the released material in the pit• The opening of the gate and the relocation of the accumulated melt• The passive flooding and quenching of the melt in the spreading compartment• Long term cooling and heat removalDuring this sequence, a transformation of the molten corium into a coolable and cooledconfiguration is achieved on the basis of simple physics and without requiring further operatoraction or the use of internal or external active systems. The above-listed stages of the CMSSare described in detail in the following sections.2.4.1.1. Assessment of the general robustness of the concept2.4.1.1.1. Tolerance to melt release conditions from the RPVIn a postulated severe accident, the initial conditions of the melt release from the RPV aredetermined by the preceding scenario, by the course of in-vessel core degradation, relocationand quenching, and finally by the failure sequence of the lower head. All these processesinvolve a significant degree of uncertainty.To make the EPR CMSS tolerant to these uncertainties, the design allows for an initial phase oftemporary melt retention and accumulation in the reactor pit. This strategy, which incorporates asacrificial concrete layer has a favourable, self-adjusting characteristic, which is based on thefact that a well-defined amount of energy is needed to melt and incorporate a concrete-layer ofgiven type and thickness.As this energy must come from the decay heat produced within the released melt, a postulatedlower initial release fraction, or a later time of release at correspondingly lower decay powerlevel, will result in longer retention times in the pit and thus also compensate for the longerperiod of melt release from the RPV under these conditions.The sensitivity to the conditions of the initial melt release from the RPV is further reduced by thefact that the lower RPV is not only heated from the inside by the residual core debris in the lowerhead, but also from the outside by thermal radiation from the released melt.In the pit, the interaction of the melt with the concrete results in vigorous gas production andintense gas-induced mixing. This favours an isotropic heat flux distribution inside the MoltenCore Concrete Interaction (MCCI) pool and adjusts the heat flux radiated off the pool surfacerelative to that carried into the surrounding concrete. Consequently, the heat-up of the RPVlower head is coupled to the progression of the melt front into the sacrificial concrete in such away that, for all relevant scenarios, the lower RPV will be thermally destroyed before theconcrete above the gate is ablated [Ref].