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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 136 / 295Document ID.No.UKEPR-0002-162 Issue 04The total amount of sacrificial concrete ablated during the retention period is limited and largelydetermined by the position of the surrounding refractory layer. As the amount of concrete in themolten corium determines the properties of the melt, these properties - including temperatureand mass - become adjusted into narrow, well-defined ranges at the end of the retention period.Therefore, the addition of sacrificial concrete not only results in the inclusion of most of the coreinventory into the molten pool prior to the release of the melt from the pit, but also leads to anequalisation of the spectrum of possible melt states. In consequence, all subsequentstabilisation measures become widely independent of the inherent uncertainties related to themelt release conditions from the vessel.2.4.1.1.2. Tolerance to loads during RPV failureThe core catcher is located in a dedicated compartment adjacent to the pit. The connectionbetween pit and spreading compartment is normally closed and will only be opened by the meltin case of a severe accident. Due to this separation, the core-catcher in the spreading room issafe from potentially critical loads related to the failure of the RPV.The design element that provides this functional separation is the melt plug at the bottom of thepit. This plug closes the entrance of the transfer channel and avoids an early release of meltbefore the end of the accumulation process as described in the previous section. The function ofthe melt plug must not be endangered by loads potentially connected with the failure of the RPVfor any relevant scenario.The design of the CMSS is based on the assumption that after primary depressurisation andafter reaching a non-coolable melt state, there will be no late active injection of water into theprimary circuit. As a consequence, the RPV will fail under dry conditions at an internaloverpressure significantly below 5 bar, which strongly reduces the loads on the surroundingstructures.Higher RPV failure pressures are only possible following a postulated late active injection. Insuch cases, the open depressurisation valve will limit the pressure in the primary circuit tovalues below 20 bar at the time of RPV melt-through. To demonstrate that the melt stabilisationfunction is invulnerable against late re-flood scenarios it has to be shown that RPV melt throughat this pressure level does not lead to fatal consequences caused by pressure build up in the pit,mechanical impact of the detaching lower head, erosive effects of the outflowing melt or meltdispersal. These phenomena are discussed in turn.Pressure build-up in the pitAs an upper-limit decoupling value for the pressure in the pit following RPV failure in case of latere-flood, a quasi-static internal pressure of 20 bar has been adopted. This corresponds to thepredicted maximum pressure inside the primary circuit at the time of vessel failure (see section2.2 above). Since RPV failure leads to an increase in volume, the pressure after expansion willbe correspondingly reduced, which justifies the conservatism of the chosen value.To achieve a sufficient load-carrying capacity, the surrounding walls and the melt plug havebeen provided with an adequate reinforcement. The melt plug is further provided with a steelframe, which transfers the pressure forces into the structural concrete.