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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 148 / 295Document ID.No.UKEPR-0002-162 Issue 042.4.1.2.4. Conclusion on temporary melt retention in the pitGiven the inherent characteristics of the MCCI to adjust the ablation front progression to theamount of initially released melt, combined with the fact that the MCCI forms a coherent systemwith the lower structures of the RPV, the analysis performed demonstrates that the core meltinventory will be collected in the reactor pit sufficiently before the melt attains contact with themelt gate, independent of the underlying scenario.Another important result concerns the state of the molten pool at the end of the retention phasein the reactor pit. The analysis predicts excellent spread of the oxidic melt fraction, due to its lowvolumetric solid fraction and viscosity at the time of spreading. This is consistent with theproposed phenomenological description of the MCCI [Ref].Furthermore, the composition of the melt at the end of the retention period was found to be fairlyinsensitive to the mode and time of the initial melt release from the RPV as well as to the meltinitial conditions, including the degree of Zr-oxidation and the Zr-partitioning ratio between oxideand metal. In all cases the final melt is fully oxidised and includes an amount of concretedecomposition products of the order of 50% by volume.This unification is supported by the geometrical constraints established by the refractory layer,which spatially restricts the melt front progression in radial and axial directions and thus limitsthe amount of incorporated concrete. In this sense, temporary melt retention in the pit makes allsubsequent measures independent from the initiating scenarios as well as from the inherentuncertainties related to in-vessel melt progression and RPV-failure.The retention function is sufficiently resistant to deviations from design-base conditions as ittolerates late flooding as well as highly conservative assumptions regarding the melt releasefrom the RPV.2.4.1.3. Assessment of gate failureThis sub-section describes the function of the melt gate, the tools used for its validation and themain results obtained.The implementation of a removable melt plug in the pit bottom results from the requirement toaccess the lower pit for inspection, if necessary. The melt plug consists of a sacrificial concretelayer on top of an aluminium plate, the so-called “gate”. The gate is mounted on top of a steelframe, which establishes mechanical support against potential pressure loads acting on the meltplug from above. The steel frame has the shape of an open mesh and provides the necessarycross-section for melt release after gate melt-through. The melt plug can be moved into itsposition and locked against the outer steel frame by means of a remote-controlled cart.In its locked-in state, the melt plug is functionally an integral part of the sacrificial concrete layer.The thickness of its concrete layer is identical to that covering the surrounding lower pit region.As the sacrificial concrete is not backed by protective material at the position of the melt plug themelt plug acts as the predefined weak point of the melt enclosure during the phase of temporarymelt retention and accumulation in the pit.The corresponding analysis shows that the entire melt will be collected in the MCCI pool afterless than half of the sacrificial concrete in axial direction is eroded, see section 2.4.1.2.3.1 of thissub-chapter.