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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 24 / 295Document ID.No.EPR-0002-0162 Issue 04As the molten pool in the reactor pit is compact, the potential for water ingression is highlylimited regardless of whether the pool material is in stratified layers or mixed. Only the freesurface of the melt is expected to interact with water, thus the water is kept away from the bulkof the melt. This situation is enhanced by mixing of the sacrificial material with the melt, whichincreases the melt volume whilst further diminishing the possibility of water ingression into thebulk of the melt.In conclusion, for the reasons described above recriticality is not an issue for the corium poolwithin the reactor pit.1.3.2.2.2. Assessment of the potential for recriticality in the core catcherMolten pool configurationFollowing the opening of the melt gate the molten corium spreads and undergoes interactionswith the sacrificial concrete layer covering the internal, melt-facing surface of the core catcher.As is the case for the reactor pit, the pool will either form two layers or stay mixed due toagitation by concrete decomposition gases. In contrast to the reactor pit, the corium pool withinthe core catcher is, shallow. 300 seconds after the flooding valves have been triggered by thespreading of the melt, flooding of the pool free surface with water commences, whilst the moltencorium concrete interactions continue. As a result, fragmentation of the free surface is likely tooccur and water ingress into the corresponding fragment bed is possible. At the same time themelt becomes completely enclosed by water. Therefore, it is necessary to perform an analysis toassess the likelihood of recriticality in the core catcher.AnalysisThis analysis predicts the eigenvalue (k ∞ ) as a function of melt fragmentation level, which in turndetermines the water ingression depth. A number of parameter variations are made to the basecase used in the analysis. These include enrichment, boron concentration, melt temperature andporosity of the fragment bed, which is synonymous for moderator volume/fuel volume ratio. Thefollowing table compares these parameters for the base case with the parameters for thevariation cases.Parameter Base case VariationMelt density 4680 kg/m 3 -Height of the melt 0.429 m -Average fuel enrichment 2 w/o U235 3 w/o U235Porosity of fragment bed 0.2 0.4Melt temperature 200 °C 1850°CWater conditions 20°C, 1bar -Boron concentration 0 ppm 2500 ppm B natFurther important assumptions and boundary conditions used in the analysis include:• the melt expands infinitely in the xy-direction,• a 1-dimensional model is adequate,• the top and bottom surfaces of the melt are covered with layers of water (thickness:0.2 m),• the melt contains the total UO 2 mass of the core (approximately 142 t),