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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 151 / 295Document ID.No.UKEPR-0002-162 Issue 042.4.1.4.1. Validation strategyTwo redundant tools are used to validate the assumption that the melt will relocate into anddistribute within the core catcher after gate opening. First a detailed analysis with the AREVAcode CORFLOW (see Appendix 16A) and a confirmatory assessment with thephenomenological spreading model developed at RIT [Ref], which was validated against a wideset of spreading experiments. In addition, the ability of an EPR-type melt to spread has alsobeen confirmed independently [Ref] by the CEA with the code THEMA within the framework ofthe European ECOSTAR project.Based on the accumulated knowledge base, the success of melt spreading under EPRconditions is generally considered a non-critical issue because:• as a result of the preceding MCCI in the pit, the out-flowing oxidic melt will be fullyliquid• the liquid metallic melt is expected to be released first and thus the oxide will spreadon a hot smooth surface• the surrounding structures, in particular in the melt discharge channel, have only alimited heat absorption capability• the average melt level in the core catcher after spreading will be as high as 50 cmThis evaluation is reflected in the corresponding positive assessment given by a group ofEuropean experts within the framework of the EUROCORE project [Ref].2.4.1.4.2. Modelling approach2.4.1.4.2.1. CORFLOW codeCORFLOW (Appendix 16A) uses a 3D-model in Cartesian coordinates for the reactor pit,channel and spreading compartment, as shown in Sub-section 16.2.2.4 - Figure 17. To reducethe complexity of the actual geometry, several simplifications are included:• a flat bottom, neglecting the step at the end of the melt discharge channel• each phase (oxide/metal) is released and spread separately, which neglects theadditional driving hydrostatic pressure head of the other overlying melt phases inthe reactor pit during outflow through the gateAll the above simplifications are conservative with respect to front propagation, as they reducethe mechanical energy of the fluid. With respect to the opening characteristics of the melt gate,various cases were modelled, in accordance with the findings in section 2.4.1.3.3 of this subchapter.They include:• a complete, instantaneous opening of the entire cross-section• a partial opening with a cross-section of 0.24 m² , which corresponds to about 10%of the total gate area