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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 152 / 295Document ID.No.UKEPR-0002-162 Issue 04Cases with an initial release of either an oxidic or mixed melt were included in the analysisdespite the fact that an initial release of metallic melt is predicted to be most likely, consistentwith the COSACO calculations described in section 2.4.1.5.2.1 of this sub-chapter, and thededuced release sequences:• first metal, then oxide plus slag (RS-A, after layer inversion)• suspension of metal in oxide plus slag (RS-B)• first oxide, then metal, then slag (RS-C, layer inversion suppressed)As CORFLOW is restricted to the analysis of single-component flow, the above releasesequences had to be reduced to the following simplified cases:1a Spreading of metallic corium onto concrete1b Spreading of oxidic corium with slag, onto a metallic melt2 Spreading of oxidic corium without slag, onto concrete3 Spreading of a mixed corium (oxide, slag and metal) onto concreteMixed corium spreading is considered as the best estimate release model (indexed as "Ref."),since an intense mixing of phases during the release process has also been observed in the2D-COMAS EU-4 spreading test [Ref] performed with initially layered oxidic and metallic corium.This test was simulated by CORFLOW with good accuracy by assuming a single-componentfluidwith properties of the mixture of metal and oxide [Ref].In addition to the analysis of the four cases 1 a/b, 2 and 3 listed above, the CORFLOW analysesfor the EPR [Ref], also include separate calculations to assess the effects of:• a lower initial temperature of the oxidic corium• a reduced cross-section of the gate area• a non-Newtonian rheology of the oxidic coriumThe initial melt conditions for the CORFLOW analyses [Ref] were extracted from the COSACOresults for the melt retention phase in the pit for an SB(LOCA) scenario. This scenariorepresents a conservatively low decay heat level and is thus bounding for examining the abilityof the melt to spread.2.4.1.4.2.2. RIT modelAs compared to the former approach, the methodology developed at the Royal Institute ofTechnology Stockholm (RIT) is not based on a numerical, but on an empirical-phenomenologicalapproach. It is based on an in-depth review of the state-of-the-art knowledge of melt spreading(database, simulation, analysis methods and scaling considerations) [Ref] together with acomplete description of the RIT model. The underlying methodology was first developed forspreading in 1D-channels, but has recently been extended to cover spreading in 2D-channelsand, more importantly, spreading into an open area.