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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 159 / 295Document ID.No.UKEPR-0002-162 Issue 04The exclusion of highly energetic FCI is based on the results of this analysis and on theexperience gained from an extensive set of tests with top flooding of a spread melt [Ref]. Inthese tests either prototypic, urania-based core melts or stimulant melt have been flooded withwater under the conditions of ongoing interaction with concrete. The stimulant melts usedincluded those with oxidic phases containing large amounts of aluminia, which is commonlyknown for its high likelihood to engage in FCI. No type of energetic interaction has beenobserved in any of these tests, despite the fact that the gas-induced mixing was often moreintense and the concrete content lower than predicted for the melt flooding process in the EPRcore catcher.The mass and energy release into the containment is determined using an engineering modelwhich is based on conservative assumptions with respect to quenching and superficialfragmentation.2.4.1.5.2. Modelling approach2.4.1.5.2.1. MCCIThe concrete walls of the core catcher are modelled assuming cylindrical geometry. To adjustthe model, and in particular the ratio between bottom area and circumferential length, to that ofthe real core catcher, only a sector of the full cylinder and an inner inert region are assumed. Inthe calculations a sector width of 5.125 rad, an inert radius of 5.346 m and an outer radius of9.83 m are used.At the free surface, radiant heat transfer to the overlying concrete structure is considered. Dueto the complexity of the flooding process, the presence of water at the surface is not taken intoaccount. Therefore, the determination of the gas release from the MCCI is decoupled from theevaluation of the steam production resulting from the flooding of the free surface. The latter isaddressed separately in the next section and in section 2.4.1.5.3.2The MCCI ends at the time when the residual concrete thickness at the bottom is zero.2.4.1.5.2.2. Melt flooding and quenchingAs a consequence of the preceding admixture of concrete decomposition products and theresulting density inversion between the oxidic and metallic melt fraction during and after MCCI,the water at the surface will necessarily be in contact with an oxidic-type of melt.The probability of FCI during flooding is low because of the formation of viscous oxidic layersand crusts, which inhibit the contact between liquid melt and water.The events during flooding and quenching of oxidic prototypic corium melts have beenextensively studied in the framework of the MACE and OECD MCCI projects [Ref]. The resultsobtained unanimously demonstrate that water distributes smoothly on the molten corium andthat a substantial superficial fragmentation and improved coolability can occur. Fragmentation ispromoted by the gas release from the ongoing interaction with the concrete and by thesubstantial shrinkage of the material during solidification.