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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 99 / 295Document ID.No.UKEPR-0002-162 Issue 042.3. ASSESSMENT OF HYDROGEN CONTROL2.3.1. Approach to Verifying the Efficiency of the Combustible Gas ControlSystemThe combustible gas control system (CGCS) is described in detail in section 4 ofSub-chapter 6.2. The procedure to verify the efficiency of the CGCS consists of four steps:1. Modelling of the scenarios relevant to hydrogen risk with MAAP-4 (see Appendix 16A) up tothe point of RPV failure in order to provide mass and energy release data. Representative andbounding scenarios are selected. The classification of scenarios is outlined in section 2.1 of thissub-chapter. In brief, representative scenarios form the basis for the design and the verificationof the severe accident design measures, while bounding scenarios are used to demonstratetheir robustness and to explore their limits. Mass and energy release (including hydrogenrelease) into the containment due to ex-vessel melt stabilisation is modelled with COSACO (seeAppendix 16A) in combination with an engineering model to address steam discharge due tomelt flooding and quenching. The approach and results are described in section 2.4 of this subchapter.2. Modelling of the gas and temperature distribution in the containment for the selectedscenarios with the computational fluid dynamics (CFD) code GASFLOW (see Appendix 16A),taking into account the recombiners. These calculations are necessary for the time period inwhich there is a risk of hydrogen combustion. This is typically from the start of hydrogen releaseto the achievement of a uniform gas distribution in the early ex-vessel phase. In order to applythe correct boundary conditions during the period of hydrogen combustion risk, in particularsteam concentration and wall temperatures, the history of the thermo-hydraulic parameters mustbe considered from the beginning of the accident in an appropriate way.3. The assessment of the hydrogen risk is based on GASFLOW (see Appendix 16A) results andconsists of analysing the possible Adiabatic Isochoric Complete Combustion (AICC) pressure,the potential for flame acceleration and deflagration to detonation transition (DDT). Theassessment of flame acceleration and DDT is based on experimentally proven criteria. The mostimportant of these criteria is the “sigma” criterion. This relates the non-isochoric combustionexpansion ratio (‘gas density before combustion’ divided by ‘gas density after combustion’) to athreshold value determined by experiment. The assessment of the hydrogen combustion modeis based on the gas and temperature distribution under the assumption that ignition can occur atany time and any place if the gas mixture is ignitable. Following this deterministic approach, thelikelihood of ignition is not considered and it is conservatively assumed that maximumaccumulation of hydrogen occurs prior to ignition. Explicit calculations of hydrogen combustionare necessary to assess temperature loads, in particular on the containment shell. To calculatethese temperature loads, GASFLOW is also used, as its laminar combustion model is sufficientfor this purpose. (COM3D, a CFD code for modelling fast hydrogen deflagration (see below) isnot appropriate for these calculations because it does not account for heat transfer to the walls.)