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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 194 / 295Document ID.No.UKEPR-0002-162 Issue 042.5. CONTAINMENT PRESSURE AND CONTAINMENT TEMPERATURENote: all pressures given in this section are absolute pressures.2.5.1. Methods and basic assumptions for pressure and temperaturecalculations2.5.1.1. Description of the scenariosThree severe accident scenarios are studied:• LB(LOCA) (double ended break of the surge line at the hot leg without safetyinjection).• SB(LOCA) (5 cm diameter (20 cm²) leak at the cold leg with partial and fastsecondary cooldown).• LOOP (opening of the severe accident dedicated valves when core outlet gastemperature exceeds 650°C).These scenarios were chosen as the reference cases for considering the temperature andpressure expected in the containment, particularly in the phase of corium spreading untiloperating the EVU [CHRS]. The analyses are based on representative initial and boundaryconditions.2.5.1.2. Pressure and temperature calculation codeThe COCOSYS code (Appendix 16A) has been used for the pressure and temperaturecalculations in the containment. COCOSYS is based on mechanistic models for thecomprehensive simulation of relevant processes and plant states during severe accidents.COCOSYS is a lumped parameter code that allows analysis of the local states (e.g.pressure, temperature, gas distribution) in the containment.2.5.1.3. Determination of flow ratesThe flow rate inputs for COCOSYS are generated with:• MAAP-4 (Appendix 16A) for the in-vessel phase,• COSACO (Appendix 16A) for MCCI in the reactor pit and core catcher.20% core melt fragmentation is assumed during quenching in the core catcher [Ref]. This isimportant for the steam mass flow rate - see below for details.