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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 198 / 295Document ID.No.UKEPR-0002-162 Issue 04After the start of cladding oxidation, the decay heat of volatiles is calculated according to thetime dependent fission product distribution in the containment. Effects concerning settlementof aerosols on the walls, wash-down by condensate, transportation to the IRWST andaerosol re-suspension from the boiling sump in the spreading room are taken into account inthis approach as they can influence pressure and temperature loads in the containment.2.5.2.3. Initial conditions in the containmentThe containment initial conditions are given in Sub-section 16.2.2.5 - Table 4. The initialstructure temperatures are taken to be the same as the local gas temperature, and are takenas steady state.2.5.3. LB(LOCA)2.5.3.1. Time history and input data for the in-vessel phaseThe LB(LOCA) scenario considered is defined as follows:• double ended break of the surge line,• partial secondary cooldown,• no LHSI,• no MHSI,• accumulators available,• opening of the severe accident dedicated valve when the maximum core outlettemperature exceeds 650°C.The time history of key events for the in-vessel phase from MAAP-4.04 calculations is shownin Sub-section 16.2.2.5 - Table 5.2.5.3.2. Time history and input data for the ex-vessel phaseThe time history of key events for the ex-vessel phase is shown in Sub-section 16.2.2.5 -Table 6.The RPV lower head fails at ~10,680 seconds and the core melt pours into the reactor pit.MCCI occurs and produces steam, hydrogen, CO 2 and CO which is released into thecontainment. The generation of these gases is modelled using COSACO. Because of thehigh temperatures in the pit, instantaneous combustion of the generated hydrogen and COby in a standing flame is assumed.