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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 32 / 295Document ID.No.UKEPR-0002-162 Issue 04• Time t2: Onset of core melt with eutectic interactions among the core materials,candling of the cladding, structural materials and fuel, and formation of blockagezones at the bottom of the core, followed by the appearance of a pool of corium.• Time t3: Beginning of mass relocation to the reactor vessel lower head. Thegeneric behaviour of natural convection for a molten mass heated volumetricallyfirst leads to sideways relocation via the heavy reflector which occurs before adownward relocation through the thick support plate of the core.ooThis initial relocation always takes place into the water-filled lower plenumof the reactor pressure vessel. Some or all of the relocated material maybecome converted into debris (i.e. particles or fragments of different sizes)made up of both metal and oxides.Between t3 and vessel failure (t4) a period of corium heating occurs afterthe first relocation in the water. The debris dries out, re-melts andprogressively forms a pool of corium, which includes successive downflowsfrom the melting core. A crust develops on the top of the pool and along thewall of the reactor vessel.o The calculation uses the assumption of a homogeneously mixed oxidicdebris pool which exchanges heat by natural convection with the crust. Ametallic layer is above the oxidic molten pool and particulate debris isassumed to rise to the top. A convective circulation together withconduction in the metallic layer may remove decay heat from the oxidicmolten pool and may distribute it along the interface between the metallayer and the vessel wall.• Time t4: Reactor vessel failure due to one of various possible mechanisms:o The layer of molten metal may thermally attack and weaken the reactorvessel wall,ooThe remaining internal pressure, the weight of the corium and the thermalloads may cause a creep rupture,Jet forces during the relocation phase may cause a localised failure of thereactor vessel lower head. Analysis does not predict this type of failure butinstead predicts fragmentation of the corium and the formation of a crust atthe moment of the first flow into the water-filled reactor lower head.2.1.1.2.3. Identification of Main Scenarios for Representative CasesThe main core melt scenarios to be considered are:• total loss of the RRI-SEC [CCWS-ESWS] cooling train,• total loss of offsite power (LOOP) combined with the loss of 6 diesel generators,• total loss of ultimate heat sink,• small break LOCA (SB(LOCA)),• large break LOCA or break of the surge line (LB(LOCA)),