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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 30 / 295Document ID.No.UKEPR-0002-162 Issue 04odepressurise the primary system to avoid high pressure melt ejection fromthe reactor vessel and the risk of direct containment heating.Any delay in opening the severe accident relief valves may lead to potential disadvantageswhich would increase containment loads; this is addressed in the section covering RCP[RCS] depressurisation and hydrogen risk.Bounding scenarios:Two kinds of bounding scenarios are considered:• high pressure core melt scenarios with late primary system depressurisation,which can lead to passive accumulator water injection on the partially moltencore or active water injection when the safety injection is available.• all scenarios with active reflooding due to recovery of pumps and water sources.The important parameters are the timing of the onset of water injection and the flow-rate ofinjection (dependent on the number of pumps available).The following consequences are possible:• reflooding, particularly during the first stage of oxidation with a slightly degradedgeometry, may increase hydrogen production in-vessel. The increase inhydrogen production will depend on the level of core damage, the temperaturelevel and the level of cladding oxidation prior to reflooding and the reflooding flowrate, because of the competing effects between the cooling by the water and theincrease in temperature caused by the exothermal reaction of the zirconium withthe steam.• reflooding may lead to reactor vessel pressurisation due to the corium/coolantinteraction. The degree of pressurisation depends on the level of coriumfragmentation and the effectiveness of thermal transfer. It is limited by the RCP[RCS] discharge capacity. The reflooding time may affect the phenomena ofcorium relocation and solidification and blocking in the core. The discharge ofsteam from the RCP [RCS] also contributes to the increase in the containmentpressure.• early addition of water to the core with a sufficiently high flow rate during the firstoxidation phase may quench the core and thus the core degradation may bestopped. It is possible that the molten core may be retained in the reactor vessel.However, if the water addition flow rate is insufficient, the core will heat up andreach the next stage of degradation.• if the water injection is performed too late, i.e. when the corium can no longer becooled, reactor vessel failure is inevitable and corium and water will be releasedfrom the reactor vessel into the reactor pit.Sensitivity analyses on reflooding time have been carried out. A scenario is consideredbounding when boundary conditions corresponding to the most onerous reflooding time areselected for the evaluation.