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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 78 / 295Document ID.No.UKEPR-0002-162 Issue 04For late in-vessel reflooding which may occur after the onset of relocation, vessel failuremight not be prevented. On the other hand, the capacity of one PDS line is sufficient toremove decay heat and to keep the equilibrium pressure below 20 bar at vessel failure,whatever the safety injection flow rate (1 pump or several pumps). The pressure spike in theprimary system, which occurs at reflooding, decays rapidly due to high discharge capacity ofthe PDS valves.At the time of water injection, the pressure spike can be above 50 bar but the vessel failurealways occurs the range 12 to 20 bar. The results for different in-vessel reflooding times aregiven in Section 16.2.2.2 - Table 2.Although RPV failure at 20 bar is only conceivable in case of late depressurisation or latein-vessel reflooding, 20 bar is considered as a design basis for the RPV supports and for thedesign of the pit walls against the resulting differential pressure. With the assumption ofinstantaneous circumferential vessel rupture at 20 bar, the resultant of calculated uplift forceson the vessel and support are shown in Section 16.2.2.2 - Figure 16.2.2.3.3. Identification of margins for delayed depressurisationThe available period of time to actuate the PDS valves in a high pressure scenario variesaccording to the primary circuit break size and steam generator conditions: SG dryout, failureof partial cooldown, or a scenario with success of partial cooldown only and failure of fastsecondary cooldown.For SB(LOCA) scenario with very small break size such as 2.5 cm (5 cm²), the action timeavailable to the operator will be longer due to the slow rate of water depletion. For a largebreak size, which would enable rapid injection by the accumulators, the primary pressure willalready be low when severe accident signal of 650°C at the core outlet is reached and theaccumulator water depletion will also increase the time to reach the signal. Therefore, arange of break sizes around 5 cm (20 cm²) and 7.5 cm (46 cm²) exists for which the breaksize is:• too small to take benefit of accumulator injection before reaching the signal,• but sufficient to maximise the available time before reaching the signal.The bleed capacity of one PDS line is sufficient to provide much extra time to have a delayeddepressurisation, which enables pressure to be below 20 bar at vessel failure.This period of time represents a margin for a successful mitigation in cases where the PDSline is not opened at the core exit temperature of 650°C.A delayed depressurisation can have some disadvantages such as:• higher hydrogen production due to possible accumulator water injection on a hotcore,• risk of RCP [RCS] piping rupture by creep (steam generator tubes).For the worst scenario with highest decay heat, which is expected for TLOFW, vessel failureat 170 bar is expected to occur at 3 hours 51 minutes after reactor trip if no depressurisationvalves are actuated and without consideration of other RCP [RCS] creep failures [Ref]. Forhalf an hour before vessel failure, it is possible to actuate the bleed valve to decreaseRCP [RCS] pressure below 20 bar at vessel failure.