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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 79 / 295Document ID.No.UKEPR-0002-162 Issue 04Section 16.2.2.2 - Table 1 presents the main results for TLOFW core melt scenarios.The main results of core melt scenario TLOFW with late PDS line opening are given inSection 16.2.2.2 - Figures 9 to 15. Opening of the PDS line with safety injection is actuatedafter the onset of core melt relocation into the lower head at 3 hours 33 minutes.The bleed discharge capacity enables a more tolerant design An additional period of time of1 hour 30 minutes is available after the severe accident signal (650°C), which represents anincreased margin for the operator to actuate the PDS valves to achieve bleed flow and tomanage RCP [RCS] pressure to be below 20 bar at vessel failure.2.2.3.4. Temperatures in the Primary System2.2.3.4.1. Maximum Gas Temperature• For scenarios with the opening of one PDS line at the temperature criterion of650°C (such as the LOOP scenario (see Section 16.2.2.2 - Figures 6 and 7)), thepressuriser is almost full of water so that the gas temperature equals thesaturation temperature of water, which is 350°C. Superheated steam exiting thecore is cooled down to saturation temperature while going through thepressuriser plug. This pressuriser plug is discharged partly when the PDS line isopened, but some water can still remain in it and not flow down into the hot legdue to insufficient differential pressure between pressuriser and hot leg; the gastemperature does not increase before complete pressuriser dryout. Thetemperature of the gas flow through the valves of the opened PDS line remainsbelow 350°C for a long time and reaches 600°C only very late, just before time ofvessel failure.• In the case of delayed depressurisation in the TLOFW scenario(Section 16.2.2.2 - Figures 13 and 14), the pressuriser gas temperature reaches620°C at valve opening. For valves opening at 650°C, the expected hot gastemperature through the valves should be below 600°C taking into accountMAAP 4 code uncertainties.• In the time period between the onset of core melting and the onset of corerelocation into the vessel lower head, for the LOOP scenario, gas naturalcirculation from the upper plenum increases the temperature of the hot leg of thepressuriser loop up to 880°C (Section 16.2.2.2 - Figure 8). For the TLOFWscenario with delayed RCP [RCS] depressurisation, the hot leg temperaturereaches 1000°C (Section 16.2.2.2 - Figure 15). Because of uncertainties in thecalculation of natural circulation in the primary system, 1000°C can beconsidered as a maximum gas temperature through the valves of the openedPDS line before vessel failure occurs.These temperature values meet the functional requirements of the PDS lines (long termincrease of temperature and increase of temperature followed by a cold thermal shock incase of short term spike due to a delayed depressurisation or a reflooding).2.2.3.4.2. Risk of Creep Rupture in RCS Pipes in case of Delayed DepressurisationThree distinct natural circulation patterns can be established in the RCP [RCS] under highpressure severe accident conditions:• in the reactor vessel between the core and upper plenum regions,