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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 35 / 295Document ID.No.UKEPR-0002-162 Issue 04Family 2: LB(LOCA) and SB(LOCA) with partial secondary cooldown only (Section16.2.2.1 - Table 8) [Ref]Breaks of different sizes have been analysed. The evolution of primary pressure is connectedto the steam discharge rate at the break:• in the case of large break LOCA scenarios, the break is sufficient to remove theresidual heat. Heat exchange in the water-fed SG is achieved from thesecondary side to the primary side (the secondary becomes a heat source andtransfers its energy to the primary): the primary pressure drops below thesecondary pressure.• in the case of smaller breaks, e.g. a break of 46 cm 2 (7.5 cm), since the steamdischarge rate at the break is insufficient to completely remove the heatgenerated by the core, a part of the decay heat is transferred to the secondaryside by the vaporisation of the feedwater which supplies water to the steamgenerator. The heat flux goes from the primary side of the steam generator tubesto the secondary side; the temperature of the primary side coolant is slightlyhigher than the saturation temperature of the secondary side pressure of thewater-fed SG. The primary pressure consequently stabilises at a value higherthan that of the secondary pressure which corresponds to the steam generatorrelief valve setpoint. In the case of a break of 46 cm 2 , the primary pressurefollows the evolution of the secondary pressure: during partial cooldown, thepressure falls then stabilises just below 60 bar.• For evaluation of the calculation a LB(LOCA) and a 5 cm SB(LOCA), the sameparameters as indicated above are depicted in Section 16.2.2.1 - Figures 3 to 6.Family 3: Scenarios without RCS break and with SG dryout (Section 16.2.2.1 - Table 9)Two scenarios were analysed: LOOP [Ref] and TLOFW [Ref].The description of a LOOP scenario with loss of the emergency diesel generators and loss ofemergency feedwater system is as follows:• the loss of offsite power at time t = 0 seconds causes the shutdown of theprimary pumps and the ARE [MFWS] pumps, reactor and turbine trip. Theturbine isolation valves close and a signal is sent to start up the emergencyfeedwater injection. The emergency feedwater and emergency power (dieselgenerators) are unavailable, thus preventing the use of active systems. Only thepassive components are available: discharge valves and pressuriser, steamgenerator and accumulator safety valves.• 6 seconds after accident initiation, the control rods are fully inserted into thecore, but the core heat generation is such that the primary pressure rises to theopening pressure of the pressuriser valves.• from this time, the steam generators are removing enough decay heat from thecore by secondary water vaporisation to allow the primary pressure to decreaseto around 140 bar. Subsequently, there is no longer enough water in the SGsecondary side resulting in an increase of primary system pressure to the cyclingpoint of the pressuriser valves. When the lowest opening pressure of the valvesis reached, the decay heat removal begins by the loss of primary waterinventory. The steam generators have dried out after just 2 hours.