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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 158 / 295Document ID.No.UKEPR-0002-162 Issue 04Due to the high capacity of the EVU [CHRS] coolers, one train is sufficient to achieve this target.Therefore, the second EVU [CHRS] train could still be used temporarily for containment spraysand thus for removing decay heat produced by airborne and deposited fission products from thecontainment atmosphere.Specifically, active cooling would be conducted by closing the valve for water supply to the sprayring and opening the valves for water supply to the core catcher in the relevant EVU [CHRS]train. To prevent most of the coolant from flowing back into the IRWST through the openconnection between IRWST and core catcher, a passive outflow reducer (POR) is providedbetween the IRWST and the connection to the flooding line. The POR has a high flow resistancein the direction towards the IRWST and a low resistance in opposite direction. Therefore, thiselement effectively limits direct backflow of water to the IRWST and ensures that most of thecoolant will reach the core catcher.During active core catcher cooling, the water in the spreading compartment would rise to the topof the chimney where it spreads on the heavy floor and, from there, flows back into the IRWST.As the spreading compartment and the reactor pit are connected via the open melt gate, thewater level in the pit would also rise to the approximate level of the RPV nozzles. In this way, theresidual RPV and the structural concrete around the reactor pit, which were exposed to hightemperature during the phase of temporary melt retention, would be better cooled, resulting in afaster temperature decrease in this area.The temperature at the core catcher/basemat interface and thus the temperature gradientthrough the base-slab will also be reduced relative to passive cooling, as the temperatureboundary condition will be lower than the saturation temperature.Another positive side effect is that the sub-cooled pool terminates the re-suspension of fissionproducts during boiling at the water surface. Together with the achieved pressure reduction(ultimately to ambient pressure), this leads to a decrease of the fission product inventory in thecontainment atmosphere and in consequence, a reduction of the potential activity release in theevent of leaks.For the parts of the EVU [CHRS] located outside the containment, the active mode of corecatcher cooling will imply the same requirements as the active spray mode. The water levels inthe spreading compartment, reactor pit and IRWST during active cooling are shownschematically in Sub-section 16.2.2.4 - Figure 19.2.4.1.5.1. Validation strategyThe interaction of the melt with the sacrificial concrete layer in the spreading compartment isanalysed with the code COSACO (Appendix 16A). As an initial condition, it applies the meltstate at the end of the temporary melt retention phase in the reactor pit, obtained by the analysisdescribed in section 2.4.1.5.2.1 of this sub-chapter. The time to ablate the sacrificial concrete iscompared with the time needed to flood the pipes and channels of the cooling system and to fillthe freeboard between the concrete wall and the core catcher.