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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 8 / 295Document ID.No.UKEPR-0002-162 Issue 04In addition to the spray mode, which alone is sufficient to fulfil the EPR safety objectives forsevere accidents, the EVU [CHRS] provides for active core catcher cooling, which creates asub-cooled water pool in the spreading compartment in the long-term, whereas passivecooling only allows for removal of heat from the melt by boiling and evaporation. Due to thehigh capacity of the EVU [CHRS] coolers, one train alone is sufficient to achieve this target.Therefore, the second EVU [CHRS] train can still be used for spraying and thus for removingdecay heat produced by airborne and deposited fission products directly from thecontainment atmosphere.Specifically, active cooling is achieved by closing the valve for water supply to the spray ringand opening the valves for water supply to the core catcher for the relevant EVU [CHRS]train. In order to prevent most of the coolant from flowing back into the IRWST through theopen connection between IRWST and core catcher, a passive outflow reducer (POR) isplaced between the IRWST and the connection to the flooding line. The POR has a high flowresistance in the IRWST direction and a low resistance in the opposite direction. Therefore,this element effectively limits direct backflow of water to the IRWST and ensures that most ofthe coolant will reach the core catcher.As this is basically a traditional fluid system, no R&D is required for its implementation.1.2.5. Limitation of Radiological ReleasesA fundamental contributor to the limitation of radiological releases to the environment is thecontainment, which has a maximum leak rate of 0.3 vol%/day at containment designpressure. This leak tightness is secured by the containment steel liner, hatch and airlocks,penetrations and a containment isolation which are able to withstand ambient conditionsprevailing inside the containment in severe accidents. Most leakage is collected in theannulus, which is kept at sub-atmospheric pressure by the annulus ventilation system(EDE [AVS]). This system is also used to route any leakage through HEPA and iodine filtersin series to the stack, from which they are released to the environment in a controlledmanner. Leakages which are not collected in the annulus enter the peripheral buildings andare filtered before being released.The annulus ventilation system has two operation modes. If AC power is still available, twofans with a maximum exhaust rate of 1000 m 3 /h each, keep sub-pressure in the annulus.Alternatively, the annulus ventilation in combination with the stack is designed to developsufficient natural draught to maintain sub-pressure for the case that AC power is notavailable.