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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 115 / 295Document ID.No.UKEPR-0002-162 Issue 04The initial conditions at the moment of ignition are as follows:• Time since start of the accident: 7840 seconds• Time since start of hydrogen release: 40 seconds• Accumulated amount of hydrogen in the containment: 19 kg• Accumulated amount of hydrogen in the dome: 9 kg• Pressure in the containment: 2.2 bar• Hydrogen cloud above 4% by volume: 70 m 3• Average gas temperature in the containment: 110°C• Average gas temperature in the hydrogen cloud: 200°C• Initial containment shell surface temperature (hot spot): 101°C• Average hydrogen concentration in the 4% plume: 6% by volume• Average steam concentration in the 4% plume: 65% by volumeHydrogen release rate:• Through the break: < 0.05 kg/s• Through the relief tank: 0.9 kg/sSteam release rate:• Through the break: 10 kg/s• Through the relief tank: 57 kg/sAt the time of maximum ceiling temperature (8025 seconds), the containment pressure peakedat 3.9 bar. The maximum average containment atmospheric temperature was 330°C.An amount of 566 kg hydrogen was burnt within 175 seconds (3 minutes) up to the time ofmaximum hot spot temperature. At the end of the calculation at 8500 seconds, 640 kg hydrogenwas burnt, corresponding to 76% of the released hydrogen. Thus, more than twice as muchhydrogen was burnt as in the SB(LOCA) case without delayed depressurisation.As expected, the highest containment shell temperatures occurred at the ceiling of thecontainment above the loop 2 compartments. Peak temperatures were around 320°C andoccurred at the same time as the maximum gas temperature around 7975 seconds. Thesesurface temperatures drop continuously towards 220°C at the end of the calculation (8500seconds).The containment shell surface temperature for the vertical wall was significantly lower, decliningfrom top to bottom.