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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 113 / 295Document ID.No.UKEPR-0002-162 Issue 04• Amount of hydrogen in the dome: 250 kg• Average hydrogen concentration: 7.8% by volume• Average hydrogen concentration in the dome: 9.4% by volume• Average steam concentration in the 4% plume: 16% by volume• Average steam concentration in the plume in the dome: 17% by volume• Internal wall surface temperatures: 61 to 67°C• Containment shell surface temperatures: 61 to 64°CBecause of combustion, the pressure rises within 50 seconds to 2.2 bar and the average gastemperature increases to 290°C. This is well below the corresponding AICC values (4.1 bar and770°C, respectively) and indicates that combustion is far from complete. The increase of bothpressure and temperature occurs within 50 seconds, while the respective decline to the halfvalue takes 150 seconds. At the end of the period under consideration the average gastemperature is 114°C.Only 250 kg of hydrogen is burnt immediately, and another 56 kg is burnt slowly until the end ofthe period under consideration, 1300 seconds after ignition.The maximum surface temperature of the inner walls is around 155°C, whereas the maximumcontainment surface temperature is around 82°C at half height and 140°C at the top of thecontainment. Thus, at around 20°C, the increase is rather low at half height, but may be as highas 80°C at the top of the dome.2.3.3.5.2.2. 5 cm (20 cm2) SB(LOCA) in the Cold Leg with Partial Cooldown (WithConsideration of Ex-Vessel Hydrogen) [Ref]To conservatively address the ex-vessel hydrogen risk, a scenario was selected with• minor Zr oxidation in the in-vessel phase (a large amount of Zr remains in theex-vessel melt leading to fast early dissolution of the sacrificial concrete and to highrelease rates of hydrogen), and• early vessel failure to reduce the time available for the recombination of in-vesselhydrogenA 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with only partial cooldown (p.c.) was assumed. This isa so-called “mixed release” scenario, which means that, in addition to the break, in-vesselhydrogen is also released from the relief tank by operator action (at the 650°C core outlettemperature signal) into the lower part of the containment following depressurisation.Melt-concrete interaction and the corresponding hydrogen release has been calculated with theCOSACO code (see Appendix 16A), which has been developed for the EPR core meltstabilisation system.