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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 116 / 295Document ID.No.UKEPR-0002-162 Issue 04At high elevations above 45 m, peak temperatures were around 136°C at 8220 seconds. Thetemperatures decrease slowly to about 131°C at the end of the calculation (8,500 seconds).At lower elevations the temperatures were below 122°C at all times during combustion. Thus forthe material hatch no temperature above 122°C can be expected for this extremely challengingscenario.The temperatures mentioned above are surface temperatures. Inside the containment shell thetemperatures decrease within 25 cm towards the steady state temperature that existed beforethe hydrogen combustion event.High temperatures were spatially limited to a hot spot above the SG compartments on the sidewhere combustion started and lasted only a few hundred seconds. Sub-section 16.2.2.3 - Figure13 shows the time histories of the ceiling temperature at different locations.2.3.4. Conclusions from the Verification ProcessThe EPR CGCS has been evaluated in a consistent manner utilising state-of-the-art CFD codes,using an adequate nodalisation scheme, and taking into account current knowledge oncombustion behaviour, such as criteria for the non-occurrence of flame acceleration and DDT. Asummary of corresponding current and future research and development work is presented in[Ref].Several representative and bounding scenarios have been specified to justify the EPR CGCS.The major findings are as follows:• Average hydrogen concentration does not exceed 10% by volume for any scenario.• AICC pressure is at no time > 5.5 bar for representative scenarios.• AICC pressure can reach 6.3 bar for a bounding scenario. The realistic pressure ismuch lower because combustion is neither complete (hydrogen concentration toolow) nor adiabatic (relatively slow combustion).• Recombination rate of hydrogen is several hundred kg/hour. This is sufficient toensure that hydrogen removal is sufficiently advanced when spraying might berequired (12 hours).• Temperature loads on the containment shell resulting from recombination are below120°C.• Temperature loads on the containment shell are below 170°C for representativescenarios when considering combustion of in-vessel generated hydrogen. Ex-vesselgenerated hydrogen may lead to containment shell temperature of at most 220°Cdue to continuous combustion (standing flame). These temperatures are reached inthe dome only.• Peak temperature on the containment shell from combustion is around 300°C forbounding scenarios (SB(LOCA) with delayed depressurisation, steel liner explicitlymodelled). However, this peak temperature is only reached in the dome andinvolves only an area of around 10 m 2 .