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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 111 / 295Document ID.No.UKEPR-0002-162 Issue 04This study on the thermal loads from recombination led to the conclusion that containment shelltemperatures are tolerable. Although there are local high temperatures of up to around 800°C inthe exhaust gas from the recombiners and although large areas of the dome can have gastemperatures around 200°C, the containment shell temperatures are well below the saturationtemperature of steam at the design pressure. This is due to the arrangement of the recombiners,which are all located sufficiently far away from the containment shell in the configuration of theEPR utilised for these calculations (i.e. the Olkiluoto 3 plant).2.3.3.5.2. Temperature Loads Due to CombustionHigher local containment shell temperatures can be expected from hydrogen combustioncompared to recombination, because combustion heat can be released in a much shorter periodof time. As the EPR is not equipped with igniters, ignition can only occur accidentally. On theother hand, without immediate combustion, hydrogen can accumulate locally despite thepresence of recombiners with the potential of a late ignition. Recombiners or sparks fromelectrical equipment can be regarded as potential triggers for combustion.The analysis is based on three scenarios:• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with fast secondary cooldown (f.s.c.). Thisscenario covers combustion from in-vessel generated hydrogen.• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with partial cooldown (p.c.). This scenarioalso addresses combustion of ex-vessel generated hydrogen in a “standing flame”above the un-flooded melt.• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with partial cooldown (p.c.) and delayeddepressurisation. This scenario covers combustion from in-vessel generatedhydrogen.2.3.3.5.2.1. 5 cm (20 cm 2 ) SB(LOCA) in the Cold Leg with Fast Secondary Cooldown [Ref]Accidental ignition was assumed in the dome just above the SG compartment of the affectedloop 2 to maximise thermal loads on the containment shell. Two cases were analysed:a) Continuous combustion starting just after the onset of the hydrogen release above the SG ofloop 2 and ending with the termination of hydrogen release 4000 seconds later. Thus, theduration of combustion was about 40 minutes. This calculation was performed for an additional2000 seconds after combustion to allow for heat exchange between atmosphere and walls.b) Instantaneous combustion 1500 seconds after the onset of the hydrogen release,corresponding to an accumulation of 520 kg hydrogen in the containment. The duration of globalcombustion was about 50 seconds, where 250 kg of hydrogen was burnt. After globalcombustion, the released hydrogen is continuously burnt as in case a) close to the source withno significant further energy input (combustion of only 30 kg hydrogen). This calculation wascarried out for an additional 1300 seconds to allow for heat exchange to the walls as in case a).In both cases the combustion as calculated with GASFLOW was incomplete (less than half ofthe released hydrogen was burnt).