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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 101 / 295Document ID.No.UKEPR-0002-162 Issue 04A low steam concentration in the containment also unfavourably influences both the flammabilityof the mixture and the detonation cell widths. Therefore, scenarios with fast secondarycooldown, which result in low steam concentration in the containment, must be investigated.The calculations presented here were performed for a representative configuration of the EPR(i.e. the basic design phase configuration), assuming a one-room containment and withoutconsidering the steel liner. It is a justified simplification to not explicitly model the transformationof the two-room containment into a one-room containment by means of rupture and convectionfoils and passively activated dampers, as this happens very early in the course of a severeaccident, well before hydrogen release into the containment. Consequently, at the time ofhydrogen peak release, the foils and dampers connecting the equipment rooms affected byhydrogen release to the annular rooms and the dome are already open, and efficient convectionthroughout these compartments has been established [Ref].The following section presents the results of the calculations of gas distribution, pressure andtemperature loads for the selected representative and bounding scenarios.2.3.2. Potential Pressure Loads2.3.2.1. Overview of the Analysed ScenariosIn order to determine potential pressure loads, different representative and bounding scenarioshave been analysed following the procedure described in section 2.3.1 of this sub-chapter. TheAICC pressure, which is a purely theoretical upper limit for pressure, is calculated for allscenarios, whereas the combustion process itself has been modelled only for the most importantscenarios. Sub-section 16.2.2.3 - Table 1 lists these scenarios analysed for pressure andthermal loadings. Bounding scenarios are given in the lower part of the table and are shadedwith grey. The abbreviations used are as follows: f.s.c. is fast secondary cooldown and p.c. ispartial cooldown. For some scenarios, the process of laminar combustion has also beencalculated with GASFLOW (see Appendix 16A). The main emphasis of these calculations is toquantify thermal loads. However, they also highlight the margins for AICC pressure.2.3.2.2. Hydrogen DistributionThe gas and temperature distributions, which are the prerequisites for the assessment ofcombustion loads, have been calculated for all scenarios with GASFLOW. This code has beendeveloped and validated by the Research Centre Karlsruhe and is described in Appendix 16A.The containment is divided into about 100,000 cells using cylindrical coordinates andnon-equidistant grid spacing in the radial and vertical directions [Ref].Sub-section 16.2.2.3 - Table 2 gives an overview of the main characteristics of the scenariosinvestigated. The maximum amount of hydrogen present in the containment varies from 450 to860 kg. Due to the recombiners, this value is always less than the amount of hydrogen releasedfrom the RCP [RCS] as discussed above. The extent of this difference depends on the releaserate. In case of slow release, e.g. from a 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with fastsecondary cooldown, the maximum amount of hydrogen in the containment is significantly lowerthan the cumulative release. In contrast, for a fast release, e.g. in a SB(LOCA)/D scenario(where ‘D’ signifies delayed depressurisation), the difference is marginal. The maximumhydrogen concentration also depends on the steam concentration. The hydrogen concentrationis relatively high with respect to the mass of hydrogen for scenarios with low steamconcentration, as in the case of a fast secondary cooldown, and relatively low for scenarios withhigh steam concentration, as in the case of a partial cooldown. For all scenarios the averagehydrogen concentration in the containment is always below 10% by volume. The size of theregion of high concentration (≥ 13% by volume) gives some first indication of the risk from fast