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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 100 / 295Document ID.No.UKEPR-0002-162 Issue 044. If fast combustion cannot be excluded by the criteria, an explicit calculation of the combustionprocess and the resulting pressure history is necessary. COM3D (see Appendix 16A) is used forthis purpose. In order to perform COM3D calculations, one (or more) instant(s) for ignition mustbe selected. Usually a compromise has to be found between decreasing likelihood of flameacceleration (i.e. increasing homogenisation of the atmosphere) and increasing amount ofhydrogen (i.e. potential energy) in the containment with time. For the selected cases, theGASFLOW results (gas and temperature distribution) have to be transferred as input values toCOM3D. Next, a location for the ignition must be selected, taking into account hydrogen andsteam concentrations in the vicinity and the available path for flame acceleration. Thecombustion process itself needs to be analysed only for a period of about one second. Dynamiceffects with high, sharp pressure peaks normally only occur in the first tenth of a second. It is notnecessary to calculate the total maximum pressure from the entire combustion process byCOM3D because it is covered by the AICC pressure. The calculation of the AICC pressure isbased on gas composition and temperature as a function of time.The justification process described above focuses on pressure loads resulting from globaldeflagration and on flame acceleration with the potential for DDT. It is based on particularscenarios, i.e. on calculated hydrogen release histories. For these scenarios the in-vesselproduction of 450 to 900 kg of hydrogen up until core slumping is predicted by MAAP-4 [Ref]. Ithas been shown by comparisons with other codes and by validation calculations with MAAP-4 ofQUENCH experiments [Ref], that MAAP-4 provides reasonably conservative results. In the exvesselphase, another 830 kg of hydrogen are generated by corium-concrete interactions in thereactor pit and core catcher. Consequently, the system verification considers a total amount ofhydrogen that significantly exceeds the amount that would be generated by hypotheticallyassuming 100% oxidation of core cladding.Hydrogen production is not an instantaneous process. It occurs over a time defined by the coredegradation process and by ex-vessel reactions. Generally, slow core degradation leads to alarge amount of hydrogen because the time available for oxidation is longer. Because of thedepletion by recombiners, the amount of hydrogen present in the containment is lower than thegenerated amount of hydrogen except for a short period during the beginning of release (beforethe start of recombiner operation). This difference is most pronounced for slow hydrogengeneration. This can be seen in Sub-section 16.2.2.3 - Figure 1, which shows the amount ofhydrogen generated during the core melt process, the amount of hydrogen depleted by therecombiners and the amount of hydrogen present in the containment for a SB(LOCA) scenariowith fast cooldown. An extensive programme of recombiner qualification tests has beenperformed in parallel with the development of the recombiners [Ref]. In addition, recombinermodelling in computational programs has been validated based on extensive, international,qualification testing and significant margins in the modelling of the recombiners for the CGCShave been demonstrated [Ref].. About 900 kg of hydrogen are recombined within 5.5 hours,which corresponds to an average recombination rate of 163 kg/h. The total amount of hydrogenresulting from 100% oxidation of core Zr corresponds to 1320 kg and would thus be removedwithin 8 hours. This satisfies a design aim of the EPR to complete hydrogen removal beforespraying might be required (12 hours).Hence the total generated hydrogen mass is not the most important parameter for estimating thehydrogen risk. The hydrogen release rate and the steam concentration in the containment aremore important for fast combustion.A high hydrogen release rate leads temporarily to a non-uniform hydrogen distribution with highpeak concentrations. This unfavourably affects both the possibility for flame acceleration and thedetonation cell widths. High hydrogen release rates can be associated with re-flooding of a hotcore. Therefore, re-flood scenarios must be considered.