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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 42 / 295Document ID.No.UKEPR-0002-162 Issue 042.1.2.2.4. Selection of Hydrogen Releases into the ContainmentSection 16.2.2.1 - Figure 11 shows the mass of hydrogen produced against time for differentreflooding times for the SB(LOCA) scenario, “20 cm² break in cold leg with fast secondarycooldown” [Ref], which is a scenario with high hydrogen production and low steam content.The scenarios selected for the analysis of the different hydrogen risks: dynamic pressure dueto fast deflagration, AICC pressure and thermal loads due to recombination or combustionare given in Section 16.2.2.1 - Table 13.The different masses of hydrogen produced in-vessel selected for the containment analysisare presented in Section 16.2.2.1 - Figure 10.2.1.2.3. Risk of Basemat and Reactor Pit Ablation2.1.2.3.1. Parameters of Interest to select Reference ScenariosRelevant scenarios are selected to justify the functioning of temporary melt retention in thereactor pit as a principal prerequisite for successful melt spreading and long-term meltstabilisation. The corium releases from the vessel are the boundary conditions of themitigation approach. The scenarios are chosen with reference to the following relevantparameters:Pressure at reactor vessel failure:For the main representative scenarios without in-vessel reflooding, gravity driven flow is themain release mode of the corium from the vessel because the vessel failure pressure isexpected to be below 5 bar. Only for low likelihood bounding scenarios such as late in-vesselreflooding, can a corium dispersal mode occur since the vessel failure pressure could be inthe range up to 20 bar.Modes of reactor vessel failure:In spite of remaining R&D uncertainties, there is a high likelihood that in most cases thelower vessel head failure will be as a consequence of creep which may be accompanied bypartial melting of the vessel wall.For the main core melt scenarios, the first relocation of melt into the lower head occursthrough breaches in the heavy reflector and vessel baffle.According to MAAP4 predictions, there is no vessel failure due to ablation caused by acorium jet. Instead, MAAP 4 predicts the formation of a molten oxidic pool in the lower head[Ref].Different configurations of corium phases may exist which can lead to different stratificationlayers in the lower head. A particular assumption of stratification is taken into account in thestudies: the oxidic debris pool is homogeneously mixed and the metal layer rises to the top ofthe debris pool. The same convective circulation in the metal layer which transfers the decayheat of the debris bed upward is responsible for the sideward heat transfer between the bulkmetal layer and the vessel wall, which is at the melting point.