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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 107 / 295Document ID.No.UKEPR-0002-162 Issue 042.3.3.4. Dynamic Pressure LoadsThe consequences of fast combustion on the containment shell are analysed with COM3D (seeAppendix 16A), which is a CFD code specially developed by Research Centre Karlsruhe for theanalysis of fast combustion processes. Combustion is modelled in the code on the basis of aneddy break-up model. Initial conditions for the code are taken from the respective GASFLOWcalculation; ignition time and location are selected taking into account the sigma criterion forflame acceleration.The simulation of turbulent combustion requires detailed spatial resolution of the reacting flows.Therefore, the containment is modelled with finer resolution than required for GASFLOW(distribution and laminar combustion). COM3D uses a cubic Cartesian grid (41·41·41 cm 3 ) toachieve this high resolution within the calculation. The total number of cells in the wholecontainment exceeds 2,000,000.Two representative scenarios and two bounding scenarios have been analysed to envelop allscenarios where there is the possibility of hydrogen combustion:• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with fast secondary cooldown (f.s.c.);rationale: steam depleted condition, large amount of hydrogen,• 7.5 cm (46 cm 2 ) SB(LOCA) at the top of the pressuriser with fast secondarycooldown (f.s.c); rationale: steam depleted condition, stratification,• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with partial cooldown (p.c.) and delayeddepressurisation (SB(LOCA)/D); rationale: high release rate and thus temporarilylarge local concentrations, large amount of hydrogen, mixed release mode,• 5 cm (20 cm 2 ) SB(LOCA) in the cold leg with fast secondary cooldown (f.s.c.) andre-flood (SB(LOCA)/R); rationale: steam depleted condition, large amount ofhydrogen, high release rate.These scenarios are a subset of those listed in section 2.3.2.3 of this sub-chapter.2.3.3.4.1. 5 cm (20 cm 2 ) SB(LOCA) in the Cold Leg with Fast Secondary Cooldown [Ref]Based on the time dependent hydrogen distribution as calculated with GASFLOW, the criteriafor the exclusion of flame acceleration (sigma index < 1) is violated for a short period of time inparticular rooms (local violation). Fast secondary cooldown is very onerous for hydrogencombustion because of the low steam concentration within the containment when hydrogen isreleased. The event is characterised by release only through the break, and the combustionprocess is analysed to assess the consequences of the sigma criterion being violated in eventswhich are significant contributors to core melt frequency. An accidental ignition is assumed atthe most unfavourable time (just before the maximum amount of hydrogen has been releasedinto the containment and at the end of the time period of violation of the sigma criterion) and atthe most unfavourable location (above the break and just below the upper pump room).Flame acceleration occurs in the upper part of the pump room in which the break is located andthe two adjacent SG compartments. Here pressure pulses typical of a fast deflagration arefound. On its way through the containment, the flame decelerates in the dome due to the lowerhydrogen concentration compared to the loop compartments. Only at a level close to the top ofthe SG tower, e.g. at the top of the cylindrical part of the containment, dynamic effects can beobserved. At the top and bottom of the containment the pressure rise is quasi-static.