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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 40 / 295Document ID.No.UKEPR-0002-162 Issue 04• the thermal loads on the containment and the steel liner due to recombination orcombustion; the available mass of hydrogen in the containment which may burnis the main parameter influencing these loads.2.1.2.2.2. Hydrogen Production2.1.2.2.2.1. Representative ScenariosThe amount of hydrogen produced has been determined for all scenarios studied (Section16.2.2.1 - Table 13). These "representative scenarios" are selected on the basis of theirlikelihood and are used to demonstrate the efficiency of hydrogen mitigation features.Several phases of hydrogen production are identified:• The first oxidation phase including relocation of oxidic and metallic materials inthe core. This corresponds to the highest H 2 production as predicted by theMAAP4 code [Ref] and mainly depends on the oxidation surface area, kineticrate, steam availability (dependent on the water supply and the flow blockages inthe core), and the diffusion of steam and hydrogen in the gas mixture in theuncovered part of the core.• The second phase during relocation of corium into the lower head whichdepends on the relocation flow rate and debris fragmentation level; there is nosignificant H 2 production in phase 2 according to predictions by the MAAP4 code[Ref].• The ex-vessel production phase, which depends on the interaction between meltand sacrificial concrete in the reactor pit and on the spreading area.Only the hydrogen produced during the first oxidation phase is considered to be relevantbecause the rate of recombination is greater than the production rate during later phases ofthe accident progression [Ref] and leads to good mixing of the containment atmosphere, thusreducing the risk of fast deflagration.Many sensitivity studies and code comparisons have been performed for this phase whichconfirms the suitability of MAAP4. In general, it can be stated that MAAP4 hydrogenproduction rates bound other calculations for scenarios without reflooding.Thus, by using MAAP4 results, the relevant scenarios for hydrogen production have beenchosen which have the highest amount of hydrogen produced in one phase and a high flowrate.In order to assess the local H 2 risk in different compartments, SB(LOCA) studies have beenperformed not only for different break sizes but also for different locations, with breaks in thecold leg, hot leg, and the pressuriser [Ref].In order to investigate the effect of hydrogen released after RPV failure and the effect ofthermal loads resulting from standing flames, the SB(LOCA) “20 cm² cold leg scenario withonly partial secondary cooldown” has been investigated [Ref]. This is a representativescenario which leads to the production of significant amounts of burnable hydrogen andwhich minimises the period between in-vessel and ex-vessel hydrogen production.