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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 41 / 295Document ID.No.UKEPR-0002-162 Issue 042.1.2.2.2.2. Bounding ScenariosThe production of hydrogen with in-vessel passive or active reflooding, which are consideredas bounding scenarios, have been determined in a conservative manner. Additionalproduction of hydrogen has been calculated for many reflooding times with MAAP4 code andonly the worst case, corresponding to maximum hydrogen production together with the mostunfavourable moment of reflooding is selected. MAAP4 predicts significant additionalhydrogen production rate, only for reflooding which occurs during the autocatalytic oxidationphase of the rods. Passive accumulator water injection in the case of late depressurisation ina high pressure scenario or active reflooding with MHSI or LHSI pumps running provide anadditional supply of steam, which sustains and reinforces the oxidation process.The reflooding scenarios are "bounding scenarios" selected they include phenomena thatmight aggravate the hydrogen risk. They are used to demonstrate the robustness of theconcept of preventing early containment failure with regard to “cliff-edge” effects.2.1.2.2.3. Hydrogen Release ModeDirect discharge into the containment is via the pressuriser relief tank which has dischargepipes installed lower than the pressuriser safety valves to avoid any water plug downstreamof the valves. A large pressuriser relief tank is needed to collect the pressuriser safety valvedischarge to avoid contaminating the containment in the most frequent events and to collectdischarges in case the pressuriser safety valve is stuck in the open position for a few minutesduring testing.A 40 m 3 tank is compatible with the present layout. Its location is at the 1.5 m level. The tankis in series with the discharge piping and is separated by a wall from the neighbouringgenerator blowdown system flash tank. It is equipped with two rupture disks which direct thedischarge of the pressuriser relief tank to two reactor coolant pump compartments (2 and 3).According to the behaviour of the pressuriser relief tank rupture disk, which ruptures at19 bar differential pressure, three different release modes are possible:• release only through the break as in SB(LOCA) scenarios with fast secondarycooldown because the RCP [RCS] is depressurised to below 20 bar due to thebreak size or steam generator heat removal before the core outlet temperaturereaches 650°C and a PDS line is opened. [Ref]• release only through the pressuriser valves for core melt scenario without anyRCP [RCS] break, such as LOOP with failure of 6 diesel generators or TLOFW.• release of gas mixture through the break and through pressuriser valves forscenarios such as SB(LOCA) with only secondary partial cooldown. A mixedrelease scenario could influence:o The hydrogen content in pump compartments 2 or 3 in the case ofadditional releases: break release + PDS release.oThe risk of flame propagation from areas having leaner mixtures into areaswith richer mixtures (or the contrary) in the case of two different releaselocations.