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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 199 / 295Document ID.No.UKEPR-0002-162 Issue 04At ~17,500 seconds, the melt gate fails and the liquid core melt/concrete mixture flows downinto the core catcher. Further MCCI occurs there until all the sacrificial concrete is consumedat ~22,600 seconds. During the MCCI in the core catcher steam, hydrogen, CO 2 and CO areproduced and released into the containment. As before, the generation of these gases ismodelled using COSACO. Again, because of the high temperatures in the core catcher,instantaneous combustion of the generated hydrogen and CO in a standing flame isassumed.2.5.3.3. Results of the COCOSYS calculation2.5.3.3.1. Containment pressureThe pressure time history for the LB(LOCA) case without EVU [CHRS] is shown inSub-section 16.2.2.5 - Figure 2a and with activation of two EVU [CHRS] trains after12.4 hours in Sub-section 16.2.2.5 - Figure 2b.The relevant phases are:• First pressure peak at the beginning of the event during blow-down andsubsequent pressure decrease due to strongly decreasing steam and waterrelease.• Small pressure peak after core relocation (~7,400 seconds).• Ex-vessel phase with MCCI in the reactor pit after initial vessel failure from~10,680 seconds – 17,500 seconds.• At ~17,300 seconds, a large hydrogen release due to the layer inversion in thecore melt in the pit. The subsequent combustion leads to increase incontainment pressure.• The quenching of the core melt in the core catcher (starting at ~17,800 seconds)causes a significant pressure peak because of the (assumed) high steamproduction rate of 100 kg/s until ~19,300 seconds.• Pressure decrease due to the cessation of steam production during the filling ofthe core catcher with sub-cooled water from the IRWST (from ~19,300 seconds– 25,000 seconds).• Continuation of pressure decrease in the containment during the heat-up of thewater inventory in the core catcher (without steam production) until saturation isreached at ~33,000 seconds.• Long term pressure increase after commencement of boiling and subsequentsteam production due to the decay heat of the core melt.• In the case of activation of two trains of the EVU [CHRS] after 12.4 hours, thepressure decreases immediately.The pressure development with and without activation of the EVU [CHRS] is summarised inSub-section 16.2.2.5 - Table 7.