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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 204 / 295Document ID.No.UKEPR-0002-162 Issue 042.5.5.2. Time history and input data for the ex-vessel phaseThe time history of key events for the ex-vessel phase is given in Sub-section 16.2.2.5 –Table 13.After vessel failure at ~30,360 seconds the core melt pours into the reactor pit and MCCIoccurs. Steam, hydrogen, CO 2 and CO are produced and released into the containment. Thegeneration of these gases is modelled using COSACO. Because of the high temperatures inthe pit, instantaneous combustion of the generated hydrogen and carbon monoxide in astanding flame is assumed.At ~38,400 seconds the melt gate fails and the liquid core melt/concrete mixture flows downinto the core catcher. MCCI occurs there in which steam, hydrogen, CO 2 and CO areproduced and released into the containment. Again, the generation of these gases ismodelled using COSACO. Because of the high temperatures in the core catcher,instantaneous combustion of the generated hydrogen and carbon monoxide in a standingflame is assumed. MCCI continues until all sacrificial concrete is consumed at ~44,300seconds.The activation of two EVU [CHRS] trains is considered at ~54,600 seconds (12 hours afterthe core outlet temperature reaches 650°C).2.5.5.3. Results of COCOSYS calculation2.5.5.3.1. Containment pressureThe pressure time history for the LOOP case is shown in Sub-section 16.2.2.5 - Figure 12.The relevant phases are:• First pressure increase due to release from the quench tank (2.6 bar at ~12,000seconds). Subsequently the pressure decreases due to reduced steam release.• Core melt quenching in the core catcher (starting at ~38,700 seconds) causes asignificant pressure peak (4.7 bar at ~40,100 seconds) because of the(assumed) high steam production rate of 100 kg/s.• Pressure decrease due to the cessation of steam production during the filling ofthe spreading room (from ~40,100 seconds – 45,000 seconds).• Continuation of pressure decrease in the containment during the heat-up of thewater inventory in the core catcher (without steam production) until saturation(with respect to the actual containment pressure) is reached at ~55,000 seconds.• Enhanced pressure decrease after the start of two trains of the EVU [CHRS] at~54,600 seconds.• After the beginning of steam production in the core catcher, due to the decayheat of the core melt (at ~55,000 seconds), the rate of pressure decrease isreduced to 0.01 bar/hour at ~100,000 seconds.The pressure remains well below 5.5 bar at all times. See Sub-section 16.2.2.5 - Table 14.