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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 200 / 295Document ID.No.UKEPR-0002-162 Issue 04After blow down, the local pressure differences in the containment are very small in relationto the absolute pressure.In the case of activation of two EVU [CHRS] trains after 12.4 hours, the containmentpressure remains well below 5.5 bar. The long term pressure is below 2 bar and decreasessteadily.2.5.3.3.2. Containment temperatureThe local gas temperature distribution in the containment is very inhomogeneous until~30,000 seconds (Sub-section 16.2.2.5 - Figures 3a and 3b). During hydrogen combustion,the temperature peak is highest in the upper equipment compartments, i.e. zone Eq14. Lateron, in the long term steaming phase, the temperature differences are reduced significantly.In Sub-section 16.2.2.5 - Figure 3b, the temperature history is shown for the case withactivation of 2 EVU [CHRS] trains after 12.4 hours (~44,700 seconds). After this, thetemperature decreases in contrast to the behaviour without EVU [CHRS]. The temperaturesin the inaccessible part of the containment eventually become higher than in the dome.Sub-section 16.2.2.5 - Figures 4a, 4b show the liner surface temperature for the dome (zoneAc7) and a lower annular compartment (zone Ac4) with and without activation of 2 EVU[CHRS] trains with respect to the gas temperature. The liner temperature remains below150°C. After activation of the EVU [CHRS], the temperature decreases continually, goingbelow 100°C at ~57,000 seconds.The liner temperature in the annular compartment Ac4 remains below the liner temperaturein the dome in the case without activation of the EVU [CHRS]. After activation of the EVU[CHRS] the gas and liner temperatures equalise very quickly below 100°C (see Sub-section16.2.2.5 - Table 8).The IRWST water temperature and the saturation temperature (with respect to thecontainment pressure) are shown in Sub-section 16.2.2.5 - Figures 5a, 5b. The IRWSTwater, which is fed into the core catcher in the long term steaming phase, is significantly subcooled.The sub-cooling for the bottom water layer remains > 50°C without activation of theEVU [CHRS]. In the case with two EVU [CHRS] trains in operation, the sub-cooling isreduced to 20°C. This reduction in the sub-cooling margin results from both the higherIRWST water temperature and the lower saturation temperature (because of the lowercontainment pressure). The temperature of the surface water layer is closer to the saturationtemperature when there is no activation of the EVU [CHRS].2.5.3.3.3. Containment humidityThe local humidity distribution in the containment is very inhomogeneous (Sub-section16.2.2.5 - Figures 6a and 6b). Later on, in the long term steaming phase, the humiditydifferences reduce with time. With the exception of the first phase of strong steam releaseafter LOCA, the containment atmosphere remains in the superheated state everywhere in thecontainment without activation of the EVU [CHRS]. Even in the case with activation of theEVU [CHRS] (2 trains) after 12.4 hours, the lower equipment rooms remain superheatedwhile the dome achieves a saturated state.