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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 110 / 295Document ID.No.UKEPR-0002-162 Issue 04• ignition in the dome at the elevation of the polar crane above the affected SGcompartment.In the case of the lower ignition point, flame acceleration in the SG compartment occurred,leading to flame speeds above 400 m/s at the top of the SG compartment. Deceleration in thedome reduced the flame speeds to around 100 m/s when the flame touches the containmentshell. Hence, dynamic effects were limited to the first 0.2 seconds and were well below the AICCpressure.In the case of ignition in the dome, a laminar combustion occurred with flame velocities around30 m/s. After 2 seconds, flame acceleration down into the SG compartments occurred, leadingto flame velocities of around 100 m/s. Loads on the containment shell were benign compared toignition in the pump room.However, despite the smooth pressure history on the containment shell, pressure differencesacross inner walls are important. Sub-section 16.2.2.3 - Figure 11 shows the pressure historiesfor two different locations on the inner wall of the SG tower together with a typical pressurehistory for the inner pump room walls and the annular compartments for the first 0.2 seconds.The maximum pressure difference across any wall is 0.4 bar.2.3.3.5. Temperature LoadsIn the case of a severe accident, thermal loads on the containment shell may result fromhydrogen oxidation, both by recombiners or during combustion. The following sub-sectionspresent typical results of calculations performed with GASFLOW that are relevant to thermalloads.2.3.3.5.1. Temperature Loads Due to RecombinationWhen there is no combustion, containment shell temperatures are determined by hydrogenrecombination. A specific investigation of thermal loads was performed for the LOOP scenariowith re-flood at the most unfavourable time (LOOP/R) [Ref], because this scenario leads to thelargest amount of hydrogen released into the containment (nearly 1000 kg). However, thisspecific LOOP scenario was investigated with an early version of the GASFLOW code that didnot include radiation from gas to walls. This means that gas temperatures, in particular close tothe walls, are over-predicted, while wall temperatures are under-predicted. Note that the steelliner was not modelled explicitly in any of the calculations.In general, no wall temperature above 121°C was identified in any of the calculations.Sub-section 16.2.2.3 - Table 3 lists the calculations performed and the major findings. Themaximum wall temperature depends on the amount of hydrogen recombined and thus on theamount of hydrogen available as well as other parameters such as steam concentration.Sub-section 16.2.2.3 - Figure 12 gives some time histories for containment shell temperatures atdifferent positions at the top of the vertical containment cylinder wall for the 5 cm (20 cm 2 )SB(LOCA) cold leg f.s.c. scenario. The maximum temperature observed for this scenario was90°C. For other scenarios, temperatures up to 121°C were found. The table only gives a lowerbound for the temperature in the SB(LOCA) scenario with re-flood because the calculation wasperformed for only a relatively short period of time and more hydrogen might be expected to beinvolved in the reaction than was actually accounted for in the calculation. The highestcontainment shell temperatures were found for the SB(LOCA) scenario with delayeddepressurisation.