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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 108 / 295Document ID.No.UKEPR-0002-162 Issue 04The pressure history at two locations of the containment shell is plotted in Sub-section 16.2.2.3 -Figure 10 for the top of the cylindrical region of the containment. On the side where the ignitionspot is located, pressurisation starts at the top of the cylindrical part of the containment at thesame time as at lower locations around the material hatch. On the opposite side, the pressurerise is delayed by about 0.05 seconds.Strong oscillations in pressure, more pronounced than at the elevation of the material hatch, arelimited to the time range before 0.3 seconds. Later, a quasi-static pressure rise dominates. Atthe top of the containment only weak oscillations occur. The maximum dynamic pressure foundanywhere on the containment shell is 2.7 bar. The pressure load on the containment shell isthus essentially quasi-static.In summary, the observed combustion is a mild fast deflagration starting quite violently withinthe affected loop but damped on its way towards the containment shell because of the lowerhydrogen concentration in the dome.2.3.3.4.2. 7.5 cm (46 cm²) SB(LOCA) at the Top of the Pressuriser with Fast SecondaryCooldown [Ref]This scenario has been selected for analysis with COM3D because of the presence of hydrogenstratification above the break location. Ignition has been assumed at the most unfavourable time(29,700 seconds into the accident) which is characterised by a hydrogen concentration in thedome of about 12% by volume. Hydrogen concentration is above 24% by volume in thepressuriser room and above 12% in the adjacent SG compartment, which is connected to thedome. Steam concentration within the hydrogen plume is between 20 and 25% by volume. Thesigma criterion is violated in the pressuriser and SG compartment as well as in parts of thedome at the time of ignition. Ignition is assumed at the worst location, that is in the SGcompartment at the elevation of the connection to the upper pressuriser room.Apart from short pressure spikes up to 4 bar close to the ignition location, no dynamic effects onthe containment shell are found. Therefore, combustion pressure is bounded by the AICCpressure for this scenario (4.3 bar).2.3.3.4.3. 5 cm (20 cm 2 ) SB(LOCA) in the Cold Leg with Partial Cooldown and DelayedDepressurisation [Ref]This bounding scenario results in a large release of hydrogen with high release rates due toflooding of the hot core by the accumulators. This is a result of the assumption thatdepressurisation of the RCP [RCS] by the operator is delayed. The sigma index is > 1 only for ashort period between 7900 and 8100 seconds. Later, homogenisation of the atmosphere andrecombination eliminates the possibility of flame acceleration. The maximum sigma index occursvery early between 7900 and 7950 seconds, when the amount of hydrogen in the containment isstill small. During this period the lambda criterion is also violated (leading to a risk of DDT). Themost severe consequences of combustion can be expected when the maximum amount ofhydrogen in the containment is present during the period of DDT risk. Consequently, ignitionwas assumed at 8000 seconds just above the break, where the potential for flame accelerationis highest.