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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 109 / 295Document ID.No.UKEPR-0002-162 Issue 04The calculation was performed for one second. Less than 200 kg hydrogen is burnt in this periodof time, with a peak burning rate of 240 kg/s. The maximum flame velocity was always less than60 m/s. Thus, no significant dynamic loads occurred, either on the containment shell or on theinner walls. Extrapolating the burning rate to complete combustion, a pressure increase of2.8 bar is expected. Adding the 2.8 bar to the initial pressure of 2.9 bar, the total pressure isclose to the AICC pressure. The maximum dynamic pressure found locally on the containmentshell is 3.8 bar. Note that the assumption of no heat transfer to the wall in the COM3Dcalculation is valid because of the short period of calculation time for fast deflagration.Generally, for the SB(LOCA) scenario with delayed depressurisation and initial conditions ascalculated with GASFLOW, no extensive fast deflagration is predicted and thus no significantdynamic loads on the structures occur.As no substantial fast combustions occurred under these particular conditions, a set ofparametric calculations was performed as a sensitivity study:• reducing the initial steam concentration to show the effect on flame velocity andcombustion rate,• adding additional obstacles to show the effect on flame acceleration,• changing the location of ignition to investigate the flame propagation from poor torich mixture, and• closing all radial openings of the SG compartments.From these calculations, it was concluded that:• some flame acceleration occurred in the SG compartments but was followed bydeceleration in the dome, resulting in small pressure loads on the containment shell,• radial venting has a significant effect on the process of flame acceleration, and thusthe transfer of results from closed tube experiments to a realistic containmentprobably leads to very conservative results.High local pressure peaks on the containment shell did not occur because flame accelerationwithin the SG compartment is limited by radial venting and deceleration in the dome. The latterseems to be an effect of general importance. The risk of flame acceleration with the potential tojeopardise the containment integrity only exists, if at all, in the early period after onset ofhydrogen release. At that time, the distribution is not homogeneous and regions of highconcentration exist in the equipment rooms but little hydrogen is in the dome. Later, when theconcentration in the dome rises, the overall gas distribution is quite homogeneous, withhydrogen concentration well below 10% by volume. This effect is mainly due to convection butalso supported by the recombiners.2.3.3.4.4. 5 cm (20 cm 2 ) SB(LOCA) in the Cold Leg with Fast Secondary Cooldown andRe-Flood [Ref]This is the most conservative bounding scenario analysed because it combines low steamconcentration in the containment (from fast secondary cooldown) with a high peak hydrogenconcentration (from re-flood). Two cases were investigated:• ignition in the pump room at the elevation of the main coolant lines,