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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 38 / 295Document ID.No.UKEPR-0002-162 Issue 04The consequence of reflooding the reactor vessel due to accident management proceduresare analysed in a parametric manner: the parameters are the time of onset of safety injectionand the flow rate. The variations in these parameters are studied for two groups of scenarios:• a scenario in which the RCP [RCS] is intact: LOOP and total loss of feedwater,• a core melt scenario with a break of the RCP [RCS]: SB(LOCA) with partialsecondary cooldown only for the late RCP [RCS] depressurisation andSB(LOCA) with fast secondary cooldown for active reflooding by safety injection.2.1.2. Selection of Relevant Scenarios to Design Mitigation FeaturesThe boundary conditions used in the design of mitigation measures are derived from theresults of representative or bounding scenarios which may reasonably fall within the range ofthe phenomena which must be controlled by mitigation features. These are used as a basisfor designing the mitigation measures for the following risks:• risk of reactor vessel failure at high pressure (scenarios of core melt at highpressure):ooprevention of induced steam generator tube rupture (SGTR) and the risk ofcontainment bypass by radioactive materials,preventing the corium from being dispersed in the containment under highpressure and leading to direct containment heating (DCH).• hydrogen risk:o preventing a global detonation or local transition from deflagration todetonation,o limiting the global deflagration pressure peaks to below the containmentproof pressure of 6.5 bar, for both representative and bounding scenarios.• risk of ablation of foundation raft:oavoiding corium ablation of the concrete foundation raft and in particularthe penetration of the liner.• risk of excess containment pressure for a long period of time:olimiting the containment pressure peaks to below the containment designpressure of 5.5 bar.• risk of not meeting radiological targets:oolimiting the leak rate from the internal containment to 0.3% per day of thegas volume inside the containment at the design pressure of 5.5 bar, underaccident conditions without direct leaks to the environment,prevention of containment bypass.