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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 28 / 295Document ID.No.UKEPR-0002-162 Issue 042. ANALYSIS OF CORE MELT SEQUENCES2.1. IN-VESSEL ACCIDENT PROGRESSION AND SELECTION OFRELEVANT SCENARIOS2.1.1. Analysis of Core Melt ScenariosCore degradation and in-vessel core melt progression provide the characteristics of meltrelocation from the core into the vessel lower head and the energy released from the primarysystem, before and after vessel failure, and thus the initial conditions for assessing the loadson the surrounding structures and the containment.In order to assess the effectiveness of design provisions and mitigation features intended todeal with the different consequences of a severe accident, boundary conditions and designsafety requirements have been defined based on relevant core melt scenarios according tothe following multi-step approach:• the first step is to identify the relevant scenarios from a phenomenological pointof view.• the second step is to study the scenarios with integral codes and to classifydifferent core melt scenarios which are representative for the situations whichlead to a core melt. During this step, the following parameters are determined:primary circuit pressure level, mass and energy releases, steam, hydrogen andfission products releases, and melt release into the reactor pit after vesselfailure.• the third step is to select the relevant design scenarios associated with eachsevere accident challenge and to establish representative boundary conditionsthat provide the loads the mitigation feature(s) will have to cope with. Thesechallenges are as follows:oooooprimary coolant system depressurisation,hydrogen risk in the containment,ex-vessel melt stabilisation,containment pressurisation, andradiological releases into the containment.In compliance with the safety approach for the exclusion of high-pressure core meltscenarios, the selection contains scenarios that cover a wide range of physical phenomena.Thus, a reasonable envelope for the phenomena is provided.The selection of design-relevant scenarios is mainly based on deterministic principles;however, in some cases, additional probabilistic arguments are also used.