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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 74 / 295Document ID.No.UKEPR-0002-162 Issue 042.2. ASSESSMENT OF PRIMARY SYSTEM DEPRESSURISATIONFailure of the reactor pressure vessel (RPV) under high internal pressure is of importance tosevere accident risk as missiles could be created from vessel movement. Also high pressuremelt ejection (HPME) could lead to direct containment heating (DCH) by melt dispersal insidethe containment atmosphere which could result in subsequent containment failure.The general safety requirements which apply to the primary depressurisation system (PDS)of the EPR are:• the containment should be designed so that the pressures and temperaturesattained inside the containment as a consequence of a severe accident will notresult in its uncontrolled failure,• as containment design takes into account consequences related to a severeaccident (SA) but without considering loads induced by the high pressure meltejection (HPME), rupture of the reactor coolant system (RCP [RCS]) at highpressure must be excluded by design.Therefore, the ability to reduce the primary system pressure in high pressure severeaccidents should be ensured so that occurrence of a SA at high pressure can be prevented.The primary coolant system should therefore be equipped with pressure reduction systemsthat must be able to perform their safety function appropriately under high pressure severeaccident conditions.Depressurisation could either be intentional by operator action or unintentional by creepinduced rupture of the RCP [RCS] pipework due to heat-up if steam natural circulation isestablished within the RCP [RCS].One of the main provisions of the EPR design in this respect is the severe accident primarydepressurisation system (PDS) with its dedicated severe accident valves. A short descriptionof the primary depressurisation system is provided in the following section.2.2.1. Description of the Primary Depressurisation System (PDS)Even though RPV failure at high pressure is physically unlikely, the EPR is designed with anobjective to transfer high pressure core melt sequences into low pressure sequences withhigh reliability so that a high pressure vessel breach can be practically eliminated. This isachieved through two dedicated SA depressurisation valves (PDS valves) on two dischargelines on top of the pressuriser that are part of the PDS.The PDS consists of PDS valves, discharge lines, and the pressuriser relief tank (PRT). Eachline includes two valves in series that are connected to the same pressuriser nozzle, locatedat the same elevation as the nozzles of the safety valves, and discharge into the pressuriserrelief tank via the same line as the safety valves.The total steam flow capacity of one PDS valve is 900 te/h at 17.6 MPa, under saturatedsteam conditions. For both, Feed & Bleed (F&B) and SA functions, the motor operated valvesof one line only can be opened manually during high pressure accident sequences when thesevere accident criterion (core exit temperature = 650°C) is reached, which corresponds to apressuriser steam temperature of 350°C.