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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 134 / 295Document ID.No.UKEPR-0002-162 Issue 042.4. ASSESSMENT OF MELT STABILISATION2.4.1. Basic strategyDue to the high projected power rating of the EPR and the related low margins, In-Vessel meltRetention (IVR) by outside cooling of the Reactor Pressure Vessel (RPV) has been dismissedfrom the beginning. Instead, an ex-vessel Core Melt Stabilisation System (CMSS) is beingimplemented. Its function is based on the spreading of the melt onto the surface of a watercooledmetallic and concrete core catcher, followed by subsequent quenching with waterdrained passively from the In-containment Refuelling Water Storage Tank (IRWST). Theefficiency of the stabilisation process strongly benefits from the achieved increase in thesurface-to-volume ratio of the melt [Ref].The core catcher is located in a dedicated compartment adjacent to the pit. Under normaloperating conditions the connection between pit and spreading compartment is closed. In thecase of a severe accident it will be opened by the thermal destruction of a separating plug by thecore melt. Thanks to the spatial separation between pit and spreading compartment, the corecatcheris safe from potentially critical loads related to the failure of the RPV. Furthermore,unintentional flooding of the core catcher during power operation is not critical to the safety ofthe plant. As a consequence of this decoupling, power operation and design-basis mitigationmeasures remain unaffected by the existence of the core catcher.The relocation of the melt from the pit into the core catcher is preceded by a phase of temporaryretention in the reactor pit. This measure allows for the likelihood that the release of the moltenmaterial from the RPV will not take place in one pour, but in stages. Temporary retention isbased on the provision of a layer of sacrificial concrete that must be penetrated by the melt priorto its release from the pit. The resulting delay and the admixture of sacrificial concrete make thecharacteristics of the melt during relocation and subsequent spreading and stabilisationpredictable and independent of the inherent uncertainties associated with in-vessel melt poolformation and RPV failure.Melt arrival in the core catcher initiates the gravity-driven overflow of water from the IRWSTwhich cools the core catcher from the outside and brings the melt into a stable state by meansof passive systems only. As an option, the Containment Heat Removal System (EVU [CHRS])can be used to provide water to the core catcher actively. This will completely submerge thespreading compartment and the reactor pit and stop further steam discharge into thecontainment as a pre-condition for reaching atmospheric pressure conditions in the long-termwithout the need for a venting system.The provision of the CMSS avoids the interaction of the molten core with the structural concreteand with it the risk of:(i)penetrating the embedded liner,(ii)weakening and mechanically deforming load-bearing structures and the basematitself, and(iii)sustained release of non-condensable gas into the containment atmosphere.