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

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SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 21 / 295Document ID.No.UKEPR-0002-0162 Issue 04The second configuration studies higher masses of corium relocated in the lower head andforming a molten pool. When reflooding occurs the upper part of the pool is fragmented andcooled by forming a debris bed. With this configuration, vessel failure cannot be prevented. Aspecific model is used to study the geometry in which the bottom of the pool is withoutmoderation (due to the high temperature of the pool centre, which prevents liquid waterpenetration) while the upper part is cooled and filled with water at the optimum porosity andis consequently moderated.The results show that the potential for criticality is not determined by the dry corium mass,but by the small mass of debris deposited on the crust, because this material is moderated.Recriticality does not occur for a cooled thickness under 15 cm. However, this is more thanthe maximum cooled thickness calculated by the severe accident code MAAP, This isbecause water penetration in the debris bed is limited by steam production and the limitedtime before vessel failure.Therefore, for phase 3 there is no real potential for recriticality.1.3.2.1.4 ConclusionWithout water injection in the vessel the corium has a large subcriticality margin with UO 2 fueldue to the negative void coefficient. Consequently there is no potential for recriticality.In the case of Safety Injection System (RIS [SIS]) recovery during the core degradation, aninjection at the IRWST boron concentration presents no recriticality risk in phase 1 of thecore degradation, where the available fuel surface for heat transfer is high. In phases 2 and 3there are uncertainties regarding the effective coolable mass in low pressure degradedscenarios. Therefore available estimations show that the effective coolable mass is notsufficient to reach the minimum mass required to produce a critical configuration under therelevant hypotheses described above. Even though the corium mass that may be fragmentedand cooled remains limited, the operating strategy for severe accidents promotes the abilityto add highly borated water when the corium is maintained in the vessel to allow for anyuncertainty in the assessments.
SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 22 / 295Document ID.No.UKEPR-0002-0162 Issue 04SUB-SECTION 16.2.1.3.2.1 - TABLE 1In-Vessel Recriticality Configurations and ResultsStart of core degradationPhase 1Safety injection with IRWST boron concentrationpresents no recriticality risk, when fuel geometry isquite intact.Formation of a molten pool in the corePhase 2There are uncertainties regarding the effective coolablemass in low pressure degraded scenarios. Expectedcoolable mass like that at TMI-2, uniformly fragmentedat optimal porosity, presents no recriticality risk.In addition a higher amount of corium (although unlikelyto be coolable) may present no recriticality risk sinceoptimal and uniform porosity is not assured for largemasses.Relocation of the corium into the vessel lower headPhase 3In this configuration vessel failure cannot be prevented,but reflooding may fragment corium in the upper part.The expected coolable thickness presents norecriticality risk, because water penetration in thedebris bed is limited by steam production.
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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