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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 20 / 295Document ID.No.UKEPR-0002-0162 Issue 04Before the start of quenching, the more the fuel is degraded, a greater amount of fissionproducts are likely to be released. Therefore a long fuel degradation period results in the lossof phase 1 geometry. In this scenario the assessment considers only the core fissionproducts not yet released for the delayed quench, which corresponds to the water injection atthe time of fuel geometry loss. MAAP results for LBLOCA and SBLOCA core melt transientsshow that when 10% of fuel is melted, implying phase 1 geometry loss at least 50% of thegaseous fission products remain in the fuel matrix after quenching.The assumptions addressing partial release of volatile fission products before significant fuelrod melting and incomplete degradation of the control rods lead to the conclusion that theIRWST boron concentration is sufficient to completely prevent recriticality in phase 1 withuncertainties of 125 ppm in the neutronic calculations included.Consequently there is no potential for recriticality, especially as the likelihood of in-vesselinjection during the time scale of a few minutes of phase 1, is very low.1.3.2.1.2 Assessment of the Potential for Recriticality during Phase 2When fuel melting begins in the top of the core the corium relocates and forms a molten pool.Upon reflooding, a crust forms around the molten pool preventing water ingress into asignificant fraction of the corium molten pool. Therefore only a limited amount of the debrisbed can be reflooded and cooled at the top of the molten pool, as in the TMI-2 accident [Ref].The state of art of corium debris coolability for low pressure scenarios shows that thecoolable mass of corium is limited and uncertain. A coolable mass under the 20-30 tobserved in the TMI-2 accident should be expected [Ref], due to low Critical Heat Flux at lowpressure.Neutronic calculations for this phase assume that all the control rods have been lost and thatall volatile absorbent fission products have been lost. The solid fission products areconsidered homogeneously mixed in molten matrix.By assuming a homogeneous distribution of the corium components, but without mixing withthe melted control rods, a configuration is assessed for a mass like that of TMI-2. Resultsshow that the IRWST boron concentration is sufficient to prevent recriticality for the expectedcorium mass cooled and uniformly fragmented at the optimum porosity.1.3.2.1.3 Assessment of the Potential for Recriticality during Phase 3This phase starts when sufficient corium is relocated to the lower head of the ReactorPressure Vessel. In this phase a scenario like that at TMI-2 can be expected, where only alimited corium mass is in the lower head after crust failure (relocation after quench), or ascenario with corium relocation that forms a pool later covered by cold water (relocationbefore quench). The assessment considers two different configurations for the coriumgeometry in the lower head, following the injection of water.The first configuration with small masses of corium relocated and assumed totallyfragmented in the lower head, has been studied using the same model as that used forphase 2. This assumes that all of the corium is fragmented at the optimum porosity. Theneutronic study simulates small fragmented masses in the lower head, with an equivalentcylindrical geometry, and all control rods lost. For irradiated fuel, it assumes the completeloss of all volatile absorbent fission products. The results of the study are the same as forphase 2. This demonstrates that a TMI-2 like event does not present a risk of recriticality.