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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 155 / 295Document ID.No.UKEPR-0002-162 Issue 04Results of the sensitivity analysis indicate that besides initial melt temperature and superheat,for which conservatively low values were assumed, the total mass of the melt and its dischargerate are the most important parameters. Within the range of parameters investigated, otherinfluential parameters, like scenario and phenomenological uncertainties were found to onlyhave a small impact on the results of the assessment.To summarise: the results of the assessment of core melt spreading in the EPR melt retentiondevice show that for the given melt delivery conditions the entire spreading area will be covereduniformly. This confirms the predictions of the code CORFLOW.2.4.1.5. Assessment of melt flooding and quenchingThe EPR core catcher into which the melt relocates after its release from the pit is a shallowmetallic crucible [Ref]. Its bottom and sidewalls are assembled from a large number of individualelements made of cast iron. They are connected using a slot-and-feather technique. Due tothese flexible connections, the structure is highly insensitive to thermal expansion anddeformation as a result of temperature gradients after melt contact. To enhance downward heattransfer, the bottom elements have fins that form rectangular, horizontal cooling channels.The sidewall elements are thinner than those at the bottom and are stacked inside verticalbeams for mounting purposes and for fixing to the wall. Also for the lateral elements, a flexibleconnection is provided by a similar slot-and-feather technique as used for the bottom plate. Thevertical beams are attached to the surrounding concrete walls and preserve a sufficient gap foroutside cooling and for the escape of the generated steam. The top of the steam vent is coveredwith a lid-type structure, which prevents the entry of material that was potentially splashed bylocal fuel coolant interactions during melt entry. The inside of the crucible structure is coveredwith a layer of concrete.In the reactor pit the objective of the sacrificial concrete is to achieve a sufficiently long period oftemporary melt retention to modify the properties of the melt and to oxidise excessive metallicZr. In contrast, no such strong requirements exist for the spreading area. In particular, nospecific conditioning of the melt has to be achieved at the end of the MCCI. Nevertheless, thepresence of the sacrificial concrete layer has a few advantageous consequences, namely to:• provide an easily-to-handle closed surface• mechanically protect the structure during melt spreading• reduce temperature and density of the melt• delay the contact between melt and cooling structure• promote melt fragmentation at the surface during flooding• reduce the probability of FCI by the introduction of glass-formers, namely silica• further improve the capability of the melt to retain fission productsThe preceding MCCI, in combination with the high thermal inertia of the thick metallic coolingstructure, ensures that heat fluxes to the water remain within tolerable limits for all reasonablyexpected melt states and spatial distributions with a high margin.