Heat Transfer Processes in Reactor Vessel Lower Plenum During Late Phase of In-Vessel Core Melt Progression

Abstract

AbstractLate phase in-vessel core melt progression during the course of a hypothetical severe (melt-down) accident in a light water reactor (LWR) is considered, with particular emphasis on thermal processes occurring in the lower plenum of the reactor pressure vessel (RPV). The formation of a melt pool, from the initial state of a uniform composition, dried out, debris bed, is investigated.The objective of this work is to analyze the effects of various heat transfer phenomena, which are of potential importance to the vessel thermal loading. In particular, numerical investigation of heat transfer in a metallic layer was performed for cases with different boundary conditions. A two-dimensional model was developed to simulate heat transfer in naturally-convected, internally-heated corium melt pool and in metallic layer heated from below. The model solves the energy conservation equation in general curvilinear coordinates, while taking into account homogeneous-orthotropic anisotropic heat conduction. A method is developed to describe buoyancy-induced convective and turbulent heat transport by means of effective heat, conduction and pseudo-convective (effective velocity) terms. Extensive validation was performed with analyses of simulant experiments and good agreement was obtained between calculated parameters (energy splitting, transient and steady temperature fields, local heat. flux distributions) and measured data. These included cases with different boundary conditions, and geometries, and high Rayleigh numbers. This method provides a sufficiently mechanistic modeling in an integrated manner.The method was applied, in two-dimensions, to analyze the heat transfer and phase change processes in the RPV lower plenum in several selected scenarios of core melt-down accident progression. It was found that the multi-dimensional representation of heat conduction in, and melting of, the vessel wall significantly reduce the ‘focussing effect’ of a thin metallic layer resident on top of an oxidic melt pool, compared to the earlier predicions of thermal loadings at the vessel wall obtained with 1-D representation.KeywordsHeat TransferHeat FluxHeat Transfer CoefficientNusselt NumberNatural ConvectionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.