Studies on the effects of local power peaking on heat transfer under dryout conditions in BWRs

Abstract

This paper presents the results of a series of studies, using detailed pin power distributions derived from Monte Carlo simulation results, to assess the effects of local dryout during normal operation on the cladding temperatures, heat fluxes and potentially on cladding oxidation for modern boiling water reactor (BWR) fuel. The development of a new three-dimensional LWR fuel thermo-mechanics solver, OpenFoam Fuel BEhAviour Tool (OFFBEAT), was initiated for these studies. Three-dimensional simulations of heat transfer in an axial section of a fuel pin have been conducted using OFFBEAT assuming different local dryout conditions. The results have shown that local power effects and, in particular, strong power gradients across the fuel pin can contribute significantly towards increased heat fluxes in the cladding, reducing the safety margin to dryout. Further, the studies have shown that, under the conservative assumption that the fuel pellet is not centred, but is instead touching the cladding on one side, the safety margin is further decreased. Investigations of possible neutronics feedback effects following dryout, taking into account the detailed intra-pin temperature distribution before and after dryout, have confirmed that there is no mitigating neutronics effect under dryout conditions. Finally, a basic study of the cladding oxidation under dryout conditions has shown that days to weeks of continuous dryout, or alternatively weeks of repeated dryout and rewetting would be required to obtain oxide layers thicknesses that would affect the mechanical integrity of the cladding. This work is an initial step towards developing a comprehensive multi-physics capability for simulations and studies of local three-dimensional coupled effects during dryout, and on this basis strengthen our understanding of the potential impact on the cladding integrity.