Prediction of the oxygen potential in the fuel-to-clad gap of defective fuel rods during severe accident conditions

A model has been developed to describe the multi-component transport of ste am, hydrogen and stable fission-product gas in the fuel-to-clad gap of defe ctive fuel rods during severe reactor accident conditions. The incoming ste am must diffuse into the breached rod against any counter-current flow of n on-condensable fission gases and out-flowing hydrogen that is produced from the internal reaction of the steam with the Zircaloy cladding or urania. T his treatment is used to predict the local molar distribution of hydrogen a nd steam so that the internal oxygen potential can be estimated along the g ap as a function of time. The oxygen potential will directly influence the fuel oxidation state and the fission product release characteristics. The m odel also considers the effects of mass transfer and changing boundary cond itions at the defect site, as steam in the reactor coolant system flows ove r the defective rod and reacts with the external cladding surface. (C) 1999 Elsevier Science B.V. All rights reserved.