Multi-Component Gas Transport in CANDU Fuel Rods During Severe Accidents

Abstract

The multi-component transport of steam, hydrogen and stable fission gas in the fuel-to-clad gap of defective CANDU fuel rods, during severe accident conditions, is investigated. Based on a general Stefan-Maxwell treatment this work considers how incoming steam will diffuse into a breached rod against a counter-current flow of non-condensable fission gases and out-flowing hydrogen that is produced from the internal reaction of steam with the Zircaloy cladding or urania. The ability of the oxidized clad to act as a physical barrier to either hydrogen or oxygen diffusion was further investigated in the current work with a molecular-dynamics approach, with the interactions between atoms represented by a Modified Embedded Atom Method. During the initial Zircaloy oxidation phase in the CRL experiments, the model was able to predict the reduced fission product release kinetics as well as the timing for the completion of the clad-oxidation process. In this simulation, the model (with an effective gap size of 20 um) was able to successfully predict whether singlesided or double-sided oxidation had occurred in accordance with the metallographic examination. However, in order to account for the observed release kinetics after the completion of clad oxidation, it was necessary to assume a greater atmospheric exchange due to possible cracking of the brittle oxide layer. With the assumption of cracking (by assuming a reduced path length for gas transport), the model was successfully able to reproduce the fission product release kinetics and the final fuel stoichiometry as determined from end-of-test weight gain measurements. This analysis particularly shows that local hydrogen production (from the internal fuel oxidation process) will result in a reduced local oxygen potential in the fuel-to-clad gap compared to that which occurs in the bulk coolant.