Multi-component gas transport in the fuel-to-clad gap of candu fuel rods during severe accidents

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 in the present work based on a general Stefan–Maxwell treatment. In this analysis, incoming steam must 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. A solution of the transport equations yields the local molar distribution of hydrogen and steam so that the internal oxygen potential can be estimated along the length of the gap as a function of time. These equations are numerically solved using finite-difference and control-volume methods with a sparse matrix approach. The model has been used to interpret the fission product release and fuel-rod (i.e., Zircaloy and urania) oxidation behavior observed in high-temperature annealing experiments that were conducted at the Chalk River Laboratories with spent fuel samples with Zircaloy cladding.