Oxygen Potential in the Gap of Defective CANDU Fuel Rods During Severe Accident Conditions

Abstract

A model has been developed to describe the multi-component transport of steam, hydrogen and stable fission-product gas in the fuel-to-clad gap of defective CANDU fuel rods during severe reactor accident conditions. The incoming steam must diffuse into a breached rod (i.e., with a defect located at the down stream end) against any counter-current flow of non-condensable-fission gases and out-flowing hydrogen that is produced fiom the internal reaction of the steam with the Zircaloy cladding or urania. This treatment is used to predict the local molar distribution of hydrogen and steam so that the internal oxygen potential can be estimated along the gap as a function of time. The present analysis demonstrates that fuel oxidation does not occur because of significant hydrogen production due to both external and internal Zircaloy oxidation. Moreover. even after clad oxidation is complete, the oxygen potential in the gap is significantly reduced from that in the bulk atmosphere of the reactor coolant due to reduced steam penetration into the gap and local internal hydrogen production from urania oxidation.