Fission Gas Release from UO_2+x in Defective Fuel Rods

The literature dealing with fission gas release from UO_2+x is reviewed. A simplified semiempirical model predicting fission gas release from UO_2+x fuel to the fuel rod plenum as a function of stoichiometry excess x is developed to apply to the fuel of a defective light water reactor fuel rod in operation. An effective diffusion coefficient including a parabolic dependence of x is obtained based on existing data in the literature. The new diffusion coefficient is a composite expression of intrinsic, fission-enhanced, and nonstoichiometry-induced diffusion. The effective diffusion coefficient is incorporated into the Booth model to assess the time-dependent fractional fission gas release. The new model predictions are compared with existing data.