Framework for sheath hydriding model for defective nuclear fuel

In a nuclear reactor the fuel and gaseous products of fission are separated from the coolant by a zirconium based Zircaloy sheath. This sheath is susceptible to breaches, a rare event, which exposes fission products to the coolant. A breach (or defective fuel element) allows coolant to interact with both the inner layer of the sheath and the fuel in oxidation reactions. These reactions produce hydrogen which can lead to secondary hydriding with the formation of delta-hydride on the inner layer of the Zircaloy sheath. The delta-hydride is brittle which can lead to secondary defects in the sheath and associated fission product release into the coolant.;In the first area, thermodynamic models of the Zr-H, Zr-O and Zr-O-H systems have been developed using Gibbs energy minimization which provide boundary conditions to the kinetics of sheath hydriding. The ternary model predicts that virtually pure hydrogen (i.e. with less than 1% water vapour) is required to hydride the sheath. In addition, a kinetic model of the Sawatzky and Vogt experiment of transient hydrogen distribution in a sample of Zircaloy has been developed using the finite-element method and confirms the a-Zr hydrogen diffusion coefficient. This model serves as a first step to the development of a detailed mechanistic hydriding model.;In the second area of research, a remedy to the hydriding problem has been introduced and involves a minor change in the final step of fuel processing of CANDU pellets in order to slightly oxidize the fuel. This thin layer of oxidation on the fuel pellets (UO2+x) could maintain the passivity of the oxide layer on the inner surface of the sheath through interactions with either CANLUB or hydrogen present in the fuel-to-sheath gap. The oxide layer is thought to mitigate secondary hydriding by blocking direct hydrogen absorption in the sheath. Keywords:;Research with respect to this phenomenon has been focused on the following two areas: (i) description of the problem through thermodynamic and kinetic modelling. and (ii) the proposal of solutions to the problem.