Axial crack velocity in failed zircaloy cladding caused by UO2 to U3O8 spent fuel oxidation in a decreasing temperature history

The full text follows. An assessment of safety, design, and cost tradeoff issues for short (ten to fifty years) and longer (fifty to hundreds of years) interim dry storage of spent nuclear fuel in Zircaloy rods shall address potential axial crack propagation in the cladding resulting from volume changes as spent fuel oxidizes from UO2 to U4O9 to U3O8. If such assessment analyses are to be done rigorously, they will be necessarily complex because the oxidation response of uranium oxide spent nuclear fuels is a stochastic functional of fission product/actinide burnup concentrations, oxygen fugacity, temperature, pellet microcrack/fragment structures, and flaw sizes of the cladding. Thus, there are, and probably always will be, issues to analytically and experimentally resolve concerning the consistency, the completeness, and the certainty of models, data, initial and boundary conditions, and tradeoffs for assessing risks and consequences of axial crack propagation induced by spent fuel oxidation during dry storage time intervals. One of the issues to address is the crack propagation velocity functional dependence on Arrhenius' activation energy parameter and the grain sizes of confined (in cladding) spent fuel oxidation relative to those of unconfined spent fuel oxidation. In this case, axial crack propagation speeds estimated from axial crack growth data have an Arrhenius' value for confined oxidation driven cracks that is roughly one half of that found in unconfined spent fuel oxidation data. A model that addresses this issue is developed based on an extension of existing models for the brittle fracture kinematics and grain size oxidation kinetics dependence. The primary fracture kinematic assumption of this model, after crack propagation is initiated, is that the crack propagates at a velocity such that the relative inter-atomic displacement, in a neighborhood of the crack-tip, is invariant when moving at the crack speed. An expression for the axial crack speed is derived as a function of the induced oxidation opening rate of the cladding and available material properties of Zircaloy cladding. (authors)