WAPD-T-3173, " The Corrosion of Zircaloy-Clad Fuel Assemblies in a Geologic Repository Environment."

Recently analyzed long-term Zircaloy autoclave corrosion data were used to develop new Zircaloy corrosion correlations. Calculations were performed to estimate the additional quantity of corrosion that may be experienced by spent Zircaloy-c1ad fuel assemblies from long-term exposure to geologic repository conditions. These estimates indicate that the level of general corrosion is very small and will not be expected to affect the integrity of nonfailed fuel elements with Zircaloy-4 cladding through the life of a repository. Twenty-two different autoclave tests were analyzed. The tests included specimens from 46 different heats. of both Zircaloy-2 and Zircaloy-4. The material conditions included different heat treatments and various prefilms. Maximum exposure time was 10,507 days (-29 years) in a 600°F (316°C) test and maximum weight gain was 1.665 mg/dm2 (-4.5 mils of oxide film) for a 640°F (338°C) experiment. The new kinetic data, with various conservative assumptions. were used in conjunction with a projected fuel temperature profile during disposal to generate a bestestimate prediction of -0.3 mils of additional oxide film growth after one million years exposure in the repository. This additional oxide film growth should not pose a threat to the integrity of the Zircaloy cladding. A review of the fundamental factors affecting corrosion indicates that the extrapolation from the test database to repository times is reasonable, in part because the database covers the thickness of oxides expected for repository disposal. It also is shown that the ionic constituents of typical ground water, in the anticipated concentrations and pH levels, will have no discernible detrimental effect on the corrosion behavior of Zirc.aloy in the repository environment.