Low temperature spent fuel oxidation under tuff repository conditions

The Nevada Nuclear Waste Storage Investigations Project is studying the suitability of tuffaceous rocks at Yucca Mountain, Nye County, Nevada, for high level waste disposal. The oxidation state of LWR spent fuel in a tuff repository may be a significant factor in determining its ability to inhibit radionuclide migration. Long-term exposure at low temperatures to the moist air expected in a tuff repository is expected to increase the oxidation state of the fuel. A program is underway to determine the spent fuel oxidation mechanisms which might be active in a tuff repository. Initial work involves a series of TGA experiments to determine the effectiveness of the technique and to obtain preliminary oxidation data. Tests were run at 200{sup 0} and 225{sup 0}C for as long as 720 hours. Grain boundary diffusion appears to open up a greater surface area for oxidation prior to the onset of bulk diffusion. Temperature strongly influences the oxidation rates. The effect of moisture is small but readily measurable.