Spent-Fuel Special-Studies Progress Report: Probable Mechanisms for Oxidation and Dissolution of Single-Crystal UO sub 2 Surfaces

摘要

Due to the complexity of the structural, microstructural and compositional characteristics of spent fuel, basic leaching and dissolution mechanisms were studied with UO sub 2 matrix material, specifically with single-crystal UO sub 2 , to isolate individual contributory factors. The effects of oxidation and oxidation-dissolution were investigated in different oxidation conditions, such as in air, oxygenated solutions and deionized water containing H sub 2 O sub 2 . In addition, the effects of temperature on dissolution of UO sub 2 were studied in autoclaves at 75 and 150 exp 0 C. Also, oxidation and dissolution measurements were investigated via electrochemical methods to determine if those techniques could be applied to the characterization of leaching and dissolution of spent fuel in a hot cell. Finally, the effects of radiation were explored since the radiolysis of water may create a localized oxidizing condition at or near the spent fuel-solution interface, even in neutral or reducing conditions as commonly found in deep geological environments. The oxidation and oxidation-dissolution mechanisms for UO sub 2 are proposed as follows: The UO sub 2 surface is first oxidized in solution to form a UO/sub 2+x/ surface layer several angstroms thick. This oxidized surface has a high dissolution rate since the UO/sub 2+x/ reacts with the dissolved O sub 2 , or H sub 2 O sub 2 , to form uranyl complex ions in a U(VI) state. As the uranyl ions exceed the solubility limits in solution, they become hydrolyzed to form solid deposits and suspended particles of UO sub 3 hydrates. The thickness and porosity of the deposited UO sub 3 hydrate surface-film is dependent on temperature, pH and deposition time. A long-term dissolution rate is then determined by the nature of the surface film, such as porosity, solubility and mechanical properties. (ERA citation 06:016555)