Identification of Secondary Phases Formed During Unsaturated Reaction of UO 2 with EJ-13 Water
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IDENTIFICATION OF SECONDARY PHASES FORMED DURING
UNSATURATED REACTION OF U02 WITH EJ-13 WATER J. K. Bates, B. S. Tani, E. Veleckis, and 0. J. Wronklewicz, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439. ABSTRACT A set of experiments, wherein U02 has been contacted by dripping water, has been conducted over a period of 182.5 weeks. The experiments are being conducted to develop procedures to study spent fuel reaction under unsaturated conditions that are expected to exist over the lifetime of the proposed Yucca Mountain repository site. One half of the experiments have been terminated, while one half are ongoing. Analyses of solutions that have dripped from the reacted U02 have been performed for all experiments, while reacted U02 surfaces have been examined for the terminated experiments. A pulse of uranium release from the U02 solid, combined with the formation of schoepite on the surface of the U02 , was observed between 39 and 96 weeks of reaction. Thereafter, the uranium release decreased and a second set of secondary phases was observed. The latter phases incorporated cations from the EJ-13 water and include boltwoodite, uranophane, sklodowskite, compreignacite, and schoepite. The experiments are continuing to monitor whether additional changes in solution chemistry or secondary phase formation occurs. INTRODUCTION The unsaturated tuff beds of Yucca Mountain, Nevada, are being investigated as a potential site for the permanent disposal of high-level nuclear waste. A description of the repository horizon and of issues that need to be considered when evaluating the performance of waste forms has been provided in the Site Characterization Plan [1]. Contact of the waste by humid air and by small amounts of liquid water after container breach is one scenario that has received particular attention because of the small volumes of water anticipated to exist in the waste package environment. The Unsaturated Test Method [2] has been developed to investigate waste form reactions under such conditions. This method, which involves periodically dripping repository water on the waste form, has been applied extensively to study the reaction of glass [3,4]. With glass, it is found that the increased SA/V (surface area of the waste form to volume of liquid) at which the test is performed, accentuates interactions that occur to transform the glass into secondary phases. While the goal of the method is to provide descriptive detail of several reaction processes (e.g., reaction between waste and humid air, the reaction between the waste and small quantities of intermittently recharged water, as well as the reactions between the waste, water, and metal container), measurements of total elemental releases from the waste are also provided. It is also possible that spent reactor fuel will be exposed to unsaturated conditions. Reactions of the U02 matrix under such conditions could be significantly different from those observed in saturated spent fuel leach tests because of the oxidizing environment and reduced
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