Oxidative Alteration of Spent Fuel in a Silica-Rich Environment: SEM/AEM Investigation and Geochemical Modeling.

摘要

Correctly identifying the possible alteration products and accurately predicting their occurrence in a repository-relevant environment are the key for source-term calculations in a repository performance assessment. Uraninite in uranium deposits has long been used as a natural analog to spent fuel in a repository because of their chemical and structural similarity. In this paper, a SEIWAEM investigation has been conducted on a partially alterated uraninite sample from a uranium ore deposit .of Shinkolobwe of Congo. The mineral formation sequences were identified: uraninite yields uranyl hydrates yields uranyl silicates yields Ca-uranyl silicates or uraninite yields uranyl silicates yields Ca-uranyl silicates. Reaction-path calculations were conducted for the oxidative dissolution of spent fiel in a representative Yucca Mountain groundwater. The predicted sequence is in general consistent with the SEM observations. The calculations also show that uranium carbonate minerals are unlikely to become major volubility-controlling mineral phases in a Yucca Mountain environment. Some discrepancies between model predictions and field observations are observed. Those discrepancies may result from poorly constrained thermodynamic data for uranyl silicate minerals.