Hydrothermal interactions between calcine, glass, spent fuel, and ceramic-waste forms with representative shale-repository rocks

A prototype waste glass (PNL 76-68), a prototype ceramic (SPC-2), a raw PW-7a calcine, and a simulated spent fuel were reacted individually with six shales in the presence of water in sealed gold capsules. Conditions were (usually) 1:1:1 waste:shale:water by weight, P = 30 MPa, T = 100, 200, 300 and 4000C. Reaction products were characterized by optical microscopic and SEM examination, EDX analysis, x-ray diffraction analysis, and Gandolfi X-ray analysis. The shales react with the waste forms presumably by dissolution in water followed by reaction between the solution and the shale minerals. Degree of reaction ranges from barely detectable at 1000C to almost complete alteration of the waste forms and shales at 4000C. Cesium extracted from the waste forms reacts with alumino-silicate minerals to form a pollucite-analcime solid solution at lower temperatures. It is shown that the observed gradual disappearance of pollucite in higher temperature runs can be explained by the decomposition of the pollucite-analcime solid solution into a cesium-rich pollucite plus feldspar. The seeming disappearance of pollucite from high temperature runs need not imply loss of cesium fixation. The uranium reacts with silicate minerals under oxidizing conditions to form alkali uranyl silicates (of which weeksite is the most common example). Shales containing sulfide minerals or organic materials maintain a reducing environment which keeps uranium (and presumably transuranic elements) in the far less soluble tetravalent state. Powellite, CaMoO4, appears frequently as the reaction product of molybdenum. 23 figures, 15 tables