Investigating the Thermal Limit of Clay Minerals for Applications in Nuclear Waste Repository Design

Clay minerals are likely candidates to aid in nuclear waste isolation due to their low permeability, favorable swelling properties, and high cation sorption capacities. Establishing the thermal limit for clay minerals in a nuclear waste repository is a potentially important component of repository design, as flexibility of the heat load within the repository can have a major impact on the selection of repository design. For example, the thermal limit plays a critical role in the time that waste packages would need to cool before being transferred to the repository. Understanding the chemical and physical changes that occur in clay minerals at various temperatures above the current thermal limit (of 100 °C) can enable decision-makers with information critical to evaluating the potential trade-offs of increasing the thermal limit within the repository. Most critical is gaining understanding of how varying thermal conditions in the repository will impact radionuclide sorption and transport in clay materials either as engineered barriers or as disposal media. A variety of clays (illite, mixed layer illite/smectite, montmorillonite, and palygorskite) were heated for a range of temperatures between 100-500 °C. These samples were characterized by a variety of methods, including nitrogen adsorption, x-ray diffraction, thermogravimetric analysis, barium chloride exchange for cation exchange capacity (CEC), and iodide sorption. The nitrogen porosimetry shows that for all the clays, thermally-induced changes in BET surface area are dominated by collapse/creation of the microporosity, i.e. pore diameters < 17 angstroms. Changes in micro porosity (relative to no heat treatment) are most significant for heat treatments 300 °C and above. Alterations are also seen in the chemical properties (CEC, XRD, iodide sorption) of clays, and like pore size distribution changes, are most significant above 300 °C. Overall, the results imply that changes seen in pores size distribution correlate with cation exchange capacity and cation exchange processes. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's Nation Nuclear Security Administration under contract DE-AC04-94AL85000. SAND Number: 2013-6352A.