Understanding the Behavior and Stability of Some Uranium Mineral Colloids
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Submitted to Mat. Res. Soc. Proc., Boston, November, 2002 Understanding the Behavior and Stability of Some Uranium Mineral Colloids Carol J. Mertz, Jeff A. Fortner, and Yifen Tsai Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 ABSTRACT Colloids are believed to be a dominant mode of transport for radionuclides in the unsaturated zone and thus the characterization of the attributes of the colloids is important for understanding and predicting subsurface transport. As uranium-based spent nuclear fuels will be prevalent at the high-level waste repository at Yucca Mountain, we have examined the colloidal properties of a mixture of two uranium minerals. This paper presents results of analyses on colloidal suspensions of meta-schoepite, (UO2)4)(OH)6·5H2O, and UO2+x, in 10 mM uranyl nitrate or J-13 groundwater (from Yucca Mountain, NV). A suite of techniques was used to characterize the colloids. The colloids detected by dynamic light scattering in the 10 mM uranyl nitrate solution exhibited a mean size of ~200 nm and a colloid concentration on the order of 1012 particles/L. Likewise, large 200 nm colloids of meta-schoepite and UO2+x were dominant in the transmission electron microscopy analyses. The colloids detected in the J-13 groundwater exhibited a bimodal distribution; large globular 100-200 nm UO2+x colloids and needle-like colloids of meta-schoepite were observed (with extremely fine microstructure exhibited for the meta-schoepite colloids). Electrophoretic mobility measurements of the meta-schoepite and UO2+x suspension in 10 mM uranyl nitrate indicated that the colloids are not stable at pH values less than or equal to five. While at pH values ≥ 5.5, the colloids of meta-schoepite and UO2+x in 10 mM uranyl nitrate are stable, exhibiting zeta potentials of -30 to -60 mV. The results in this paper show that stable colloidal suspensions of uranium substrate colloids are possible under aqueous, oxidizing conditions. INTRODUCTION Colloids generated during waste form corrosion have the potential to represent an important variable in the performance assessment of the waste form in the repository. Colloids may transport radioactivity from a waste storage or disposal site in a form that is not accounted for based solely on radionuclide elemental solubility limits. Recent studies at the Nevada Test Site (an unsaturated environment similar to Yucca Mountain) have shown the mobility of plutonium to be associated with colloids composed of clays, zeolites, and cristobalite [1]. Numerous studies on nuclear waste glass corrosion have shown that clay colloids form and are prevalent in silica saturated groundwater [2-4]. However, few colloids from commercial spent nuclear fuel corrosion tests have been identified [5-6]. Similarly, colloids were not found to be a major mechanism for the transport of uranium at many natural analogue sites [7-8]. However, experimental work on the formation of uranium oxide sols at pH and temperatures likely for the Yucca
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