The Dissolution of Uranophane in CaCl 2 -SiO 2 (aq) Test Solutions
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The Dissolution of Uranophane in CaCl2-SiO2(aq) Test Solutions James D. Prikryl1 and William M. Murphy2 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute®, San Antonio, TX 78228-0210, U.S.A., E-mail address: [email protected] 2 Department of Geological and Environmental Sciences, California State University, Chico, CA 95929-0205, U.S.A. 1
ABSTRACT Uranophane [Ca(UO2)2(SiO3OH)2 · 5H2O] is a corrosion product of long-term leaching of spent fuel under oxidizing conditions and is a weathering product of uraninite in uranium ore deposits hosted by siliceous rocks. Incorporation of radionuclides into uranophane by coprecipitation may occur as a result of spent fuel alteration. Dissolution of uranophane leading to release of these radionuclides may therefore influence the longterm dissolved concentration and mobility of radionuclides at the proposed nuclear waste repository at Yucca Mountain, Nevada. In this study, the dissolution of uranophane in Ca- and Si-rich test solutions was investigated. Batch dissolution experiments were designed to approach uranophane equilibrium from undersaturated solutions at nearneutral pH (~6.0). Test solutions had initial U concentrations of 0.0 and 10-7 mol/L in matrices of ~10-2 mol/L CaCl2 and ~10-3 mol/L SiO2(aq). The test solutions were reacted with synthetic uranophane (confirmed by XRD and chemical analyses) and analyzed periodically over 10 weeks. Reaction quotients (Log Qs) derived from aqueous speciations of experimental solutions considered to be near equilibrium with uranophane ranged from 10.54 to 11.06 for the dissolution reaction: Ca(UO2)2(SiO3OH)2 · 5H2O + 6H+ ⇔ Ca2+ + 2UO22+ + 2SiO2(aq) + 9H2O. INTRODUCTION Studies at the Nopal I uranium (U) deposit in Chihuahua, Mexico (a natural analog to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada) indicate that uranophane is the dominant end product of U mineralization hosted by siliceous volcanic rocks [1]. Similarly, long-term leaching studies of synthetic UO2 and spent UO2 fuel designed to simulate conditions in a Yucca Mountain repository indicate that the alkali uranyl silicates uranophane and boltwoodite [K(UO2)(SiO3OH) · 1.5H2O] are the dominant end products of spent fuel alteration [2,3]. Crystallographic theory and recent experimental data suggest that spent fuel waste species (e.g., Np) may be incorporated into uranyl compounds, most notably uranophane and Na-compreignacite [Na2((UO2)3O2(OH)3)2 · 7(H2O)], that form as alteration products of spent nuclear fuel [4,5]. Dissolution of uranophane leading to release of incorporated spent fuel species could therefore influence the source term for long-term radionuclide release at a Yucca Mountain repository. Predictive modeling of uranophane formation and dissolution requires reliable thermodynamic data, which is lacking for uranophane and other uranyl phases. In this study, the dissolution of uranophane was investigated by reacting uranophane in Ca- and Si-rich solutions. Because natural uranophane samples are
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