Uranium (VI) Solubility in Carbonate-Free ERDA-6 Brine
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1265-AA01-04
Uranium (VI) Solubility in Carbonate-Free ERDA-6 Brine Jean-Francois Lucchini, Hnin Khaing and Donald T. Reed Los Alamos National Laboratory, Earth and Environmental Sciences Division, 115 N. Main, Carlsbad, NM 88220, U.S.A.
ABSTRACT When present, uranium is usually an element of importance in a nuclear waste repository. In the Waste Isolation Pilot Plant (WIPP), uranium is present in significant quantities, with about 647 metric tons to be placed in the repository [1]. Therefore, the chemistry of uranium, and especially its solubility, needs to be determined under WIPP-relevant conditions. Long-term experiments were performed to measure the solubility of uranium (VI) in carbonate-free ERDA-6 brine, a simulated WIPP brine, at pCH+ values between 8 and 12.5. These data, obtained from the over-saturation approach, were the first WIPP repository-relevant data for the VI actinide oxidation state. The solubility trends observed pointed towards low uranium solubility in WIPP brine and a lack of amphotericity. At the expected pCH+ in the WIPP (~ 9.5), measured uranium solubility approached 10-7 M. The objective of these experiments was to establish a baseline solubility to further investigate the effects of carbonate complexation on uranium solubility in WIPP brines, during the ongoing research program in actinide solubility under WIPP-relevant conditions. INTRODUCTION In the anoxic and strongly reducing environment expected in the WIPP, tetravalent uranium is predicted to be the dominant oxidation state. As a consequence, the uranium solubility will be very low (about 10-8M) [2]. However, some uranium (VI) phases and aqueous species, although not expected to predominate in the WIPP, could be present due to the localized effects of radiolysis. The WIPP Performance Assessment (PA) currently makes the conservative assumption that U(VI) species predominate in 50% of the PA vectors [1]. The solubility of U(VI) in the WIPP is expected to be defined by the combined contribution of two processes: hydrolysis with oxyhydroxide phase formation, and carbonate complexation with uranium carbonate phase formation. In the absence of WIPP-specific experimental data, the solubility of U(VI) is conservatively set at 10-3 M in WIPP PA for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [3]. This paper summarizes the results of long-term experiments to establish the solubility of U(VI) in a simulated carbonate-free WIPP brine from an over-saturation approach as a function of pH. More information on these experiments and their results can be found elsewhere [4, 5]. These data in carbonate-free brine provided a baseline for carbonate effects on U(VI) solubility, during the ongoing research program in actinide solubility under WIPP-relevant conditions.
EXPERIMENT The general experimental approach was to investigate uranium (VI) solubility from oversaturation, as described by Nitsche [6]. This consisted of sequentially adding dissolved uranium, as a function of experimental condi
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