The Long-Term Stability of Becquerelite
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THE LONG-TERM STABILITY OF BECQUERELITE. 4 3 2 R.J. FINCH', J. SUKSI , K. RASILAINEN AND R.C. EWING
I University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada 2 University of Helsinki, Unioninkatu 35, FIN-00014, Helsinki, Finland 3 VTT Energy, P.O. Box 1604, FIN-02044, Helsinki, Finland 4 University of New Mexico, Albuquerque, New Mexico, 87131, U. S.A.
ABSTRACT Uranium-series disequilibria data, in conjunction with petrographic analyses, indicate that the uranyl oxide hydrate becquerelite can persist for hundreds of thousands of years, possibly longer. Becquerelite probably forms continuously as ground water compositions permit and is resistant to U leaching by ground water. On the time scale of interest for the geologic disposal of spent U0 2 nuclear fuel, becquerelite is a long-lived sink for uranium in oxidizing, U and Ca-bearing ground waters. Such long-term stability also supports recent solubility experiments that indicate natural becquerelite has a lower solubility product than that determined for synthetic becquerelites. INTRODUCTION Becquerelite, CaU 60 19-11H 20, is a common alteration product formed during the weathering of uraninite (nominally UO2÷,). Becquerelite is structurally related to a large number of uranium (VI) minerals, the uranyl oxide hydrates, which precipitate from uraniumrich ground waters under oxidizing conditions. The temporal stability of the uranyl oxide hydrates has important implications for understanding which phases control the long-term behavior of uranium in oxidizing ground waters. The solubility of becquerelite, both natural and synthetic, has been examined [1,2,3] because of its role in controlling uranium concentrations in uranium-rich ground waters. In addition to becquerelite, Ba, Sr, Pb, and K-bearing minerals are known, and these phases may be important as fission product hosts produced by the oxidative corrosion of spent fuel [4].
EXPERIMENTAL A small (approx. 1 cm 3) sample of altered uraninite with numerous euhedral crystals of becquerelite and uranophane, CaU 2Si 2O11-6H 2O from the Shinkolobwe mine, Shaba, Zaire, was examined by a range of analytical methods. The bulk sample was initially analyzed by scanning electron microscopy (SEM), and qualitative chemical analyses were conducted using energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) was conducted on several single crystals of the becquerelite that were separated by hand under a binocular microscope and ground by hand in acetone. A thin section (approx. 30 ýtm thick) was made from a portion of the sample and examined optically with a petrographic microscope and with SEM using back-scattered electron imaging (BSEI). Uranium-series data for several of the
Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
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becquerelite crystals were also determined. These crystals were dissolved in hot nitric acid and analyzed by anion exchange, CeF 3 counting standard and alpha spectrometry [5].
SAMPLE DESCRIPTION The uraninite was polycrystalline, highly fractured, and subst
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