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 UO2 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.
α-Recoil induced isotopie fractionation of radioactive decay chain 4n+2 nuclides in rock material has been investigated experimentally and theoretically. The fractionation studied here manifests itself in U-234 and Th-230 surplus in U-poor mineral phase adjacent to U-rich phase, and in U-234 deficit in readily dissolved U-rich phase in rock pores. In four samples out of five our modelling concept (continuous accumulation or release of U-234 and Th-230 due to α-recoil) gave an explanation in line with independent observations, indicating potential of α-recoil for dating application in U migration studies.
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