Browsing by Author "Williams, CT"

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    Petrography and chemistry of tungsten-rich oxycalciobetafite in hydrothermal veins of the Adamello contact aureole, northern Italy
    (Springer Nature, 2017-09) Lumpkin, GR; Gieré, R; Williams, CT; McGlinn, PJ; Payne, TE
    Tungsten-rich oxycalciobetafite occurs in complex Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern Italy, where it occurs as overgrowths on zirconolite. The betafite is weakly zoned and contains 29–34 wt% UO2. In terms of end-members, the betafite contains approximately 50 mol% CaUTi2O7 and is one of the closest known natural compositions to the pyrochlore phase proposed for use in titanate nuclear waste forms. Amorphization and volume expansion of the betafite caused cracks to form in the enclosing silicate mineral grains. Backscattered electron images reveal that betafite was subsequently altered along crystal rims, particularly near the cracks. Electron probe microanalyses reveal little difference in composition between altered and unaltered areas, except for lower totals, suggesting that alteration is primarily due to hydration. Zirconolite contains up to 18 wt% ThO2 and 24 wt% UO2, and exhibits strong compositional zoning, but no internal cracking due to differential (and anisotropic) volume expansion and no visible alteration. The available evidence demonstrates that both oxycalciobetafite and zirconolite retained actinides for approximately 40 million years after the final stage of vein formation. During this time, oxycalciobetafite and zirconolite accumulated a total alpha-decay dose of 3.0–3.6 × 1016 and 0.2–2.0 × 1016 α/mg, respectively. © 2024 Springer Nature
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    Prediction of the long-term performance of crystalline nuclear waste form phases from studies of mineral analogues
    (Institution of Engineers, Australia, 1994-05-01) Lumpkin, GR; Smith, KL; Blackford, MG; McGlinn, PJ; Gieré, R; Williams, CT
    This investigation outlines the use of natural analogues as a means of assessing the long-term performance of actinide host phases in crystalline nuclear waste forms. We employed several analytical techniques to study the mineral zirconolite, an important actinide host phase in Synroc and tailored ceramics. The following conclusions were reached: 1) Natural zirconolite experiences a crystalline to aperiodic transformation at doses of approximately 10 to 10 alpha/mg (0.08-1.0 dpa), consistent with the results of accelerated damage testing on synthetic zirconolite. 2) Damage microstructures are consistent with the accumulation and overlap of alpha-recoil collision cascades, resulting in aperiodic domains which increase in volume as a function of dose. 3) There is evidence for long-term annealing of alpha-recoil damage in zirconolite at a rate 2-5 times slower than in the structurally related mineral pyrochlore. 4) Analysis of data from the only documented natural zirconolites to show hydrothermal alteration [resulting from F- and P-rich aqueous fluids at T = 500-600 degrees C and P = 2 kb, Giere and Williams (14)], suggests that Th and U were released to the fluid phase. 5) The effects of low temperature alteration of radiation damaged zirconolite by a Si-rich ground water or fluid phase were identified in the oldest (2.5 x 10 yr) sample in our research collection. Alteration involved incorporation of Si along microfractures at the expense of Ca, Ti, and Fe, but Th and U remained immobile. In this same sample, there is evidence for migration of at least 50% of the radiogenic Pb from both unaltered and altered areas of zirconolite. Most of the Pb may have been retained within the sample as fine precipitates of galena (PbS).
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    The role of Th-U minerals in assessing the performance of nuclear waste forms
    (Camberidge University Press, 2014-10-05) Lumpkin, GR; Yan, G; Gieré, R; Williams, CT; Mariano, AN; Geisler, T
    Materials designed for nuclear waste disposal include a range of ceramics, glass ceramics and glass waste forms. Those with crystalline phases have provided the momentum for studies of minerals as a means to understand aspects of waste-form crystal chemistry, behaviour in aqueous systems and radiation damage over geological periods of time. Although the utility of natural analogue studies varies, depending upon the degree of analogy to the proposed geological repository and other factors such as chemical composition, the available data suggest that Th-U host phases such as brannerite, monazite, pyrochlore, zircon and zirconolite are resistant generally to dissolution in aqueous fluids at low temperatures. Geochemical durability may or may not extend to hydrothermal systems depending on the specifics of fluid composition, temperature and pressure. At elevated temperatures, for example, davidite may break down to new phase assemblages including titanite, ilmenite and rutile. Perovskite is generally less resistant to dissolution at low temperatures and breaks down to TiO2, releasing A-site cations to the aqueous fluid. Studies of radiation damage indicate that the oxide and silicate phases become amorphous as a result of the gradual accumulation of alpha-recoil collision cascades. Monazite tends to remain crystalline on geological time scales, a very attractive property that potentially eliminates major changes in physical properties such as density and volume, thereby reducing the potential for cracking, which is a major concern for zircon. In spite of recent success in describing the behaviour of Th-U minerals in geological systems, considerable work remains in order to understand the P-T-X conditions during alteration and T-t history of the host rocks.© The Mineralogical Society of Great Britain & Ireland.