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Chemical extraction techniques and scanning electron microscopy were used to study the distribution and behavior of actinides and rare earth elements (REE) in hydrothermal veins at Adamello, (Italy). The six samples discussed in this paper were from the phlogopite zone, which is one of the major vein zones. The samples were similar in their bulk chemical composition, mineralogy, and leaching behavior of major elements (determined by extraction with 9M HCl). However, there were major differences in the extractability of REE and actinides. The most significant influence on the leaching characteristics appears to be the amounts of U, Th and REE incorporated in resistant host phases. Uranium and Th are very highly enriched in zirconolite grains. Actinides were more readily leached from samples with a higher content of U and Th, relative to the amount of zirconolite. The results show that REE and actinides present in chemically resistant minerals can be retained under aggressive leaching conditions.
Confidence in the ability of repository systems to isolate high level wastes from the environment can be strengthened by placing greater reliance on robust designs for the repository system, and by using multiple lines of evidence to demonstrate that parameters, models and predictions developed for the repository are relevant. A particularly useful approach is to demonstrate that models and predictions incorporate processes that have been shown to be important in existing natural systems.
Polished tiles (7×7×2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of ≥ 1 × 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0–1, 1–7, 7–14 and 14–28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.
Synroc containing 20 wt% simulated high level waste (HLW) was subjected to
two sets of leach tests at 150°C where the leachant was and was not replaced
during the test (replacement and non-replacement testing). The leachant was
a KH-phthalate buffered solution (pH 4.2). Samples were characterised before
and after leach testing using SEM, AEM and SIMS. Elemental concentrations in
leachates were measured using ICP-MS. In concert with the findings of i) a
dissolution study of perovskite in a flowing leachant and ii) a previous
Synroc dissolution study (wherein Synroc containing 10 wt% simulated HLW was
subjected to periodic replacement, leach testing in deionised water at
150°C), the results of this study show that when the leachant replacement
frequency is varied from 7 d to the duration of the test, there is no effect
on leach rate or leaching mechanisms.
Synroc-C containing 10wt% simulated PW-4b-D HLW including 0.62 wt% 239Pu was
subjected to MCC-1 type leach tests at 70°C in deionised water, silicate and
carbonate leachates for 53 d and deionised water for 2472 d. The normalised
total (i.e. unfiltered leachate + vessel wall) Pu leach rates in deionised
water, silicate and carbonate leachates for periods up to 53 d were found to
be of the order of 10-5, 10-4 and 10-4 g
m-2 d-1 respectively. After 2472 d, the
differential, normalised, Pu leach rate in deionised water dropped to ∼5 ×
10-6 (total) and ∼5 × 10-8 (solution - after
filtration through a 1000NMW filter) g m-2 d-1. SEM
and AEM were used to characterise our starting material and investigate the
secondary phases on the surfaces of leached Synroc-C discs. Calculated and
measured normalised Pu leach rates are compared and the partitioning of Pu
between zirconolite and perovskite is discussed.
Np-doped Synroc was subjected to modified MCC-1 leach tests in demineralised water at 70°C. Normalised total differential Np leach rates were about 10−4 g m−2 d−1 initially, decreased to about 10−5 g m−2 d−1 after 60 days and stayed approximately constant for periods totalling up to 2283 d. Scanning electron microscope (SEM) and analytical transmission electron microscope (AEM) examination of discs leached for periods totalling 56 d or longer showed that they were covered with crystalline anatase, brookite and ilmenite. The crystalline secondary phases initially nucleated in polishing scratches and on perovskite grains. However, after leaching for a total of 422 d most of the surface was covered. None of the secondary phases contained observable amounts of Np (< 0.2 wt%). These data are discussed in relation to previous work.
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