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In this study, the oxidation state of uranium was investigated with Electron Energy-Loss Spectroscopy on the Transmission Electron Microscope. The oxygen K edge in a series of uranium-bearing phases was investigated and this technique was compared to an alternative method of using the relative intensities of the M4,5- edges of the actinides. Under controlled conditions, it is possible to obtain reliable oxidation state data on uranium solids; however, mixtures of oxidation states in a single compound cannot be distinguished with certainty.
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.
Optical emission spectra in the 300-700 nm range were collected from zirconolite and rutile specimens irradiated with a 3 μs pulsed electron beam using a Febetron 706 variable energy pulsed electronbeam generator. The long-lived emissions (up to microseconds after the electron pulse) consist of broad (halfwidths ~ 100 nm) bands centred around ~400 nm. Over the range 0.2 MeV to 0.6 MeV, the emission intensity per unit dose versus electron beam energy data from the rutile sample showed a single stage dependence on electron beam energy, whereas the zirconolite data suggested a two stage dependence. Rutile has a threshold of 0.23 ½ 0.02 MeV, which gives an Ed value of 39 ½ 4 eV for oxygen. Zirconolite has a threshold of 0.26 ½ 0.02 MeV, which gives an Ed value of 45 ½4 eV for oxygen. These data are discussed in the context of previously measured and calculated Ed values for other oxides.
Preliminary analytical and transmission electron microscopy (AEM and TEM) results for a small suite of natural perovskites are reported in this paper and discussed in relation to previous work. We show that perovskite compositions in Synroc and tailored ceramics plot within the known fields of natural perovskite compositions. AEM analyses and electron diffraction work on selected samples indicate that they are predominantly stoichiometric variants of the cubic perovskite structure. Geochemical alteration was observed in one sample of loparite from Bratthagen, Norway. The primary result of this alteration was leaching of Na from the A-site. Although sufficient alpha-decay dose levels for complete amorphization are not realized in this suite of samples, the available data bracket the beginning of the crystalline-amorphous transformation at doses that are ∼ 2-4 times greater than those of zirconolite of similar age. These results may be due to fundamental differences in the damage annealing rates of perovskite and zirconolite.
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.
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