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Heavy-Ion Irradiation Effects in Pyrochlore Structures for the Immobilization of Actinide-Rich WastesB. D. Begg,(a) W. J. Weber,(b,c) J. P. Icenhower,(b,d) and S. Thevuthasan Supported by Environmental Management Science Program. Several rare-earth titanates, RE2Ti2O7, with the pyrochlore structure were prepared by a sol-gel route using an aqueous mix of titanium iso-propoxide and the appropriate rare-earth nitrate. High-density samples were prepared with a hot isostatic press at 1500° C for 2 hours under 200 MPa. Specimens were cut for leach testing (~10 mm x 10 mm x 1 mm) and polished with 0.5 m m diamond paste on both surfaces. The specimens were subsequently irradiated on both sides with 2 MeV Au+ to an ion fluence of 5.0 ions/nm2 using the accelerator facilities within the EMSL. This ion fluence was more than three times the dose needed to amorphize the surface and resulted in an amorphous layer from the surface to depths of 360, 380, and 400 nm for RE = Lu, Gd, and Y, respectively. Additional specimens for characterization were irradiated to lower fluences of 0.3, 0.75, and 3.0 ions/nm2, which produced discontinuous and continuous buried amorphous layers, respectively. Glancing-incidence x-ray diffraction and SEM within the EMSL were used to examine all samples. Glancing-incidence XRD measurements were taken, using a range of glancing angles from 0.5 degrees to 10 degrees to characterize the structure as a function of depth. Integrated intensities of the major (222), (440) and (622) reflections were subsequently measured and plotted as a function of the glancing angle. The profile of this plot was fitted with an amorphous and crystalline component to determine the depth of the amorphous layer. The amorphous component was modeled by an absorption term, while the crystalline component of the fit accounted for both the absorption and diffracted intensity of the crystalline material. For Gd2Ti2O7 irradiated to an ion fluence of 3 Au+/nm2, the thickness of the amorphous layer was determined to be ~340 ± 20 nm, in excellent agreement with the ~360 nm predicted by TRIM calculations. Glancing-incidence XRD of the Gd2Ti2O7 sample irradiated to 0.75 Au+/nm2 indicated that it contained an ~220-nm-thick, buried amorphous layer, again in excellent agreement with TRIM calculations. Examination of the Gd2Ti2O7 sample irradiated to a fluence of 0.3 Au+/nm2 also found evidence for buried amorphous material, consistent with the discontinuous buried amorphous layer expected at this dose. In addition, the sample irradiated to 0.3 Au+/nm2 contained a sub-surface fluorite-structured phase, which was seen at glancing angles greater than 0.5 degrees. The formation of a fluorite structure is consistent with TEM results (Wang et al. 1999) that indicate Gd2Ti2O7 undergoes a structural transformation from pyrochlore to fluorite prior to amorphization and with Raman results (Weber and Hess 1993) that show cation disordering at low fluences in Gd2Ti2O7 irradiated with 3 MeV Ar+ ions. The Y2Ti2O7 and Lu2Ti2O7 pyrochlore samples, which had been irradiated to a fluence of 0.75 Au+/nm2, contained amorphous surface layers that were ~380 nm and ~300-nm-thick, respectively. Each of the pyrochlore samples, which had irradiated to 0.3 Au+/nm2, contained a fluorite-structured phase, providing further evidence that pyrochlore undergoes an irradiation-induced structural transformation to fluorite prior to amorphization. A single pass flow-through apparatus measured the leach rates of irradiated and unirradiated specimens. A comparison of the initial and final leach rates are shown in Table 9.1 for amorphous (irradiated) and crystalline (unirradiated) Gd2Ti2O7 and Y2Ti2O7 specimens. Initial dissolution rates yield qualitative information about the relative magnitudes of reactivities between samples, but cannot be considered quantitative, as the system is yet to reach steady-state.
The apparent initial dissolution rates for Gd and Ti obtained from amorphous Gd2Ti2O7 specimens were a factor of 15 higher than those measured from the crystalline specimens. The final rates, measured after 21 days, for the amorphous specimens were a factor of 10 and 2 higher for Gd and Ti, respectively. The significant drop in Ti leach rate resulted from the precipitation of anatase, which was only found on the surfaces of the amorphous samples. There was no evidence for Gd precipitation. The Gd leach rates from the crystalline samples were found to be approximately a factor of 5 higher than the Ti leach rates, providing some evidence for incongruent leaching. Characterization of the leached surfaces of the amorphous Gd2Ti2O7 samples revealed that the amorphous surface layer had been fully dissolved over the 21-day leach period, although evidence for a fluorite-structured surface layer was present. Fluorite would be expected to be present at depths between ~380-420 nm based on TRIM calculations, and its detection here has provided an objective measure of the dissolution depth in Gd2Ti2O7 under our experimental conditions. The order of magnitude increase in the leach rate for the amorphous material relative to the crystalline material is consistent with the 20- and 50-fold increase in Cm and Pu release, respectively, measured in amorphous Cm-doped Gd2Ti2O7 relative to the fully crystalline state (Weber et al. 1985; Wald and Weber 1984). In contrast to the behavior of Gd in Gd2Ti2O7, the initial and final Y dissolution rates from the amorphous and crystalline Y2Ti2O7 samples were identical. Thus, there is no apparent effect of amorphization on the Y leach rates. The initial Ti leach rates from the amorphous samples were, however, a factor of 6 higher than those measured from the crystalline samples, which was about half the factor observed in Gd2Ti2O7. On face value these results would suggest that the leaching behavior of Y2Ti2O7 was less susceptible to radiation damage than Gd2Ti2O7. However, examination of the leached amorphous samples revealed that anatase was present on only one of the leached surfaces, which would imply that only one side of the samples had been amorphized. No anatase was found on the surfaces of the leached crystalline samples. This might explain the factor-of-two difference observed between the initial Ti leach rate from the amorphous Gd2Ti2O7 and Y2Ti2O7 pyrochlores when compared to their crystalline counterparts, although it does not explain the failure to see any effect of amorphization in the Y leach rates. Significantly, the Y leach rates from crystalline Y2Ti2O7 were at least a factor of 50 higher than those for Ti throughout the experiment, indicating that Y2Ti2O7 is leaching incongruently. In fact, the Y leach rates from crystalline Y2Ti2O7 were equivalent to the Gd leach rates from amorphous Gd2Ti2O7. Therefore, despite their comparable crystal structures, the leaching behavior of crystalline Gd2Ti2O7 and Y2Ti2O7 would suggest that Gd is more strongly bonded into the pyrochlore structure than Y. It is important to note that the final Ti leach rates from crystalline Gd2Ti2O7 and Y2Ti2O7 were identical. Glancing-incidence XRD of the leached amorphous surface of Y2Ti2O7 revealed that the amorphous surface layer and the accompanying fluorite layer had been fully dissolved after the 23-day leach test. ReferencesWald, J. W., and W. J. Weber, in Nuclear Waste Management, edited by G. G. Wicks and W. A. Ross (Amer. Ceram. Soc., Advance in Ceramics, Vol. 8, Columbus, Ohio) pp. 71-75 (1984). Wang, S. X., L. M. Wang, R. C. Ewing, G. S. Was, and G. R. Lumpkin, Nucl. Instrum. and Methods B 148, 704-709 (1999). Weber, W. J., and N. J. Hess, Nucl. Instrum. and Methods, B 80/81, 1245 (1993). Weber, W. J., J. W. Wald, and H. J. Matzke, Materials Letters, 3, 173-180 (1985).
William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: April 17, 2000 Security & Privacy PNNL-13147 |
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