C. R. Bradley

Obsidians and tektites: Natural analogues for water diffusion in nuclear waste glasses

Projected scenarios for the proposed Yucca Mountain repository include significant periods of time when high relative humidity atmospheres will be present, thus the reaction processes of interest will include those known to occur under these conditions. The ideal natural analog for the proposed Yucca Mountain repository would consist of natural borosilicate glasses exposed to expected repository conditions for thousands of years; however, the prospects for identifying such an analog are remote, but an important caveat for using natural analog studies is to relate the reaction processes in the analog to those in the system of interest, rather than a strict comparison of the glass compositions. In lieu of this, identifying natural glasses that have reacted via reaction processes expected in the repository is the most attractive option. The goal of this study is to quantify molecular water diffusion in the natural analogs obsidian and tektites. Results from this study can be used in assessing the importance of factors affecting molecular water diffusion in nuclear waste glasses, relative to other identified reaction processes. In this way, a better understanding of the long-term reaction mechanism can be developed and incorporated into performance assessment models. 17 refs., 4 figs.

Water diffusion in tektites: An example of the use of natural analogues in evaluating the long-term reaction of glass with water

Abstract A natural analogue approach is used to relate water diffusion in natural rhyolitic glasses of great age to water diffusion in nuclear waste glasses. Tektites are glasses of excellent durability with approximately 74 wt% SiO2. They have a resistance to water diffusion similar to that for nuclear waste glasses where the diffusion coefficients are approximately 2 × 10−24 m2/s at 25°C. The results of a series of experiments with tektite glass in water vapor atmospheres between 150 and 225°C for up to 400 days are presented. Water diffusion was found to be the rate-determining process in all experiments. The reaction resulted in the formation of a birefringent hydration layer, that increased in thickness up to 4.8 μm as a function of the square root of time. The temperature dependence of the reaction was quantified, allowing the experimental results to be extrapolated to repository-relevant conditions for nuclear waste glass. These calculations indicate that the water diffusion reaction process is slower than the reaction observed in nuclear waste glass experiments.

AEM analyses of SRL 131 glass altered as a function of SA/V

The goal of the present study was to consider the effects of SVT (svt=surface area-to-volume ratio multiplied by reaction time) scaling on glass/water reactions, particularly with respect to characterizing the alteration layer formed at different values of SVT. Preliminary testing indicated that a Na-rich borosilicate glass, SRL 131, would achieve significant reaction in a relatively short period of time. While 131 glass is not expected to be produced at the defense waste processing facility (DWPF) its composition falls in the range for consideration. The results presented here include a comparison of the solution concentrations and detailed descriptions of the alteration layers that formed, analyzed using Analytical Electron Microscopy (AEM). The examination of solution concentrations and solids changes simultaneously allows a more complete assessment of the effects of surface area to volume ratio (SA/V) on glass reaction. 20 refs., 2 figs., 1 tab.

Shaping particles for ultramicrotomy

A technique for preshaping large particles of reacted glass for ultramicrotomy is described. This technique preserves delicate surface layers and results in more sections per block extending deeper into the sample than other techniques.

Analytical electron microscopy examination of solid reaction products in long-term test of SRL 200 waste glasses

Alteration phases, found on the leached surfaces and present as colloids in the leachates of 200-based frit (fully active and simulated) nuclear waste glass, reacted under static test conditions, at a surface area to leachate volume ratio of 20,000 m{sup {minus}1} for 15 days to 728 days, have been examined by analytical electron microscopy. The compositions of the secondary phases were determined using x-ray energy dispersive spectroscopy and electron energy loss spectroscopy, and structural analysis was accomplished by electron diffraction. Long-term samples of simulated glass, which had undergone an acceleration of reaction after 182 days, possessed a number of silicate secondary phases, including; smectite (iron silicate and potassium iron alumina-silicate, weeksite (uranium silicate), zeolite (calcium potassium alumino-silicate), tobermorite (calcium silicate), and a pure silica phase. However, uranium silicates and smectite have also been observed in tests, which have not undergone the acceleration of reaction, in both the leachate and leached layer, suggesting that these phases are not responsible for the acceleration of reaction.