James J. Mazer

Aqueous corrosion of natural and nuclear waste glasses II. Mechanisms of vapor hydration of nuclear waste glasses

Abstract Results of recent vapor phase hydration experiments performed on nuclear waste glasses at various temperatures and relative humidities are presented. Hydration rates were determined using a variety of techniques, including specimen weight gain measurement, alteration layer thickness measured using optical microscopy or scanning electron microscopy (SEM), and near-surface concentration profiles obtained using secondary ion mass spectrometry (SIMS). For the three nuclear waste glass formulations studied, previously observed differences in reaction products and rates of hydration in liquid and vapor environments were confirmed. At 100% relative humidity (RH) and at temperatures between 75° and 240°C, the rate of hydration of SRL 131 glass followed and Arrhenius-type rate law with an activation energy of 17.9 kcal/mol. A significantly higher apparent activation energy was estimated for SRL 131 glass hydrated at 95% RH. The vapor hydration rate of SRL 131 glass decreased with RH and was negligible below humidity of 70% RH at 202° C. Potential mechanisms that may govern the vapor hydration of glass are reviewed and several lines of evidence including the parametric dependence of vapor hydration rate observed in the present work are consistent with a molecular water diffusion model. In the context of such a model, the dependence of hydration rates on RH is explained through a relationship between p H 2 O and concentration of absorbed molecular water at the outermost surface of hydrated glasses.

High-Level Nuclear-Waste Borosilicate Glass: A Compendium of Characteristics

With the imminent startup, in the United States, of facilities for vitrification of high-level nuclear waste, a document has been prepared that compiles the scientific basis for understanding the alteration of the waste glass products under the range of service conditions to which they may be exposed during storage, transportation, and eventual geologic disposal. A summary of selected parts of the content of this document is provided. Waste glass alterations in a geologic repository may include corrosion of the glass network due to groundwater and/or water vapor contact. Experimental testing results are described and interpreted in terms of the underlying chemical reactions and physical processes involved. The status of mechanistic modeling, which can be used for long-term predictions, is described and the remaining uncertainties associated with long-term simulations are summarized.

Experimental Hydration Studies of Natural and Synthetic Glasses

The results of a series of hydration experiments on natural glasses (Hawaiian basalt, obsidian) and the nuclear waste glass WV-44 done to examine laboratory methods of accelerating reaction processes are summarized. The glasses were reacted in hydrothermal solution and in saturated vapor water. It was found that different reaction rates and processes were found using the differing conditions, and that laboratory efforts to accelerate and duplicate natural processes must account for the physical processes that occur naturally.

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.

Laboratory Obsidian Hydration Rates

The development of laboratory hydration rates is considered to be the most promising approach for the chronometric dating of obsidian artifacts. The technical aspects of accelerated hydration, hydration rim measurement, and the determination of effective hydration temperature and soil relative humidity are reviewed. It is proposed that glass hydration is controlled primarily by the amount of intrinsic water contained within the unhydrated obsidian and that rates of hydration may be estimated once the concentration level is known. The ability of the intrinsic water model to produce age determinations compatible with other chronometric methods is examined with a case example from Xaltocan, Mexico.

Simulation of Natural Corrosion by Vapor Hydration Test: Seven-Year Results

We have investigated the alteration behavior of synthetic basalt and SRL 165 borosilicate waste glasses that had been reacted in water vapor at 70 {degrees}C for time periods up to seven years. The nature and extent of corrosion of glasses have been determined by characterizing the reacted glass surface with optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). Alteration in 70 {degrees}C laboratory tests was compared to that which occurs at 150-200 {degrees}C and also with Hawaiian basaltic glasses of 480 to 750 year old subaerially altered in nature. Synthetic basalt and waste glasses, both containing about 50 percent wt SiO{sub 2} were found to react with water vapor to form an amorphous hydrated gel that contained small amounts of clay, nearly identical to palagonite layers formed on naturally altered basaltic glass. This result implies that the corrosion reaction in nature can be simulated with a vapor hydration test. These tests also provide a means for measuring the corrosion kinetics, which are difficult to determine by studying natural samples because alteration layers have often spelled off the samples and we have only limited knowledge of the conditions under which alteration occurred.

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.

Speciation of Pu(VI) in near-neutral to basic solutions via spectroscopy

The high sensitivity technique of laser photoacoustic spectroscopy and optical absorption spectroscopy were utilized to investigate the hydrolysis of Pu(VI) in perchlorate media. Absorption bands attributed to the hydrolysis of Pu(VI) were characterized in the pH = 6-8 regime which is of most interest to the proposed repository site at Yucca Mountain. Evidence was presented that supported the assignment of the polynuclear species (640 nm species) reported previously (1) as PuO{sub 3}(OH){sub 5}{sup +1}. Some temperature-dependent LPAS spectra are reported. The results obtained were compared to the predicted speciation of the geochemical code EQ3NR. 14 refs., 4 figs., 3 tabs.

NNWSI waste form testing at Argonne National Laboratory; Semiannual report: January-June 1987

The Nevada Nuclear Waste Storage Investigation (NNWSI) Project is investigating the tuff beds of Yucca Mountain, Nevada, as a potential location for a high-level radioactive waste repository. As part of the waste package development portion of this project, experiments are being performed by the Chemical Technology Division of Argonne National Laboratory to study the behavior of the waste form under anticipated repository conditions. These experiments include the development and performance of a test to measure waste form behavior in unsaturated conditions and the performance of experiments designed to study the behavior of waste package components in an irradiated environment. Previous reports document developments in these areas through 1986. This report summarizes progress during the period January--June 1987, 19 refs., 17 figs., 20 tabs.