William L. Ebert

Measurement of kinetic rate law parameters on a NaCaAl borosilicate glass for low-activity waste

Abstract The dissolution kinetics of a NaCaAl borosilicate glass, being studied for immobilization of low-activity waste, were measured between 20 and 90°C and solution pH between 6 and 12 using the single-pass flow-through method. Dissolution kinetics measurements are needed to parameterize a mechanistic model that is being used to compute the corrosion rate of the glass waste form as a function of temperature, pH, and the concentrations of the other glass components in water percolating through a proposed shallow-land disposal facility. The key factors that were found to influence the test results include test duration and background subtraction of the raw data. Background subtraction is shown to be important to prevent a non-physical increase in the computed rate with increasing flow rate, particularly in tests run at higher flow rates. Experimental factors that were found to have no detectable influence on the test results included the glass particle size and buffer type. We also illustrate how flow rate variations can be used to obtain information about the reaction order and equilibrium constant parameters in a conventional transition-state theory rate law.

The hydration of borosilicate waste glass in liquid water and steam at 200 °C☆

Abstract Simulated borosilicate waste glass was hydrated in steam at 200 °C for times up to 40 days to assess the effect of a very high glass surface area/leachant volume (SA/V) ratio on the reaction. The reactions in steam attained an SA/V in excess of 4000 m−1 due to the limited amount of water that was available to condense on the glass surface. Experiments in liquid water were performed at an SA/V of 40 m−1 for comparison. A solid reaction layer formed on the glass surface in both environments, and the thickness of this layer was used as a measure of the reaction progress. Other secondary phases formed on top of (and within) the layer on the steam-reacted samples after a few days of reaction but not on samples reacted in liquid water. The rate (layer thickness/time) measured in experiments with liquid water slows with time while the reaction in steam is very slow initially but then proceeds at a high rate after secondary phases form. The secondary phases are believed to increase the reaction rate by lowering the solution concentrations of glass species (probably most importantly silicon) which control the reaction affinity. The glass reaction is accelerated in a steam environment relative to liquid environment because, in steam, the small solution volume becomes saturated and precipitates are formed after much less glass has reacted. The experimental technique described allows secondary phases to be generated within short time periods at elevated temperatures in a steam environment. Knowledge of the phases formed is necessary to predict the long-term reaction rate. Precipitates formed on the steam-reacted samples were identified using SEM/EDS analysis and XRD. The EQ3/6 computer code was used to predict secondary phases formed at 200 °C for comparison to the observed phases. Differences in the assemblage predicted by the computer simulation and that produced in the experiments are attributed to the limited data base use by the simulation.

The Raman spectra of several uranyl-containing minerals using a microprobe

Abstract The Raman spectra of uranophane, sodium boltwoodite, soddyite, weeksite, carnotite, and dehydrated schoepite were collected using a microprobe. The dominant feature of all spectra was a strong peak between 738 and 842 cm −1 which was assigned to the symmetric stretch of the uranyl ion. The peak position varies with the environment of the uranyl ion and is helpful in the identification of uranium-bearing phases that form during the water-assisted alteration of high-level nuclear waste.

The Effects of the Glass Surface Area/Solution Volume Ratio on Glass Corrosion: a Critical Review

This report reviews and summarizes the present state of knowledge regarding the effects of the glass surface area/solution volume (SA/V) ratio on the corrosion behavior of borosilicate waste glasses. The SA/V ratio affects the rate of glass corrosion through the extent of dilution of corrosion products released from the glass into the leachate solution: glass corrosion products are diluted more in tests conducted at low SA/V ratios than they are in tests conducted at high SA/V ratios. Differences in the solution chemistries generated in tests conducted at different SA/V ratios then affect the observed glass corrosion behavior. Therefore, any testing parameter that affects the solution chemistry will also affect the glass corrosion rate. The results of static leach tests conducted to assess the effects of the SA/V are discussed with regard to the effects of SA/V on the solution chemistry. Test results show several remaining issues with regard to the long-term glass corrosion behavior: can the SA/V ratio be used as an accelerating parameter to characterize the advanced stages of glass corrosion relevant to long disposal times; is the alteration of the glass surface the same in tests conducted at different SA/V, and in tests conducted with monolithic and crushed glass samples; what are the effects of the SA/V and the extent of glass corrosion on the disposition of released radionuclides? These issues will bear on the prediction of the long-term performance of waste glasses during storage. The results of an experimental program conducted at ANL to address these and other remaining issues regarding the effects of SA/V on glass corrosion are described. 288 refs., 59 figs., 16 tabs.

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.

Issues affecting the prediction of glass reactivity in an unsaturated environment

Abstract Prior to the licensing of a high-level waste repository, the US Department of Energy (DOE) will seek to initiate hot startup of the high-level waste glass processing facilities at Savannah River and West Valley. To provide confidence that issues related to glass reactivity and disposal are evaluated prior to startup, the Environmental Restoration and Waste Management branch of DOE is studying important physical parameters that will affect glass reaction in a repository environment and is developing a modeling approach to predict glass performance. In this report, results are presented comparing the reactivity of fully radioactive glass with simulated glass of the same nominal composition. While differences in elemental releases between the glass types are observed, the differences are moderated with time and can be related to the dominant reaction mechanism. Additionally, the relationship between glass surface area and the volume of leachate (SA/V) is evaluated. The reactivity at differentSA/V is dominated by solution pH and reaction affinity, with an increase in reactivity observed at long reaction times. Finally, data are presented which provide necessary parameters for the application of predictive modeling. In particular, forward reaction rate constants are presented, for a 165-frit based glass and a simple analogue, and a list of secondary phases observed during glass reaction under a variety of conditions is compiled.

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.

The precision of product consistency tests conducted with a glass-bonded ceramic waste form

The product consistency test (PCT) that is used for qualification of borosilicate high-level radioactive waste (HLW) glasses for disposal can be used for the same purpose in the qualification of the glass-bonded sodalite ceramic waste form (CWF). The CWF was developed to immobilize radioactive salt wastes generated during the electrometallurgical treatment of spent sodium-bonded nuclear fuels. An interlaboratory study was conducted to measure the precision of PCTs conducted with the CWF for comparison with the precision of PCTs conducted with HLW glasses. The six independent sets of triplicate PCT results generated in the study were used to calculate the intralaboratory and interlaboratory consistency based on the concentrations of Al, B, Na, and Si in the test solutions. The results indicate that PCTs can be conducted as precisely with the CWF as with HLW glasses. For example, the values of the reproducibility standard deviation for Al, B, Na, and Si were 1.36, 0.347, 3.40, and 2.97 mg/l for PCT with CWF. These values are within the range of values measured for borosilicate glasses, including reference HLW glasses.

An interlaboratory study of a standard glass for acceptance testing of low-activity waste glass

An interlaboratory study was conducted to determine the precision with which the composition and chemical durability of a borosilicate glass could be measured and to generate a data base of expected values for that glass. The study was conducted with a low-activity reference material (LRM) glass that was developed for use as a standard material for acceptance testing of immobilized low-activity waste (ILAW) products, including those to be made with Hanford tank wastes. The study provided nine independent measurements of the LRM glass composition and eight independent sets of triplicate product consistency tests (PCTs) at 40°C and 90°C. Statistical analysis of these data indicates that LRM glass is suitable for use as a composition and test standard. The results from this study can be used to evaluate the accuracy of composition analyses and PCTs conducted with LRM glass at other laboratories in conjunction with acceptance tests conducted with ILAW products.

Modeling Surface Area to Volume Effects on Borosilicate Glass Dissolution

We simulated the reaction of SRL-131 glass with equilibrated J-13 water in order to investigate the effects of surface area to volume ratio (SAN/) on glass dissolution. We show that glass-fluid ion exchange causes solution pH to rise to progressively higher values as SA/V increases. Because the ion exchange is rapid relative to the duration of the glass dissolution experiment, the pH effect does not scale with (SA/V)*time. Experiments compared at the same (SA/V)*time value therefore have different pHs, with higher pHs at higher SA/V ratios. Both experimental data and our simulation results show similar trends of increasing reaction rate as a function of SAN ratio when scaled to (SA/V)*time. Glasses which react in systems of differing SA/V ratio therefore follow different reaction paths and high SAN ratios cannot be used to generate data which accurately scales to long time periods unless the ion exchange effect is taken into account. We suggest some simple test designs which enable more reliable high SAN accelerated tests.