James J. Neeway

Argon Cluster Sputtering Source for ToF-SIMS Depth Profiling of Insulating Materials: High Sputter Rate and Accurate Interfacial Information

AbstractThe use of an argon cluster ion sputtering source has been demonstrated to perform superiorly relative to traditional oxygen and cesium ion sputtering sources for ToF-SIMS depth profiling of insulating materials. The superior performance has been attributed to effective alleviation of surface charging. A simulated nuclear waste glass (SON68) and layered hole-perovskite oxide thin films were selected as model systems because of their fundamental and practical significance. Our results show that high sputter rates and accurate interfacial information can be achieved simultaneously for argon cluster sputtering, whereas this is not the case for cesium and oxygen sputtering. Therefore, the implementation of an argon cluster sputtering source can significantly improve the analysis efficiency of insulating materials and, thus, can expand its applications to the study of glass corrosion, perovskite oxide thin film characterization, and many other systems of interest. Graphical Abstractᅟ

A Strategy to Conduct an Analysis of the Long-Term Performance of Low-Activity Waste Glass in a Shallow Subsurface Disposal System at Hanford

The federal facilities located on the Hanford Site in southeastern Washington State have been used extensively by the U.S. government to produce nuclear materials for the U.S. strategic defense arsenal. Currently, the Hanford Site is under the stewardship of the U.S. Department of Energy (DOE) Office of Environmental Management (EM). A large inventory of radioactive and mixed waste resulting from the production of nuclear materials has accumulated, mainly in 177 underground single- and double-shell tanks located in the central plateau of the Hanford Site (Mann et al., 2001). The DOE-EM Office of River Protection (ORP) is proceeding with plans to immobilize and permanently dispose of the low-activity waste (LAW) fraction onsite in a shallow subsurface disposal facility (the Integrated Disposal Facility [IDF]). Pacific Northwest National Laboratory (PNNL) was contracted to provide the technical basis for estimating radionuclide release from the engineered portion of the IDF (the source term) as part of an immobilized low-activity waste (ILAW) glass testing program to support future IDF performance assessments (PAs).

Getters for improved technetium containment in cementitious waste forms

A cementitious waste form, Cast Stone, is a possible candidate technology for the immobilization of low activity nuclear waste (LAW) at the Hanford site. This work focuses on the addition of getter materials to Cast Stone that can sequester Tc from the LAW, and in turn, lower Tc release from the Cast Stone. Two getters which produce different products upon sequestering Tc from LAW were tested: Sn(II) apatite (Sn-A) that removes Tc as a Tc(IV)-oxide and potassium metal sulfide (KMS-2) that removes Tc as a Tc(IV)-sulfide species, allowing for a comparison of stability of the form of Tc upon entering the waste form. The Cast Stone with KMS-2 getter had the best performance with addition equivalent to ∼0.08wt% of the total waste form mass. The observed diffusion (Dobs) of Tc decreased from 4.6±0.2×10-12cm2/s for Cast Stone that did not contain a getter to 5.4±0.4×10-13cm2/s for KMS-2 containing Cast Stone. It was found that Tc-sulfide species are more stable against re-oxidation within getter containing Cast Stone compared with Tc-oxide and is the origin of the decrease in Tc Dobs when using the KMS-2.

Silver-based getters for 129I removal from low-activity waste

Abstract A prominent radionuclide of concern in nuclear wastes, 129I, is present in low-activity wastes (LAW) at the Hanford site. Several Ag-containing materials were tested as immobilization agents, or “getters”, for I (as iodide, I−) removal from deionized (DI) water and a liquid LAW simulant: Ag impregnated activate carbon (Ag–C), Ag exchanged zeolite (Ag–Z), and argentite. In anoxic batch experiments with DI water, the Ag–C and argentite were most effective, with maximum Kd values of 6.2 × 105 mL/g for the Ag–C and 3.7 × 105 mL/g for the argentite after 15 days. Surface area and Ag content were found to influence the performance of the getters in DI water. In the anoxic batch experiments with LAW simulant, Ag–Z vastly outperformed the other getters with Kd values of 2.2 × 104 mL/g at 2 h, which held steady until 15 days, compared with 1.8 × 103 mL/g reached at 15 days by the argentite. All getters were stable over long periods of time (i.e. 40 days) in DI water, while the Ag–Z and argentite were also stable in the LAW simulant. Ag–Z was found to have consistent I removal upon crushing to a smaller particle size and in the presence of O2, making it a strong candidate for the treatment of LAW containing I.

Corrosion Behavior and Microstructure Influence of Glass-Ceramic Nuclear Waste Forms

Glass-ceramic waste forms present a potentially viable technology for the long-term immobilization of liquid nuclear wastes arising from used nuclear fuel reprocessing. Through control of chemistry during fabrication, such waste forms can have designed secondary crystalline phases within a borosilicate glass matrix. In this work, a glass-ceramic containing crystalline powellite and oxyapatite secondary phases was tested for its corrosion properties in dilute conditions using single-pass flow-through testing. Three glass-ceramic samples were prepared using different cooling rates to produce samples with varying microstructure sizes. In testing at 90°C in buffered pH(RT) 7 and pH 9 solutions, it was found that increasing solution pH and decreasing microstructure size (resulting from rapid cooling during fabrication) both led to a reduction in overall corrosion rate, indexed by boron release from the glass matrix. On the other hand, the corrosion rate of crystalline phase decreased with a decrease in pH. The...

Impacts of glass composition, pH, and temperature on glass forward dissolution rate

Nuclear waste glasses dissolve at the forward dissolution rate (rf) in very dilute aqueous solutions, which can isolate the impact of the glass composition from solution feedback and alteration product effects. While it has long been known that pH and temperature (T) strongly impact rf, the impacts of glass composition have remained uncertain. In this work, rf data from 19 nuclear waste glasses were used with the aim of identifying the effect of glass composition on rf. The rf values were modeled as: rf = k010–ηpHexp(−Ea/RT), with k0, η, Ea, and R, respectively, being the intrinsic rate constant, pH coefficient, apparent activation energy, and gas constant. However, no predictive correlation could be established between the individual model parameters (log[k0], η, and Ea) and glass composition for the glasses considered in this study, an outcome that was attributed to the strong positive correlation between the log[k0] and Ea parameters. Therefore, a model was fitted directly to the combined rf from all 19 glasses. This approach showed that 90% of the variation in rf data could be accounted for solely by T and pH effects. Therefore, any composition effects must be relatively small. After normalizing for differences in pH and T, the only notable differences in rf between the glasses were found to correlate with variations in the fraction of glass forming tetrahedra contributed by tetrahedral boron, f([4]B), with an abrupt threshold at a high value of f([4]B) (~0.22), where higher rf are predicted with no discernable composition effects below the threshold.Glass dissolution: Comprehending composition effectsThe effect that environment and composition has on the rate of decay of nuclear waste storage glasses in aqueous solution has been studied. Borosilicate glasses are often used as containment matrices for the disposal of radioactive waste. Their interaction with water is the likeliest method by which they could release their radioactive cargo, therefore it is important to understand the conditions under which glasses corrode. Now John Vienna and colleagues from Pacific Northwest National Laboratory, have studied how pH, temperature and glass composition affect the rate at which various glasses dissolve. By measuring the dissolution rate of a test glass, comparing it with other glasses, and carrying out some modeling, they showed that 90% of the variation in dissolution rate could be accounted for by temperature and pH effects – thus any compositional effects are relatively small.

Method for the in situ Measurement of pH and Alteration Extent for Aluminoborosilicate Glasses Using Raman Spectroscopy

Characterization of long-term processes occurring during alteration of aluminoborosilicate glasses is relevant for natural as well as man-made materials. Static dissolution tests are a common setup for such studies, but the obtained results and related errors are impacted by the frequency and protocol of samplings performed to determine release via solution analysis, e.g., ICP-OES. A noninvasive method was developed to continuously monitor glass alteration based on in situ Raman spectrometry of the solution contained in the alteration vessel. The alteration of a benchmark glass, the environment assessment (EA) glass, for 7 days at 90 °C showed that the pH and boron concentration results obtained from solution monitoring and ICP-OES quantification were similar to the pH and boron results obtained from chemometric modeling of the Raman spectra and within error of previously published results in similar conditions. The errors on altered amounts of glass based on B release were similar for both in situ Raman and ICP-OES. The new Raman method provides a more detailed picture of real time monitoring of an alteration experiment, with intervals between monitoring times as short as dozens of seconds. The in situ Raman method also helps to reduce perturbation to experiments caused by the physical sampling of aliquots (including temperature excursions, re-equilibration with atmosphere, volume variation, and potential chemical contamination) by limiting their number and frequency.