Hao Gan

Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Abstract Raman spectroscopy has been used to characterize sulfur environments in a variety of borosilicate glass formulations developed for long-term radioactive waste storage. The spectra of these glasses all have S–O symmetrical stretch modes (ν1) near 1000 cm −1 from tetrahedral SO4 (sulfate) environments. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO4 tetrahedra in the glass are surrounded by network modifying cations, such as Na. By changing the type of network modifying cations in the borosilicate glass, the ν1 peak shifts, in such a way that larger cation charge densities correlate to higher ν1 frequencies. The ν1 peak for Li, Ca, Na, K, and Cs borosilicate glasses is broadened and shifted to lower frequencies with respect to ν1 for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicates that sulfate tetrahedra in borosilicate glass are more disordered and more weakly bonded to their surrounding environments than sulfate tetrahedra in the corresponding sulfate crystals.

X-ray absorption studies of manganese valence and local environment in borosilicate waste glasses

Abstract X-ray absorption data were collected and analyzed to characterize the manganese environments in borosilicate glass formulations to be used for immobilization of nuclear wastes. Mn can become a significant constituent in some radioactive wastes, because of the use of Mn-compounds in waste pretreatment processes. Sixteen borosilicate glasses were investigated, which were synthesized to simulate the Mn environments in the anticipated waste glasses, where MnO concentrations range from 0.4 to 13.6 wt%. The X-ray absorption near edge structure (XANES) for all glasses investigated indicate that most of the manganese within these samples is divalent. The extended X-ray absorption fine structure (EXAFS) analysis results for the glasses show average Mn–O distances near 2.07 A, coordination numbers between 4.3 and 5.2, and large first-shell Debye–Waller factors. The EXAFS findings indicate that Mn2+ in borosilicate glass is most likely within a distribution of environments that include 4- and 5-coordinated sites. EXAFS data and fitting results also show that the average manganese environments in these glasses are statistically invariant with respect to composition as well as to synthesis conditions.

X-ray absorption studies of chlorine valence and local environments in borosilicate waste glasses

Chlorine (Cl) is a constituent of certain types of nuclear wastes and its presence can affect the physical and chemical properties of silicate melts and glasses developed for the immobilization of such wastes. Cl K-edge X-ray absorption spectra (XAS) were collected and analyzed to characterize the unknown Cl environments in borosilicate waste glass formulations, ranging in Cl-content from 0.23 to 0.94 wt.%. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data for the glasses show trends dependent on calcium (Ca) content. Near-edge data for the Ca-rich glasses are most similar to the Cl XANES of CaCl2, where Cl is coordinated to three Ca atoms, while the XANES for the Ca-poor glasses are more similar to the mineral davyne, where Cl is most commonly coordinated to two Ca in one site, as well as Cl and oxygen nearest-neighbors in other sites. With increasing Ca content in the glass, Cl XANES for the glasses approach that for CaCl2, indicating more Ca nearest-neighbors around Cl. Reliable structural information obtained from the EXAFS data for the glasses is limited, however, to ClACl, ClAO, and ClANa distances; ClACa contributions could not be fit to the glass data, due to the narrow k-space range available for analysis. Structural models that best fit the glass EXAFS data include ClACl, ClAO, and ClANa correlations, where ClAO and ClANa distances decrease by approximately 0.16 Å as glass Ca content increases. XAS for the glasses indicates Cl is found in multiple sites where most Cl-sites have Ca neighbors, with oxygen, and possibly, Na second-nearest neighbors. EXAFS analyses suggest that ClACl environments may also exist in the glasses in minor amounts. These results are generally consistent with earlier findings for silicate glasses, where Cl was associated with Ca and Na in network modifier sites. 2010 Elsevier B.V. All rights reserved.

Optimization of Savannah River M-Area Mixed Waste for Vitrification

Vitrification studies of actual Savannah River M-Area mixed wastes have shown that the limiting factor for high waste loading of this waste stream is its chemical durability as defined by the toxicity characteristics leaching procedure (TCLP). As part of the optimization study of Savannah River M-Area wastes, a number of additives were examined including Na{sub 2}O, Li{sub 2}O, B{sub 2}O{sub 3}, ZrO{sub 2}, and TiO{sub 2}. This paper reports on the effect of varying the boron to total alkali ratio and on the effect of substitutions such as ZrO{sub 2} for waste and TiO{sub 2} for SiO{sub 2} on the chemical durability and processability of M-Area waste glasses.

Corrosion of Inconel-690 electrodes in waste glass melts

Inconel-690, a Cr-Ni-Fe-based “superalloy,” has become the material of choice for electrodes in joule-heated waste glass melters and is currently employed in the high-level nuclear waste vitrification systems at West Valley and DWPF, as well as in GTS Durateks privatized M-Area mixed waste vitrification facility at Savannah River. Future applications of joule-heated vitrification technologies will necessitate an assessment of the limits of performance of this material under more demanding conditions than have been studied previously. In this work, Inconel 690 electrodes were tested in several simulated sodium-rich aluminosilicate waste glasses in wide ranges of AC current density, electrical waveform, temperature, and glass composition.

Corrosion of Chromium-Rich Oxide Refractories in Molten Waste Glasses

The DOE is faced with a wide variety of waste treatment problems throughout the complex. The diversity in physical, chemical, and radiological characteristics of these waste streams will necessitate an array of treatment technologies since, at present, there exists no single solution. Thermal treatment technologies have an important, but by no means singular, role to play in addressing this problem since they generally offer the potential for significant volume reductions, leach resistant waste forms, considerable versatility, and are relatively well developed. In particular, DOE has made significant investments in the development and deployment of vitrification technologies for the treatment of high-level nuclear wastes and, more recently, for mixed wastes. The general area of materials of construction is especially important for thermal processes due to the inherently high-temperature and the often-corrosive environments involved. The performance of these materials directly impacts treatment costs since this determines maintenance downtime and the useful service life of the treatment unit.

Mechanism of Vapor Phase Hydration In High-Sodium Waste Glasses from Computer Simulations

The kinetics of vapor phase hydration (VHT) of high-sodium waste glasses often exhibits incubation, fast-rising , and final stages. The incubation stage is characterized by the slow and gradual appearance of transformed regions on the surface of the glass, signaling the initiation of the phase transformation. This is followed by a fast-rising stage, during which most of the transformation takes place. The final stage is characterized by the slower alteration of the remaining glass phase, which can have a duration orders of magnitude longer than those of the previous stages. While the Avrami equation provides a good representation of these features, the origin of the late-stage slow-down in the Avrami model is unrealistic for the VHT process. Computer simulations based on cell models have been used to investigate possible mechanistic origins. The models consider the formation of multiple alteration phases and simulate the complex interplay among the several microscopic mechanisms involved in the hydration process. The models assume that the transformation from the original pristine glass to the final stable altered phase occurs in two steps: from glass to a metastable phase, and then from a metastable to a stable phase. The results indicate that the late-stage slow-down can be reproduced as a result of the build-up of species that are incompatible with the stable phase or by the occlusion of the surface due to the growth of a surface phase that is less permeable to water. The simulation results are in good general agreement with the features of the experimental data.

Baseline LAW Glass Formulation Testing

The major objective of the baseline glass formulation work was to develop and select glass formulations that are compliant with contractual and processing requirements for each of the LAW waste streams. Other objectives of the work included preparation and characterization of glasses with respect to the properties of interest, optimization of sulfate loading in the glasses, evaluation of ability to achieve waste loading limits, testing to demonstrate compatibility of glass melts with melter materials of construction, development of glass formulations to support ILAW qualification activities, and identification of glass formulation issues with respect to contract specifications and processing requirements.

Glass Formulation and Testing with TWRS LAW Simulants, Final Report to Duratek Inc. and BNFL Inc.

This report presents the results of glass formulation development with TWRS LAW simulants that was conducted at the Vitreous State Laboratory of The Catholic University of America during TWRS Phase I.

Final Report - IHLW PCT, Spinel T1%, Electrical Conductivity, and Viscosity Model Development, VSL-07R1240-4

This report is the last in a series of currently scheduled reports that presents the results from the High Level Waste (HLW) glass formulation development and testing work performed at the Vitreous State Laboratory (VSL) of the Catholic University of America (CUA) and the development of IHLW property-composition models performed jointly by Pacific Northwest National Laboratory (PNNL) and VSL for the River Protection Project-Waste Treatment and Immobilization Plant (RPP-WTP). Specifically, this report presents results of glass testing at VSL and model development at PNNL for Product Consistency Test (PCT), one-percent crystal fraction temperature (T1%), electrical conductivity (EC), and viscosity of HLW glasses. The models presented in this report may be augmented and additional validation work performed during any future immobilized HLW (IHLW) model development work. Completion of the test objectives is addressed.