John D. Vienna

Nepheline Precipitation in High-Level Waste Glasses : Compositional Effects and Impact on the Waste Form Acceptability

The impact of crystalline phase precipitation in glass during canister cooling on chemical durability of the waste form limits waste loading in glass, especially for vitrification of certain high-level waste (HLW) streams rich in Na 2 O and Al 2 O 3 . This study investigates compositional effects on nepheline precipitation in simulated Hanford HLW glasses during canister centerline cooling (CCC) heat treatment. It has been demonstrated that the nepheline primary phase field defined by the Na 2 O-Al 2 O 3 -SiO 2 ternary system can be used as an indicator for screening HLW glass compositions that are prone to nepheline formation. Based on the CCC results, the component effects on increasing nepheline precipitation can be approximately ranked as Al 2 O 3 > Na 2 O > Li 2 O ≈ K 2 O ≈ Fe 2 O 3 > CaO > SiC 2 . The presence of nepheline in glass is usually detrimental to chemical durability. Using x-ray diffraction data in conjunction with a mass balance and a second-order mixture model for 7-day product consistency test (PCT) normalized B release, the effect of glass crystallization on glass durability can be predicted with an uncertainty less than 50% if the residual glass composition is within the range of the PCT model.

LIQUIDUS TEMPERATURE-COMPOSITION MODEL FOR MULTI-COMPONENT GLASSES IN THE Fe, Cr, Ni, AND Mn SPINEL PRIMARY PHASE FIELD

Abstract We developed an extensive T L database of simulated high-level waste (HLW) glasses within the spinel primary phase field. Partial-molar T L s, T i , were determined for all components that were systematically varied in database glasses ( i =Al, B, Ca, Cr, Fe, K, Li, Mg, Mn, Na, Ni, Si, Ti, U, and Zr). A clear relationship was found between the T i values and ion potential. This led to a new model that can accurately predict the T L of glasses within component concentration ranges of the database. The model gives slightly better predictability than a first-order expansion of T L in composition while using only half of the fitted parameters and offers an improvement in predictability over previously published models. The success of this model gives insight to the nature of component effects on T L , which warrants further investigation. Specifically, the concentrations of glass components appear to be influential on T L in proportion to the character of their bonds or their ionic potential.

Chalcogen -based aerogels as a multifunctional platform for remediation of radioactive iodine

Aerogels employing chalcogen-based (i.e., S, Se, and/or Te) structural units and interlinking metals are termed chalcogels and have many emerging applications. Here, chalcogels are discussed in the context of nuclear fuel reprocessing and radioactive waste remediation. Motivated by previous work on removal of heavy metals in aqueous solution, we explored the application of germanium sulfide chalcogels as a sorbent for gas-phase I2 based on Pearsons Hard/Soft Acid–Base (HSAB) principle. This work was driven by a significant need for high-efficiency sorbents for 129I, a long-lived isotope evolved during irradiated UO2 nuclear fuel reprocessing. These chalcogel compositions are shown to possess an affinity for iodine gas, I2(g), at various concentrations in air. This affinity is attributed to a strong chemical attraction between the chalcogen and I2(g), according to the HSAB principle. The high sorption efficiency is facilitated by the high porosity as well as the exceptionally large surface area of the chalcogels. This paper briefly discusses the current and alternative waste forms for 129I, elaborates on preliminary work to evaluate a Pt-Ge-S chalcogel as a I2(g) sorbent, and discusses the unknown chalcogel properties related to these materials in waste form.

The effects of melting reactions on laboratory-scale waste vitrification

At the U.S. Department of Energy`s Hanford Site, processes are being developed to vitrify waste generated during nuclear materials processing. One of the wastes slated for vitrification is known as neutralized current acid waste (NCAW). The batch chemistry of simulated NCAW was varied with oxidants and reductants. Untreated, formated, nitrated, or sugar-added samples were combined with frit to produce melter feed. Offgas measurements of the formated melter feed showed that formates decomposed at temperatures too low for participation in melt redox reactions. Sugar pyrolized and produced CO and H{sub 2} at temperatures exceeding 665 {degree}C. For the sugar-added samples, the glass quenched from 1200 {degree}C produced an Fe{sup 2+}/{summation}Fe of 0.79. The measured iron redox ratios from the glasses made from untreated, formated, and nitrated wastes were essentially indistinguishable (0.0024 at 1000 {degree}C and 0.032 at 1200 {degree}C). However, the batch chemistry affected volume expansion and the reaction paths.

High-Level Waste Melter Study Report

At the Hanford Site in Richland, Washington, the path to site cleanup involves vitrification of the majority of the wastes that currently reside in large underground tanks. A Joule-heated glass melter is the equipment of choice for vitrifying the high-level fraction of these wastes. Even though this technology has general national and international acceptance, opportunities may exist to improve or change the technology to reduce the enormous cost of accomplishing the mission of site cleanup. Consequently, the U.S. Department of Energy requested the staff of the Tanks Focus Area to review immobilization technologies, waste forms, and modifications to requirements for solidification of the high-level waste fraction at Hanford to determine what aspects could affect cost reductions with reasonable long-term risk. The results of this study are summarized in this report.

The effect of experimental conditions and evaluation techniques on the alteration of low activity glasses by vapor hydration

Abstract The vapor hydration test (VHT) is used to study the reaction between waste forms and water with the aim to gain insight into their alteration behavior. In nuclear-waste immobilization, the test is primarily used as a screening tool to identify durable/non-durable glasses and as a convenient method to generate and identify alteration products for use in performance modeling. The lack of a standard procedure for conducting this test has resulted in a number of reported test methods, thus decreasing the ability to directly compare test results from different sources. To optimize the VHT procedure, a series of tests was conducted on simulated low activity waste (LAW) glasses at temperatures ranging from 150 to 300 °C with different volumes of water, specimen holders, specimen-preparation techniques, and data-evaluation methods. Reaction progress was monitored by measuring the thickness of the remaining glass and the alteration layer. The alteration rate at which glass is converted into the alteration products was determined by linear regression from the remaining glass thickness plotted as a function of time. The resulting procedure for conducting VHT eliminates problems associated with the measurement of alteration layers and enables direct comparison of alteration rates for different materials.

X-ray absorption spectroscopic investigation of the environment of cerium in glasses based on complex cerium alkali borosilicate compositions

Abstract We present the results of cerium L III X-ray absorption near edge structure (XANES) studies of two series (-I and -II) of alkali boro-aluminosilicate based glasses prepared from compositions containing different amounts of CeO 2 and P 2 O 5 , and melted at a variety of temperatures and redox conditions. Model XANES spectra for Ce 3+ in the glass, Ce 4+ in the glass, and crystalline CeO 2 , were used to fit the Ce L III XANES spectra obtained from the glasses. This analysis allowed both the [Ce 4+ ]/[Ce total ] ratio and the distribution of Ce environments to be determined for each glass. The [Ce 4+ ]/[Ce total ] ratios, which varied from 0.00 to 0.58, determined independently by XANES spectroscopy were found to have a correlation, r >0.9, with those previously obtained using wet chemical methods. Furthermore, using this method, these two series of glasses, which differed in the amounts of BaO, Bi 2 O 3 , SnO 2 , and ZrO 2 , had different distributions in their cerium environments. The Ce L III XANES spectra obtained from series II glasses (1.0 wt% BaO, 1.0 wt% Bi 2 O 3 , no SnO 2 , and 1.0 wt% ZrO 2 ) melted at 1350°C were able to be fit using only Ce glass 3+ and Ce glass 4+ environments; whereas, those from series I glasses (2.7 wt% SnO 2 , no BaO or Bi 2 O 3 , and 5.1 wt% ZrO 2 ) melted at 1120°C could only be fit when an additional Ce 4+ CeO 2 component was included, confirming the previous assessment that the series I glasses exhibited CeO 2 related heterogeneities.

Glass Property Data and Models for Estimating High-Level Waste Glass Volume

This report describes recent efforts to develop glass property models that can be used to help estimate the volume of high-level waste (HLW) glass that will result from vitrification of Hanford tank waste. The compositions of acceptable and processable HLW glasses need to be optimized to minimize the waste-form volume and, hence, to save cost. A database of properties and associated compositions for simulated waste glasses was collected for developing property-composition models. This database, although not comprehensive, represents a large fraction of data on waste-glass compositions and properties that were available at the time of this report. Glass property-composition models were fit to subsets of the database for several key glass properties. These models apply to a significantly broader composition space than those previously publised. These models should be considered for interim use in calculating properties of Hanford waste glasses.

Database and Interim Glass Property Models for Hanford HLW and LAW Glasses

This report discusses a methodology for increasing the efficiency and decreasing the cost of vitrifying nuclear waste by optimizing waste-glass formulation. This methodology involves collecting and generating a property-composition database (for glass properties that determine waste-glass processability and acceptability) and relating these properties to glass composition via property-composition models. The report explains how the property-composition models are developed, fitted to data and evaluated, validated using additional data, used for glass-formulation optimization, and continuously updated in response to changes in waste-composition estimates and processing technologies. Further, the report describes a waste-glass property-composition database compiled from literature sources and presents the results from a critical evaluation and screening of the data for applicability to Hanford waste glasses. Finally, the report provides interim property-composition models for melt viscosity, liquidus temperature (with spinel and zircon primary crystalline phases), and Product Consistency Test normalized releases of B, Na, and Li. Models were fitted to a subset of the database deemed most relevant for the anticipated Hanford waste-glass composition region.

Database and Interim Glass Property Models for Hanford HLW Glasses

The purpose of this report is to provide a methodology for an increase in the efficiency and a decrease in the cost of vitrifying high-level waste (HLW) by optimizing HLW glass formulation. This methodology consists in collecting and generating a database of glass properties that determine HLW glass processability and acceptability and relating these properties to glass composition. The report explains how the property-composition models are developed, fitted to data, used for glass formulation optimization, and continuously updated in response to changes in HLW composition estimates and changes in glass processing technology. Further, the report reviews the glass property-composition literature data and presents their preliminary critical evaluation and screening. Finally the report provides interim property-composition models for melt viscosity, for liquidus temperature (with spinel and zircon primary crystalline phases), and for the product consistency test normalized releases of B, Na, and Li. Models were fitted to a subset of the screened database deemed most relevant for the current HLW composition region.