Immobilized Low-activity Waste Research Articles

Application of the immobilized low-activity waste glass corrosion model to the static dissolution of 24 statistically-designed alkali-borosilicate waste glasses

Glass corrosion models that capture the complex mechanisms of the glass-water reaction enable the prediction of nuclear waste glass durability in disposal scenarios. Parameterization of such models is challenging because of the need to capture changes in corrosion behavior with time, reaction conditions, and glass composition. Here, we describe and employ the ILAW (immobilized low-activity waste) glass corrosion model (IGCM) in geochemical simulations of static dissolution tests, at two temperatures (40 °C and 90 °C), for a matrix of 24 enhanced low-activity waste (eLAW) glasses statistically designed to cover a processable composition space defined by 8 major glass components (Al2O3, B2O3, CaO, Na2O, SiO2, SnO2, ZrO2, and Others as defined in the text). The IGCM includes a first-order chemical affinity term and an ion-exchange term that represents the net exchange of Na+ ions in the pristine glass with protons in aqueous solution. Constant, time-dependent, and time- and pH-dependent functional forms of the ion-exchange term are evaluated to reproduce the change in corrosion behavior with time in saturated, static dissolution tests. The agreement with measured aqueous concentrations of the main glass components (B, Na, and Si) improved significantly upon addition of a time-dependent term, which therefore constitutes a simple representation of the glass-water reaction progress. Due to the limited changes in pH in the static dissolution tests, past a short initial period of rapid increase, addition of a pH-dependent term did not appreciably improve the fits, indicating that comprehensive model parameterization requires more than one type of glass corrosion test to capture a wide range of solution chemistries. IGCM parameters were found to be dependent on glass composition, and the parameter sets generated in this work will enable the development of composition–parameter correlation models that offer the promise of predicting IGCM parameters, and thus glass corrosion behavior, solely based on glass composition.

Measurement of Solute Diffusion Behavior in Fractured Waste Glass Media

Determination of aqueous diffusion coefficients of solutes through fractured media is essential for understanding and modeling contaminant transport at many hazardous waste disposal sites. Development of experimental methods and measurements for the characterization of diffusion in fractured glass media is necessary for the design and performance assessment of glassified radionuclear waste disposal facilities. We report on the use of time-lag diffusion experimental method to assess the diffusion behavior of three different solutes (Cs, Sr, and pentafluoro benzoic acid) in fractured, immobilized low activity waste (ILAW) glass forms. A fractured media time-lag diffusion experimental apparatus, which allows the measurement of diffusion coefficients, has been designed and built for this purpose. Use of time-lag diffusion method, a considerably easier experimental method than the other available methods, was not previously demonstrated for measuring diffusion in fractured waste glass media. Hydraulic conductivity, porosity, and diffusion coefficients of a solute were experimentally measured in fractured glass blocks using this method for the first time. Results agree with the range of properties reported for similar rock media earlier, indicating that the time-lag experimental method can effectively characterize the diffusion coefficients of fractured ILAW glass media. Data presented are rare and useful for the design of vitrified glass disposal facilities.

An experimental study of the dissolution rates of simulated aluminoborosilicate waste glasses as a function of pH and temperature under dilute conditions

Abstract Single-pass flow-through tests were conducted to determine the pH (7–12) and temperature (23–90 °C) dependence of kinetic rate law parameters; ko, η, and Ea, for the dissolution of glass in aqueous solution. Experiments were performed with three prototypic nuclear waste glasses that span a wide compositional range, which covers, with high probability, the expected processing composition range for candidate immobilized low-activity waste (ILAW) glasses. Comparison of the B to Na release rates for one glass was incongruent at 23 and 40 °C, and pH (23 °C) = 7.0 and 8.0, suggesting two distinct mechanisms are responsible for the Na+ release, namely Na+–H+ ion-exchange and matrix dissolution. Matrix dissolution became the dominant dissolution mechanism for all glasses at the forward rate and pH values greater than 9.0 as evident by the congruent release of Al, B, Na and Si to solution. By combining the results collected for each ILAW glass at pH values greater than 9.0, pH and temperature-dependent rate law parameters were determined for Al, B, Na and Si release. A comparison of the pH power-law coefficient for Al, B, Na and Si at each temperature suggest that η does not depend on temperature within experimental error and suggests the release of these elements into solution is controlled by the same dissolution mechanism at the forward rate of reaction. The activation energies (Ea), based on B release, range from 52 ± 4 to 56 ± 6 kJ/mol which suggest that dissolution is a surface-controlled reaction mechanism. The data presented in this manuscript suggest that for these three ILAW glasses the chemical durability for each glass is similar under these test conditions. A lack of compositional dependence on the forward dissolution rate is observed even though there is as much as a 39 kJ/mol difference in the free energy of hydration (ΔGhyd) among the borosilicate waste glasses tested. This similarity in the forward dissolution rate despite the large ΔGhyd difference is almost certainly because these glasses have similar, if not identical, polymerization states. This is evident from the almost identical 29Si chemical shifts for each of these glasses. The polymerization state is an indication of the number of framework SiO4 linkages contained in the glass network. In general, the greater the number of framework SiO4 linkages the more durable the glass. Finally, in agreement with previous work, these results suggest breakage of the Si–O bond is the rate-determining dissolution mechanism under alkaline conditions [pH (23 °C) > 9.0] far from saturation with respect to an alteration phase or phases.

The Accelerated Weathering of a Radioactive Low-Activity Waste Glass under Hydraulically Unsaturated Conditions: Experimental Results from a Pressurized Unsaturated Flow Test

To predict the long-term fate of low- and high-level waste forms in the subsurface over geologic timescales, it is important to understand how the formation of an alteration phase or phases will affect radionuclide release from the corroding waste forms under repository-relevant conditions. To generate data to conduct performance assessment calculations for the low-activity waste (LAW) integrated disposal facility at the Hanford Site in southeastern Washington state, accelerated weathering experiments are being conducted with the pressurized unsaturated flow (PUF) test method to evaluate the long-term release of radionuclides from immobilized LAW (ILAW) glasses. The radionuclide release rate is a key parameter affecting the overall performance of the LAW disposal facility.Currently, there are three other accelerated weathering test methods being used to evaluate the long-term durability of glasses: product consistency test, vapor hydration test, and unsaturated drip test. In contrast to these test methods, PUF tests mimic the hydraulically unsaturated open-flow and transport conditions expected in the near-field vadose zone environment, allow the corroding waste form to achieve its final reaction state, and accelerate the hydrolysis and aging processes by as much as 50 times over conventional static tests run at the same temperature.In this paper, we discuss the results of an accelerated weathering experiment conducted with the PUF apparatus to evaluate the corrosion rate of an ILAW glass, LAWAN102, made with actual Hanford waste taken from Tank 241-AN-102 (U). Results from this PUF test with LAWAN102 glass showed that after 1.5 yr of testing, the corrosion rate, based on B release, reached a steady-state release of 0.010 ± 0.003 g m–2 day–1, which is approximately eight times lower than other glasses previously tested. These results indicate that 99Tc is being released from the glass congruently, whereas U is being controlled by the formation of a solubility-limiting phase or phases. These results also highlight the importance of being able to predict, with some level of certainty, the alteration phase or phases that will form and how the formation of these phases may impact the release, retention, and transport of radionuclides from the glass under the hydraulically unsaturated open flow and transport conditions that are expected in the LAW integrated disposal facility.

Measurement of effect of chemical reactions on the hydrologic properties of fractured glass media using a tri-axial flow and transport apparatus

Summary Understanding the effect of chemical reactions on the hydrologic properties of sub-surface media is critical to many natural and engineered sub-surface systems. Methods and information for such characterization of fractured media are severely lacking. Influence of glass corrosion (precipitation and dissolution) reactions on fractured glass blocks HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted in such randomly and multiply fractured ILAW glass blocks, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig (1379 KPa) pressure, causing the precipitation of alteration products. A tri-axial fractured media flow and transport experimental apparatus, which allows the simultaneous measurement of flow and transport properties and their anisotropy, has been designed and built for this purpose. Such apparatus for fractured media characterization are being reported in the literature only recently. Hydraulic properties of fractured blocks were measured in different orientations and along different cardinal directions, before and after glass corrosion reactions. Miscible displacement experiments using a non-reactive dye were also conducted, before and after glass corrosion reactions, to study the tracer transport behavior through such media. Initial efforts to analyze breakthrough curve (BTC) data using a 1D advection dispersion equation (ADE) solution revealed that a different fractured media transport model, which accurately accounts for the heterogeneous transport behavior in 3D, may be necessary for such interpretation. It was found that glass reactions could have a significant influence on the hydrologic properties of fractured ILAW glass media. The methods and results are useful to better understand the effect of chemical reactions on the hydrologic properties of fractured geomedia in general and glass media in particular.

Effect of chemical reactions on the hydrologic properties of fractured and rubbelized glass media

Understanding the effect of chemical reactions on the hydrologic properties of geological media, such as porosity, permeability and dispersivity, is critical to many natural and engineered sub-surface systems. Influence of glass corrosion (precipitation and dissolution) reactions on fractured and rubbelized (crushed) forms HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted using fractured and rubbelized forms, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig pressure, causing the precipitation of alteration products. Data were analyzed using analytical expressions and CXTFIT, a transport parameter optimization code, for the estimation of the hydrologic characteristics before and after VHT. It was found that glass reactions significantly influence the hydrologic properties of ILAW glass media. Hydrologic properties of rubbelized glass decreased due to precipitation reactions, whereas those of fractured glass media increased due to reaction which led to unconfined expansion of fracture aperture. The results are unique and useful to better understand the effect of chemical reactions on the hydrologic properties of fractured and rubbelized stony media in general and glass media in particular.

Influence of mineral precipitation and dissolution on hydrologic properties of porous media in static and dynamic systems

A critical component in determining the suitability of disposing glassified, low activity waste is the identification of key mineral assemblages affecting the porosity and permeability of both the glass and near- and far-field materials. In this study, two different classes of geochemical models are used to identify mineral precipitation and dissolution potentials for an immobilized low-activity waste (ILAW) disposal facility in Hanford, Washington. The first is a static geochemical model that does not consider the effects of transport. The second model is dynamic, and combines geochemical reactions with hydrogeological processes such as advection, diffusion and dispersion. This reactive transport model also includes an innovative application of a depositional film model for determining changes in permeability due to mineral precipitation and dissolution reactions. Although both models describe solid-aqueous phase reactions kinetically, the two models identify two different sets of mineral assemblages affecting the porosity and permeability of the media. These markedly different results are due to transport considerations, the most significant of which are the spatial variability in aqueous concentrations, and advection and diffusion of dissolved glass constituents into the backfill materials. This work shows that for the prediction of geochemical behavior of engineered systems, such as the ILAW disposal facility, the traditional reaction path modeling approach is not sufficient for an accurate assessment of the precipitation of key mineral assemblages and their effect on the geochemical and hydraulic behavior of the waste glass. Reactive transport modeling improves this assessment significantly. The static model is useful in identifying potential minerals to be included in the reactive transport simulations. The dynamic model, however, ultimately determines the key mineral assemblages affecting both the geochemical behavior and the hydraulic properties of the waste glass in the presence of a flowing aqueous phase.

Immobilized Low-Activity Waste Performance Assessment:2001 Version

ABSTRACTThe Hanford Immobilized Low-Activity Waste Performance Assessment[1] examines the longterm environmental and human health effects associated with the planned disposal of the vitrified low-activity fraction of waste presently contained in Hanford Site tanks. The tank waste is the byproduct of separating special nuclear materials from irradiated nuclear fuels over the past 50 years. This waste is stored in underground single- and double-shell tanks. The tank waste is to be retrieved, separated into low-activity and high-level fractions, and then immobilized by vitrification. The U.S. Department of Energy (DOE) plans to dispose of the low-activity fraction in the Hanford Site 200 East Area. The high-level fraction will be stored at the Hanford Site until a national repository is approved.This report provides the site-specific long-term environmental information needed by the DOE to modify the current Disposal Authorization Statement for the Hanford Site[2]. The original Disposition Authorization Statement was based on the 1998 version[3] of this performance assessment, which was conditionally accepted by DOE [4].The calculations in this performance assessment show that a “reasonable expectation” exists that the disposal of the immobilized low-level fraction of tank waste from the Hanford Site can meet environmental and health performance objectives. As shown by the sensitivity studies, this conclusion remains valid despite the conceptual designs of the disposal facility and the ILAW packaging having undergone changes.

Waste Form Release Calculations for Performance Assessment of the Hanford Immobilized Low-Activity Waste Disposal Facility Using a Parallel, Coupled Unsaturated Flow and Reactive Transport Simulator

ABSTRACTA set of reactive chemical transport calculations was conducted with the Subsurface Transport Over Reactive Multi-phases (STORM) code to evaluate the long-term performance of a representative low-activity waste glass in a shallow subsurface disposal system located on the Hanford Site. Technetium, the main contributor to a drinking water dose, is assumed to be released congruently with the dissolution of the glass. Sodium is released at a higher rate via a kinetic ion-exchange reaction. Aqueous equilibrium reactions involving sodium and other dissolved glass constituents increase the pH, and hence the rate of glass dissolution. The precipitation of secondary minerals can also lower the amount of aqueous dissolved silica, which can increase the rate of glass dissolution. Predicted technetium release rates, however, remain several orders of magnitude lower than required by drinking water regulations.

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.