[en] To predict the long-term fate of low- and high-level waste forms in the subsurface over geologic time scales, it is important to understand the behavior of the corroding waste forms under conditions the mimic to the open flow and transport properties of a subsurface repository. Fluidized bed steam reformation (FBSR), a supplemental treatment technology option, is being considered as a waste form for the immobilization of low-activity tank waste. To obtain the fundamental information needed to evaluate the behavior of the FBSR waste form under repository relevant conditions and to monitor the long-term behavior of this material, an accelerated weathering experiment is being conducted with the pressurized unsaturated flow (PUF) apparatus. Unlike other accelerated weathering test methods (product consistency test, vapor hydration test, and drip test), PUF experiments are conducted under hydraulically unsaturated conditions. These experiments are unique because they mimic the vadose zone environment and allow the corroding waste form to achieve its final reaction state. Results from this on-going experiment suggest the volumetric water content varied as a function of time and reached steady state after 160 days of testing. Unlike the volumetric water content, periodic excursions in the solution pH and electrical conductivity have been occurring consistently during the test. Release of elements from the column illustrates a general trend of decreasing concentration with increasing reaction time. Normalized concentrations of K, Na, P, Re (a chemical analogue for 99Tc), and S are as much as 1 x 10⁴ times greater than Al, Cr, Si, and Ti. After more than 600 days of testing, the solution chemistry data collected to-date illustrate the importance of understanding the long-term behavior of the FBSR product under conditions that mimic the open flow and transport properties of a subsurface repository

[en] When developing a performance assessment model for the long-term disposal of immobilized low-activity waste (ILAW) glass, it is desirable to determine the durability of glass forms over very long periods of time. However, testing is limited to short time spans, so experiments are performed under conditions that accelerate the key geochemical processes that control weathering. Verification that models currently being used can reliably calculate the long term behavior ILAW glass is a key component of the overall PA strategy. Therefore, Pacific Northwest National Laboratory was contracted by Washington River Protection Solutions, LLC to evaluate alternative strategies that can be used for PA source term model validation. One viable alternative strategy is the use of independent experimental data from archaeological studies of ancient or natural glass contained in the literature. These results represent a potential independent experiment that date back to approximately 3600 years ago or 1600 before the current era (bce) in the case of ancient glass and 106 years or older in the case of natural glass. The results of this literature review suggest that additional experimental data may be needed before the result from archaeological studies can be used as a tool for model validation of glass weathering and more specifically disposal facility performance. This is largely because none of the existing data set contains all of the information required to conduct PA source term calculations. For example, in many cases the sediments surrounding the glass was not collected and analyzed; therefore having the data required to compare computer simulations of concentration flux is not possible. This type of information is important to understanding the element release profile from the glass to the surrounding environment and provides a metric that can be used to calibrate source term models. Although useful, the available literature sources do not contain the required information needed to simulate the long-term performance of nuclear waste glasses in a near-surface or deep geologic repositories. The information that will be required include (1) experimental measurements to quantify the model parameters, (2) detailed analyses of altered glass samples, and (3) detailed analyses of the sediment surrounding the ancient glass samples.

[en] The interest in the long-term durability of waste glass stems from the need to predict radionuclide release rates from the corroding glass over geologic time-scales. Several long-term test methods have been developed to accelerate the glass-water reaction (drip test, vapor hydration test, product consistency test-B, and pressurized unsaturated flow (PUF)). Currently, the PUF test is the only method that can mimic the unsaturated hydraulic properties expected in a subsurface disposal facility and simultaneously monitor the glass-water reaction. PUF tests are being conducted to accelerate the weathering of glass and validate the model parameters being used to predict long-term glass behavior. One dimensional reactive chemical transport simulations of glass dissolution and secondary phase formation during a 1.5-year long PUF experiment was conducted with the subsurface transport over reactive multi-phases (STORM) code. Results show that parameterization of the computer model by combining direct laboratory measurements and thermodynamic data provides an integrated approach to predicting glass behavior over geologic-time scales.

[en] To quantify the rate of glass dissolution, a series of experiments have been conducted using the single-pass flow-through (SPFT) apparatus. The SPFT apparatus allow for the transfer of fresh input solution from a reservoir bottle into a Teflon reactor and finally into a sample collection vial. These experiments were conducted under varying conditions such as: temperature, from 23 to 90 C, and solution pH, from 7 to 12. Results from these experiments show that as the temperature and solution pH increases the glass dissolution rate also increases. For example, the dissolution rate at 90 C was approximately 56 times higher at pH (23 C) 12.0, 5.66± 1.08 g m-2 d-1, compared to pH (23 C) 7.0, 0.016 ±0.003 g m-2 d-1. Performing a linear regression as a function of pH at each temperature resulted in a slope [power law coefficient (plc)] of 0.50 ±0.05 indicating that plc does not depend on temperature within experimental error. Temperature also affects the dissolution rate, evident by as much as a 14x increase in the rate with a 30 degree increase in temperature. Applying an Arrhenius expression to the data obtained at each pH suggested that the dominant mechanism of dissolution was a surface-controlled process, evident by an overall activation energy (Ea) of 60 ±6 kJ mol-1. Although translation and interpretation of these SPFT results to long-term glass dissolution under repository conditions requires further analysis, these results provide the rate-law parameters needed to conduct source-term release calculations using reactive transport models

[en] The long-term performance of solid radioactive waste is measured by the release rate of radionuclides into the environment, which depends on corrosion or weathering rates of the solid waste form. The reactions involved depend on the characteristics of the solid matrix containing the radioactive waste, the radionuclides of interest, and their interaction with surrounding geologic materials. This chapter describes thermo-hydro-mechanical and reactive transport models related to the long-term performance of solid radioactive waste forms, including metal, ceramic, glass, steam reformer and cement. Future trends involving Monte-Carlo simulations and coupled/multi-scale process modeling are also discussed.

[en] New insights from laboratory experiments coupled with field observations indicate that pore water solutions that eventually breach containment materials in disposal systems will interact with sodium-excess borosilicate waste glass in an unexpected way. Because many glass waste forms are relatively sodium-rich, they are especially vulnerable to Na⁺-H⁺ exchange (ion exchange or simply, IEX). Although the kinetics of this process has been previously investigated for early-stage glass reactions, the implications of IEX for long-term dissolution resistance have not yet been realized. Non-radioactive glass with major- and minor-element chemical compositions similar to Hanford high-Na waste glass were subjected to dissolution experiments to quantify the rates of matrix dissolution and IEX rates. Single-Pass Flow-Through (SPFT) tests quantified the IEX rate at 40 C pH = 8 and silica saturation and showed a dependence upon the fraction of excess sodium in the glass. The equation for the rate (in moles of sodium released per meter squared per second) dependence on excess sodium is: log10rate(mol/(m2 · s)) = 0.63R + (-11.0); r2 = 0.86 where R = molar Na⁺/(M3+). Further, rates of Na release are slower by (ge)30% in D2O-based solutions compared to those in H2O. These results are the hallmark of IEX reactions. Our results are compared against those from a lysimeter field experiment consisting of glasses buried in Hanford sand and to dissolution experiments conducted with a Pressurized Unsaturated Flow (PUF) apparatus. These longer-term tests indicate an initial decrease in dissolution rate by a factor of 10x, and then a constant steady-state rate thereafter. Thus, these data show that IEX reactions are important at near-saturation conditions and effectively prevent dissolution rates from falling below a minimum value. In sum, IEX modifies the long-term behavior of glass dissolution and models cannot assume that dissolution of Na-rich borosilicate glass will decrease by a factor of 100x to 1000x, as argued for minerals and less sodic glasses.

[en] Pacific Northwest National Laboratory was contracted by CHPRC to evaluate the release of 99Tc from spent resin used to treat water from well 299-W15-765 and stored for several years. The key questions to be answered are: (1) does 99Tc readily release from the spent ion exchange resin after being in storage for several years; (2) if hot water stripping is used to remove the co-contaminant carbon tetrachloride, will 99Tc that has been sequestered by the resin be released; and (3) can spent resin be encapsulated into a cementitious waste form; if so, how much 99Tc would be released from the weathering of the monolith waste form? The results from the long term stability leach test results confirm that the resin is not releasing a significant amount of the sequestered 99Tc, evident by the less than 0.02% of the total 99Tc loaded being identified in the solution. Furthermore, it is possible that the measured 99Tc concentration is the result of 99Tc contained in the pore spaces of the resin. In addition to these results, analyses conducted to examine the impact of hot water on the release of 99Tc suggest that only a small percentage of the total is being released. This suggest that hot water stripping to remove carbon tetrachloride will not have a significant affect on the resin's ability to hold-on to sequestered 99Tc. Finally, encapsulation of spent resin in a cementitious material may be a viable disposal option, but additional tests are needed to examine the extent of physical degradation caused by moisture loss and the effect this degradation process can have on the release of 99Tc.

[en] Single-pass flow-through experiments were conducted with aluminoborosilicate waste glasses to evaluate how changes in solution composition affect the dissolution rate (r) at 40 C and pH(23 C) = 9.0. The three prototypic low-activity waste (LAW) glasses; LAWE-1A, -95A, and -290A, used in these experiments span a wide range covering the expected processing composition of candidate immobilized low-activity waste (ILAW) glasses. Results suggest incongruent release of Al, B, Na, and Si at low flow-rate (q) to sample surface area (S) [log10 (q/S) < -8.9 (m/s)] whereas congruent release was observed at high q/S [log10 (q/S) > -7.9 (m/s)]. Forward dissolution rates, based on boron release, are the same irrespective of glass composition, evident by the dissolution rates being within the experimental error of one another [r1A = 0.0301 ± 0.0153 g/(m2 d), r95A = 0.0248 ± 0.0125 g/(m2 d), and r290A = 0.0389 ± 0.0197 g/(m2 d)]. Finally these results support the use of a chemical affinity based rate law to describe glass dissolution as a function of solution composition

[en] Human activities have altered trace metal distributions globally. This is especially true for the trace metal mercury (Hg), a pervasive global pollutant that can be methylated to form highly toxic methylmercury (MeHg), which bio-accumulates in aquatic food webs, endangering humans and other biota. Currently there are more than 3,000 mercury-contaminated sites identified worldwide and the United Nations Environment Programme has recently highlighted the risk of this contamination to human health. The Oak Ridge Reservation (ORR) represents an example of one of these mercury-contaminated sites. Unlike other contaminants-metals, radionuclides, and organic solvents-that impact the Department of Energy Office of Environmental Management (DOE-EM) cleanup program at the ORR and other DOE sites, mercury has several unique characteristics that make environmental remediation of the Y-12 National Security Complex one of the most formidable challenges ever encountered. These distinctive physicochemical properties for mercury include the following: it is a liquid at ambient temperature and pressure; it is the only metal that bio-magnifies; and it is the only contaminant transported as a cation, as a dissolved or gaseous elemental metal (similar to an organic solvent), or as both a cation and a dissolved or gaseous elemental metal under environmental conditions. Because of these complexities, implementing cost effective and sustainable solutions that reduce mercury flux from various primary and secondary contamination sources will require linking basic science understanding and applied research advancements into Oak Ridge Office of Environmental Management's (OREM) cleanup process. Currently, DOE is investing in mercury-related research through a variety of programs, including the Office of Science-sponsored Critical Interfaces Science Focus Area, EM headquarters-sponsored Applied Field Research Initiative, OREM-sponsored Lower East Fork Poplar Creek (LEFPC) Mercury Technology Development Program, Small Business Innovative Research (SBIR), and EM's Minority Serving Institutions Partnership Program. Collectively, these multi-institutional and multidisciplinary programs are generating new tools, knowledge, and remediation approaches that will enable efficient cleanup of mercury contaminated systems locally and globally. In this talk we will highlight the progress made to date in addressing key knowledge gaps required to solve this watershed-scale conundrum. (authors)

[en] This article tells in images and chunky captions the story of a study into the durability of four component glass. The information from this study is of interest to vitrification experts and geochemists.