B. P. McGrail

Near-Field Performance Assessment for a Low-Activity Waste Glass Disposal System: Laboratory Testing to Modeling Results

Abstract Reactive chemical transport simulations of glass corrosion and radionuclide release from a low-activity waste (LAW) disposal system were conducted out to times in excess of 20 000 yr with the subsurface transport over reactive multiphases (STORM) code. Time and spatial dependence of glass corrosion rate, secondary phase formation, pH, and radionuclide concentration were evaluated. The results show low release rates overall for the LAW glasses such that performance objectives for the site will be met by a factor of 20 or more. Parameterization of the computer model was accomplished by combining direct laboratory measurements, literature data (principally thermodynamic data), and parameter estimation methods.

Radiation Effects in Crystalline Oxide Host Phases for the Immobilization of Actinides

Radiation effects from alpha-decay events in crystalline oxides, which are proposed for the immobilization of actinides, often lead to amorphization, macroscopic swelling and order-of-magnitude increases in dissolution rates for all of the phases currently under consideration. However, the results of systematic experimental studies using short-lived actinides and ion-beam irradiations, studies of radiation effects in U- and Th-bearing minerals, and the development of new models of the damage process over the past 20 years have lead to a substantial increase in the understanding of the processes of damage accumulation in apatite, zircon, perovskite, zirconolite, and pyrochlore/fluorite structures. This fundamental scientific understanding now provides a basis for the selection of nuclear waste forms based on their predicted performance in a radiation field. One of the recent successes of these studies has been the discovery of a class of radiation-resistant pyrochlore/fluorite structures that can serve as highly durable, radiation-resistant host phases for the immobilization of actinides.

Forsterite [Mg2SiO4)] Carbonation in Wet Supercritical CO2: An in Situ High-Pressure X-ray Diffraction Study

Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg(2)SiO(4)) exposed to variably wet supercritical CO(2) (scCO(2)). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO(2) containing dissolved water. Under modest pressures (90 bar) and temperatures (50 °C), scCO(2) saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO(3) · 3H(2)O), and magnesite (MgCO(3)) after 72 h. However, comparable tests with scCO(2) at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO(3)). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen ((18)O), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO(2).

Single-pass flow-through experiments on a simulated waste glass in alkaline media at 40°C.: I. Experiments conducted at variable solution flow rate to glass surface area ratio

Dissolution of a borosilicate glass containing a complex mixture of simulated fission product oxides has been investigated in single-pass flow-through (SPFT) experiments in moderately alkaline media at 40°C. The experiments were designed such that the ratio of the solution flow rate to the glass surface area varied extensively over the experimental matrix. Dissolution of the glass is surface reaction-controlled under these experimental conditions. In Si-rich, alkaline solutions, the concentration of Al is moderated to very low levels by the development of secondary aluminosilicate gels. Rates of dissolution decrease linearly as the solution activity product increases. A rate law containing a mixed Al/Si affinity term gives good agreement with the experimental data.

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.

Numerical Simulation of Methane Hydrate Production from Geologic Formations via Carbon Dioxide Injection

Scientific and technological innovations are needed to realize effective production of natural gas hydrates. Whereas global estimates of natural gas hydrate reservoirs are vast, accumulations vary greatly in nature and form. Suboceanic deposits vary from disperse concentrations residing at low saturations in the pore space of unconsolidated sediments with sand-sized particles to higher concentrations residing in the fractures of sediments with clay-sized particles. Conventional methods for gas hydrate production include depressurization, thermal stimulation, and inhibitor injection. For suboceanic accumulations in sandy sediments, depressurization has been shown, through numerical simulation, to be the most feasible production technology. However, recovery efficiencies are too low to justify pursuing these energy reservoirs. Under high pressure, low temperature suboceanic conditions the hydrate structure can accommodate small molecules other than methane (CH4), such as carbon dioxide (CO2) and nitrogen (N2) in both the small and large cages. Although CO2 and N2 clathrates generally are not naturally as abundant as those of CH4, their occurrence forms the foundation of an unconventional approach for producing natural gas hydrates that involves the exchange of CO2 with CH4 in the hydrate structure. This unconventional concept has several distinct benefits over the conventional methods: 1) the heat of formation of CO2 hydrate is greater than the heat of dissociation of CH4 hydrate, providing a low-grade heat source to support additional methane hydrate dissociation, 2) exchanging CO2 with CH4 will maintain the mechanical stability of the geologic formation, and 3) the process is environmentally friendly, providing a sequestration mechanism for the injected CO2. An operational mode of the STOMP simulator has been developed at the Pacific Northwest National Laboratory that solves the coupled flow and transport equations for the mixed CH4-CO2 hydrate system under nonisothermal conditions, with the option for considering NaCl as an inhibitor in the pore water. This paper describes the numerical simulator, its formulation, assumptions, and solution approach and demonstrates, via numerical simulation, the production of gas hydrates from permafrost accumulations in sandstone formations with high gas hydrate saturations and suboceanic accumulations in sandy sediments with low hydrate saturations using the CO2-CH4 exchange technology. Introduction Gas hydrates are clathrate compounds in which water molecules encapsulate a guest molecule within a lattice structure. The lattice structure of gas hydrates form under low temperature, high pressure conditions via hydrogen bonding between water molecules. Gas hydrates with methane (CH4) guest molecules are abundant as geologic accumulations in offshore and permafrost environments where sufficiently low temperature and high pressure conditions exist. From an energy resource perspective, these geologic accumulations of natural gas hydrates represent a significant component of the world’s organic carbon sources. Recent surveys by the United States Geological Survey (USGS) have estimated that reserves of methane in hydrate form exceed the all other fossil fuel forms of organic carbon (Booth et al., 1996). Under geologic environmental conditions, the lattice structure of a gas hydrate depends primarily on the guest molecule (Englezos, 1993; and Sloan, 1998). Interestingly, the two most prevalent emitted greenhouse gases (U.S. EPA, 2006) carbon dioxide (CO2) and methane (CH4) both form sI hydrate structures under geologic temperature and pressure conditions. Whereas their clathrate structures are similar, CO2 hydrates form at higher temperatures and have a higher enthalpy of formation compared with CH4 hydrates (Sloan, 1998). Natural gas can be produced from geologic accumulations of natural gas hydrates either by dissociating the clathrate structure, yielding liquid water and gaseous methane, or by replacing the CH4 molecule with another guest. Conventional

Single-pass flow-through experiments on a simulated waste glass in alkaline media at 40°C.: II. Experiments conducted with buffer solutions containing controlled quantities of Si and Al

Abstract Additional single-pass flow-through (SPFT) experiments have been conducted with a complex, simulated waste glass at 40°C in moderately alkaline media. Results are compared to those obtained in the experiments described in Part I, and with published data obtained in long-term, static, batch dissolution experiments with this glass formulation. Dissolution rate laws for the glass must account for the rate influencing effects of both dissolved Si and Al species. These experiments have shown that on a mole per mole basis, dissolved Al has a more significant influence on the glass dissolution rate than dissolved Si under these experimental conditions. The very low ‘long-term’ dissolution rates reported in static batch dissolution experiments reflect near saturation conditions that are not attained in SPFT tests as a result of solution flow-through.

High-Level Nuclear-Waste Borosilicate Glass: A Compendium of Characteristics

With the imminent startup, in the United States, of facilities for vitrification of high-level nuclear waste, a document has been prepared that compiles the scientific basis for understanding the alteration of the waste glass products under the range of service conditions to which they may be exposed during storage, transportation, and eventual geologic disposal. A summary of selected parts of the content of this document is provided. Waste glass alterations in a geologic repository may include corrosion of the glass network due to groundwater and/or water vapor contact. Experimental testing results are described and interpreted in terms of the underlying chemical reactions and physical processes involved. The status of mechanistic modeling, which can be used for long-term predictions, is described and the remaining uncertainties associated with long-term simulations are summarized.

Waste Form Release Calculations for the 2001 Immobilized Low-Activity Waste Performance Assessment

A set of reactive chemical transport calculations was conducted with the Subsurface Trans-port Over Reactive Multiphases (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. 1-D simulations were conducted out to times in excess of 20,000 y. A 2-D simulation was run to 2,000 y. The maximum normalized, decay-corrected Tc release rate from a trench type conceptual design under a constant recharge rate of 4.2 mm/y is 0.76 ppm/y. Factors that were found to significantly impact the predicted release rate were water recharge rate, chemical affinity control of glass dissolution rate, diffusion coefficient, and disposal system design (trench versus a concrete-lined vault). In contrast, corrosion of the steel pour canister surrounding the glass waste, and incorporation of chemical conditioning layer of silica sand at the top of the trench had little impact on Tc release rate.

Waste package component interactions with Savannah River defense waste glass in a low-magnesium salt brine

Interactive leaching experiments were performed with Savannah River 165 defense waste glass at 90/sup 0/C in a low-magnesium salt brine (approx. =100 mg/iota) with various amounts of steel present to simulate interactions between the metal container and the glass. Synergistic interactions occurred between the container material and the glass, which increased the glass dissolution rate. Measured quasi-steady-state solution concentrations of /sup 239/Pu, /sup 237/Np, and /sup 243/Am were reasonably consistent with published solubilities for the respective oxides. Decreasing /sup 238/U concentrations were observed with increasing solution pH, inconsistent with the behavior of uranyl carbonates that presumable predominate at the slightly alkaline pH of the tests. Uranium and plutonium formed pseudocolloids with hematite particles that rapidly formed during the tests. Particle size measurements showed the median size to be >5..mu..m in diameter.