Publications

Abstract not provided.

Abstract not provided.

Decommissioning high level nuclear waste tanks will leave small amounts of residual sludge clinging to the walls and floor of the structures. The permissible amount of material left in the tanks depends on the radionuclide release characteristics of the sludge. At present, no systematic process exists for assessing how much of the remaining inventory will migrate, and which radioisotopes will remain relatively fixed. Working with actual sludges is both dangerous and prohibitively expensive. Consequently, methods were developed for preparing sludge simulants and doping them with nonradioactive surrogates for several radionuclides and RCRA metals of concern in actual sludges. The phase chemistry of these mixes was found to be a reasonable match for the main phases in actual sludges. Preliminary surrogate release characteristics for these sludges were assessed by lowering the ionic strength and pH of the sludges in the manner that would occur if normal groundwater gained access to a decommissioned tank. Most of the Se, Cs and Tc in the sludges will be released into the first pulse of groundwater passing through the sludge. A significant fraction of the other surrogates will be retained indefinitely by the sludges. This prolonged sequestration results from a combination coprecipitated and sorbed into or onto relatively insoluble phases such as apatite, hydrous oxides of Fe, Al, Bi and rare earth oxides and phosphates. The coprecipitated fraction cannot be released until the host phase dissolves or recrystallizes. The sorbed fraction can be released by ion exchange processes as the pore fluid chemistry changes. However, these releases can be predicted based on a knowledge of the fluid composition and the surface chemistry of the solids. In this regard, the behavior of the hydrous iron oxide component of most sludges will probably play a dominant role for many cationic radionuclides while the hydrous aluminum oxides may be more important in governing anion releases.

The US Department of Energy (DOE) has millions of gallons of high level nuclear waste stored in underground tanks at Hanford, Washington and Savannah River, South Carolina. These tanks will eventually be emptied and decommissioned. This will leave a residue of sludge adhering to the interior tank surfaces that may contaminate groundwaters with radionuclides and RCRA metals. Experimentation on such sludges is both dangerous and prohibitively expensive so there is a great advantage to developing artificial sludges. The US DOE Environmental Management Science Program (EMSP) has funded a program to investigate the feasibility of developing such materials. The following text reports on the success of this program, and suggests that much of the radioisotope inventory left in a tank will not move out into the surrounding environment. Ultimately, such studies may play a significant role in developing safe and cost effective tank closure strategies.

Abstract not provided.

Radionuclide transport in soils and groundwaters is routinely calculated in performance assessment (PA) codes using simplified conceptual models for radionuclide sorption, such as the K{sub D} approach for linear and reversible sorption. Model inaccuracies are typically addressed by adding layers of conservativeness (e.g., very low K{sub D}'s), and often result in failed transport predictions or substantial increases in site cleanup costs. Realistic assessments of radionuclide transport over a wide range of environmental conditions can proceed only from accurate, mechanistic models of the sorption process. They have focused on the sorption mechanisms and partition coefficients for Cs{sup +}, Sr{sup 2+} and Ba{sup 2+} (analogue for Ra{sup 2+}) onto iron oxides and clay minerals using an integrated approach that includes computer simulations, sorption/desorption measurements, and synchrotron analyses of metal sorbed substrates under geochemically realistic conditions. Sorption of Ba{sup 2+} and Sr{sup 2+} onto smectite is strong, pH-independent, and fully reversible, suggesting that cation exchange at the interlayer basal sites controls the sorption process. Sr{sup 2+} sorbs weakly onto geothite and quartz, and is pH-dependent. Sr{sup 2+} sorption onto a mixture of smectite and goethite, however, is pH- and concentration dependent. The adsorption capacity of montmorillonite is higher than that of goethite, which may be attributed to the high specific surface area and reaction site density of clays. The presence of goethite also appears to control the extent of metal desorption. In-situ, extended X-ray absorption fine structure (EXAFS) spectroscopic measurements for montmorillonite and goethite show that the first shell of adsorbed Ba{sup 2+} is coordinated by 6 oxygens. The second adsorption shell, however, varies with the mineral surface coverage of adsorbed Ba{sup 2+} and the mineral substrate. This suggests that Ba{sup 2+} adsorption on mineral surfaces involves more than one mechanism and that the stability of sorbed complexes will be affected by substrate composition. Molecular modeling of Ba{sup 2+} sorption on goethite and Cs{sup +} sorption on kaolinite surfaces were performed using molecular dynamics techniques with improved Lennard-Jones interatomic potentials under periodic boundary conditions. Ba{sup 2+} was observed to have a preference for inner sphere sorption onto goethite, with the (101) and (110) surfaces representing the controlling sorption surfaces for bulk K{sub D} measurements. Large-scale simulations of Cs{sup +} sorption on kaolinite (1000's of atoms) provide a statistical basis for the theoretical evaluation and prediction of Cs{sup +} K{sub D} values. Results suggest the formation of a strong inner sphere complex for Cs{sup +} on the kaolinite edge surfaces and a weakly bound outer sphere complex on the hydroxyl basal surface.

For optical fibers used in adverse environments, a carbon coating is frequently deposited on the fiber surface to prevent water and hydrogen ingression that lead respectively to strength degradation through fatigue and hydrogen-induced attenuation. The deposition of a hermetic carbon coating onto an optical fiber during the draw process holds a particular challenge when thermally-cured specialty coatings are subsequently applied because of the slower drawing rate. In this paper, we report on our efforts to improve the low-speed carbon deposition process by altering the composition and concentration of hydrocarbon precursor gases. The resulting carbon layers have been analyzed for electrical resistance, Raman spectra, coating thickness, and surface roughness, then compared to strength data and dynamic fatigue behavior.

A better understanding of the fraction of contaminants irreversibly sorbed by minerals is necessary to effectively quantify bioavailability. Ferrihydrite, a poorly crystalline iron oxide, is a natural sink for sorbed contaminants. Contaminants may be sorbed/occluded as ferrihydrite precipitates in natural waters or as it ages and transforms to more crystalline iron oxides such as goethite or hematite. Laboratory studies indicate that Cd, Co, Cr, Cu, Ni, Np, Pb, Sr, U, and Zn are irreversibly sorbed to some extent during the aging and transformation of synthetic ferrihydrite. Barium, Ra and Sr are known to sorb on ferrihydrite in the pH range of 6 to 10 and sorb more strongly at pH values above its zero point of charge (pH> 8). We will review recent literature on metal retardation, including our laboratory and modeling investigation of Ba (as an analogue for Ra) and Sr adsorption/resorption, during ferrihydrite transformation to more crystalline iron oxides. Four ferrihydrite suspensions were aged at pH 12 and 50 °C with or without Ba in 0.01 M KN03 for 68 h or in 0.17 M KN03 for 3424 h. Two ferrihydrite suspensions were aged with and without Sr at pH 8 in 0.1 M KN03 at 70°C. Barium or Sr sorption, or resorption, was measured by periodically centrifuging suspension subsamples, filtering, and analyzing the filtrate for Ba or Sr. Solid subsamples were extracted with 0.2 M ammonium oxalate (pH 3 in the dark) and with 6 M HCl to determine the Fe and Ba or Sr attributed to ferrihydrite (or adsorbed on the goethite/hematite stiace) and the total Fe and Ba or Sr content, respectively. Barium or Sr occluded in goethite/hematite was determined by the difference between the total Ba or Sr and the oxalate extractable Ba or Sr. The percent transformation of ferrihydrite to goethite/hematite was estimated from the ratio of oxalate and HC1 extractable Fe. All Ba was retained in the precipitates for at least 20 h. Resorption of Ba reached a maximum of 7 to 8% of the Ba2+ added for samples aged in 0.01 and 0.17 M KN03 after 68 and 90 h of aging, respectively. About 3% of the Ba2+ added was readsorbed from 90 to 3424 h of aging in 0.17 M KN03. The amount of Ba sorbed by ferrihydrite or adsorbed on goethite (oxalate-extractable) decreased from 70 to 40% of the Ba2+ added after 68 h in 0.01 M KNO3 and from 80 to 20% of the Ba2+ added after 400 h in 0.17 M KN03. The Ba occluded in goethite (HCl-extractable) in 0.01 M KN03 increased rapidly to 30% of the Ba2+ added in the first 0.4 h and then to 50% of the Ba2+ added after 68 h. In 0.17 M KN03, Ba occluded in goethite increased from 60% of the Ba2+ added by 68 h and to 75% of the Ba2+ added after 3424 h. After 68 h at 70°C, ferrihydrite transformation was 99% compIete and was slightly inhibited with Sr present during the first few hours. Occlusion of Sr in ferrihydrite or Sr reversibly adsorbed decreased from 96 to 4o/0 after 86 h. Occlusion of Sr in hematite/goethite increased from 4 to 40% after 68 h. Resorption of Sr increased from 0.2 to 50% after 68 h. At least 90% of the Ba and 25% of the Sr added to the ferrihydrite suspensions were retained by the iron oxides during the aging periods in this study. At least 75% of the Ba and 15% of the Sr were irreversibly sorbed during ferrihydrite transfomnation to goethite and/or hematite.