Mechanisms and Kinetics of Organic Aging and Characterization of Intermediates in High Level Waste

The objective of this project is to characterize significant chemical degradation pathways of organic chemicals in stored high-level waste and waste treatment streams. The underlying chemistries of high-level waste are (1) the chemistry initiated by radioactive decay and the reactions initiated by heat from radioactive decay and (2) the chemistry resulting from waste management, and treatment activities (e.g., waste transfers, concentration through evaporators, caustic and other chemical additions, and separations operations). Recognizing that experiments cannot reproduce every conceivable scenario, the project works to develop predictive computational models of these chemistries. The effort is closely coordinated with a Notre Dame Radiation Laboratory project (EMSP No. 73832, ''The NOx System in Nuclear Waste'', D. Meisel, Principal Investigator) that focuses on radiolytically-induced degradation of organic complexants in waste simulants. Investigators combine experimental observations, electronic structure computations, and theoretical methods development to achieve this goal. The resulting models will facilitate accurate evaluation of the hazardous material generated, including flammable gases, and will support decision-making processes regarding safety, retrieval, treatment, and tank closure issues

Interfacial Radiolysis Effects in Tank Waste Speciation

The purpose of this project was to develop an understanding of radiolysis in systems relevant to nuclear wastes stored in tanks at DOE sites such as Hanford, Savannah River, and Idaho. Since these wastes are highly heterogeneous systems, determining the effects of particulate matter on the radiolytic yields was the main focus of interest. When translated to site-specific concerns, quantitative modeling of these processes can be developed once the fundamental phenomena are understood. Interpretation of the results and conclusions to specific issues, especially safety concerns, at the sites was a major goal. The results were transferred to site operators as soon as the experimental observations were confirmed to be reliable

NOx and Heterogeneity Effects in High Level Waste (HLW)

We summarize contributions from our EMSP supported research to several field operations of the Office of Environmental Management (EM). In particular we emphasize its impact on safety programs at the Hanford and other EM sites where storage, maintenance and handling of HLW is a major mission. In recent years we were engaged in coordinated efforts to understand the chemistry initiated by radiation in HLW. Three projects of the EMSP (''The NOx System in Nuclear Waste,'' ''Mechanisms and Kinetics of Organic Aging in High Level Nuclear Wastes, D. Camaioni--PI'' and ''Interfacial Radiolysis Effects in Tanks Waste, T. Orlando--PI'') were involved in that effort, which included a team at Argonne, later moved to the University of Notre Dame, and two teams at the Pacific Northwest National Laboratory. Much effort was invested in integrating the results of the scientific studies into the engineering operations via coordination meetings and participation in various stages of the resolution of some of the outstanding safety issues at the sites. However, in this Abstract we summarize the effort at Notre Dame

Mechanisms and Kinetics of Organic Aging in High-Level Nuclear Wastes

The goal of this project is to develop a fundamental understanding of organic aging and to assemble a model that describes and predicts the thermal and radiolytic aging of organic compounds in high-level wastes (HLW). To reach this goal, we will measure kinetics and elucidate products and mechanisms of organic reactions occurring under conditions of waste storage, retrieval, and processing. Initial emphasis will be placed on studying thermal effects, because organic reaction mechanisms and effects of varying conditions are uncertain, and because we benefit from collaborations with earlier Environmental Management Science Program (EMSP) projects that have worked on radiation effects. Organic complexants are of greatest concern regarding both safety and pretreatment because they have been found to degrade to gases, combust in dry wastes, and interfere with radionuclide separations. Therefore, efforts will focus on studying the reactions of these organic chemicals and associated degradation products. In preliminary work, the authors have used mechanistic kinetic modeling techniques to successfully model the radiolytic degradation of formate to carbonate in HLW simulants. The research will continue development of the model using an iterative process that measures degradation products and kinetics of increasingly complex molecules while adapting the model to reproduce the results each step of the way. Several mechanistic probe experiments have been designed to learn the fundamental mechanisms that operate during thermal degradations so that thermal and radiolytic processes may be integrated within the model. Key kinetic data and thermodynamic properties relating to thermal reactivity will also be acquired so that rate-controlling and product-forming reactions can be predicted. Thermochemical properties of key intermediates will be experimentally and/or theoretically determined to facilitate mechanism verification, structure/reactivity correlation, and prediction of reaction rates. We expect a comprehensive understanding of organic reactivity in HLW will accrue from the work. This understanding will be embodied in organic reaction models capable of predicting distributions of species, including gases, with respect to time, temperature, and radiation history. These models will assist waste management decisions by predicting impacts of organic chemicals on safe storage, waste retrieval, and pretreatment of HLW

The NOx System in Homogeneous and Heterogeneous Nuclear Waste

This report summarizes advances of the above-mentioned EMSP project during the period July 1, 2001-June 30, 2002. The project focuses on the effects of organic chemicals in stored nuclear waste and their impact on pretreatment and tank closure issues. Managing the tank wastes and site cleanup activities requires understanding of the chemistry of organics in aqueous basic solutions that contain high nitrate and nitrite concentrations under the field of g and b irradiation. The goal of this EMSP project is to quantitatively characterize the important chemical processes that are induced by radiolysis of the organic complexants, and their degradation products, during the storage in tanks. Furthermore, concerns that arise from pretreatment and tank closure issues, e.g., Tc speciation, can be rationalized with the mechanistic knowledge provided here. Recognizing that experiments cannot reproduce every conceivable realistic scenario, the project emphasizes computational methodologies . None of the Hanford tanks is currently on a watch list, partially due to the predictive understanding of the tanks' chemistry that was developed by these projects and their predecessor activities. The project is closely coupled to another EMSP project (81883) and exchange of information between the two related studies is frequent

Mechanisms and Kinetics of Organic Aging and Characterization of Intermediates in High Level Waste

The objective of this project is to characterize significant chemical degradation pathways of organic chemicals in stored high-level waste and waste treatment streams. Recognizing that experiments cannot reproduce every conceivable scenario, the project works to develop predictive computational models of these chemistries. The effort is closely coordinated with a Notre Dame Radiation Laboratory project (EMSP No. 73832, ''The NOx System in Nuclear Waste,'' D. Meisel, Principal Investigator) that focuses on radiolytically-induced degradation of organic complexants in waste simulants. Investigators combine experimental observations, electronic structure computations, and theoretical methods development to achieve this goal. The resulting models facilitate accurate evaluation of hazardous conditions and support decision-making processes regarding safety, retrieval, treatment, and tank closure issues

The NOx System in Homogeneous and Heterogeneous Nuclear Waste

This project focuses on the development of reliable theoretical and efficient computational descriptions of the chemistry of high-level waste simulants in highly concentrated solution and the experimental validation of this approach. It is an extension of previous work which: (a) Determined the mechanism of H{sub 2} formation in waste simulants allowing quantitative prediction of H{sub 2} generation in nuclear waste storage tanks; (b) Predicted and confirmed NO{sub 2} as a key oxidant in the tanks (c) Determined relative rates of reaction of NO{sub 2} with chelators thus helping to resolve the ''Organic Tanks'' safety issue; (d) Measured rates of NO{sub 2} and NO{sub 2}{sup -} with some model organic radicals; (e) Quantified the redox potentials of two major reductants, NO{sub 2}{sup -} and NO{sub 3}{sup -}; (f) Discovered a new radiolytic route to the reductant NO{sub 3}{sup 2-} and demonstrated its potential persistence in irradiated alkaline aqueous media; (g) Demonstrated that while radiolytic electrons escape from solid suspended silica particles to disrupt the surroundings, holes remain trapped thus storing oxidative equivalents