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--> 1 Introduction More than 167,000 cubic meters of mixed waste, waste that contains both chemically hazardous and radioactive components, are in the known inventory at U.S. Department of Energy (DOE) sites that formerly produced nuclear defense materials. The inventory contains low-level radioactive waste mixed with hazardous materials, referred to as mixed low-level waste (MLLW), and transuranic waste mixed with hazardous materials, referred to as mixed transuranic (MTRU) waste.1 Site cleanup and decommissioning activities during the next several years are expected to nearly double this inventory. The inventory will be further increased by mixed wastes retrieved as a result of DOE site remediation.2 Processing and permanent disposal of these mixed wastes is a part of the DOE Office of Environmental Management (EM) program to 1 Hazardous wastes are defined by the Resource Conservation and Recovery Act of 1976. Radioactive materials are defined by the Atomic Energy Act of 1954. Only MLLW and MTRU are dealt with in this report. Mixed high-level radioactive wastes are not included because their radiation hazard requires an infrastructure for their regulation, treatment, and disposal that is not applicable to MLLW and MTRU. Mixed wastes from uranium mining and milling are also excluded. 2 Eighteen DOE sites include waste disposal areas that must be remediated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980. CERCLA is often referred to as the Superfund act.
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--> close former DOE production sites. Within EM, the Office of Science and Technology (OST, EM-50) is charged with assuring that technologies are available to support the EM cleanup program (DOE, 1998c). OST's Mixed Waste Focus Area (MWFA) is responsible for identifying and developing technologies for processing EM's mixed wastes. The final, solid form of the waste that results from waste processing and is intended for disposal is referred to as the waste form. The waste form is a key part of an integrated waste management system. Mixed waste management and disposal are complex technical and regulatory challenges. It is a technical challenge because of both the quantity and wide variety of mixed wastes within EM's responsibility. Examples range from small amounts of laboratory wastes that contain many chemicals and relatively high-levels of radioactivity, to large volumes of soil and debris that contain only slightly contaminated material. Treatment technologies, therefore, must deal with a broad range of chemical and physical properties and volumes. Treatment objectives are generally to reduce the chemical hazards of a given waste material and to render it into a stable waste form. The long-term durability of the waste form must be assured to make it acceptable for disposal. Regulatory complexities arise because regulations have been developed by two different regulatory agencies that have taken two different approaches to protecting the environment. Regulations apply to handling, storing, transporting, and treating the waste, to characteristics of the waste form, and to the disposal facility. The U.S. Nuclear Regulatory Commission has authority to license low-level waste disposal sites. The U.S. Environmental Protection Agency has authority to regulate hazardous waste, which includes discarded materials that are corrosive, ignitable, reactive, or contain toxic compounds and metals.3 This report is the result of a one-year review of the status of waste forms for the disposal of mixed wastes under the responsibility of EM. The review was performed by the Committee on Mixed Waste of the National Research Council's Board on Radioactive Waste Management (BRWM) at the request of OST. In its statement of task, the committee 3 Although the word "metals" is used for convenience throughout this report, it is recognized that the chemical forms of these metals that exist in actual wastes include oxides, salts, and complexes. The zero valent state is not to be assumed.
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--> was requested to review and evaluate the state of development of the final forms of treated waste; to assess the characteristics (and their uncertainties) of the waste form for disposal; and identify requirements for additional research and development.4 The committee responded to the statement of task by, first of all, putting its evaluation of the state of development of waste forms into the context of EM's mixed waste inventory and the regulations that control treatment and disposal of this waste. This context is provided in Chapters 2 and 3. Technologies to treat the waste inventory and produce waste forms are described in Chapter 4. Characterization of waste forms is described in Chapter 5, and Chapter 6 describes the performance assessment (PA) methodology to evaluate the long-term safety of disposal. The committee's findings and recommendations in Chapter 7 identify needs for additional research and development. The results of this study are intended to help OST consider options in technology development to achieve waste forms that are cost-effective and safe for disposal. In carrying out the review, the committee heard formal presentations from DOE staff and other individuals and examined reports and data provided by DOE or other sources. Some committee members visited waste contractors and DOE sites to gather additional information. Because waste forms are part of an integrated waste management system, the committee evaluated their state of development in the context of EM's cleanup program and current regulatory requirements. The information considered by the committee can be divided into the following categories: scope of the mixed waste problem, including the waste inventory and characteristics; the MWFA approach and activities, including technology development and application; performance of selected waste forms in laboratory testing, as well as evaluation of specific waste forms at potential disposal sites; and regulations and waste acceptance criteria. 4 The Statement of Task is in Appendix A.
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--> A synopsis of the information presented to the committee is given in Appendix B. History of the MWFA and the Mixed Waste Committee The DOE established its Office of Environmental Management (EM) in November 1989. EM's mission was to reduce threats to health and safety posed by contamination and waste at DOE sites (DOE, 1998c). Within EM, OST5 was created to promote the development of new and improved technologies needed to effect remediation, to lower cleanup costs, and to reduce risks. In 1994, OST reorganized its work with the creation of a new management structure to focus its efforts on EM's most urgent environmental restoration and waste management problems. The following focus areas were established: high-level radioactive waste tank remediation, mixed waste characterization, treatment, and disposal, contaminant plume containment and remediation, landfill stabilization,6 and facility transitioning, decommissioning, and final disposition.7 In 1994, as an adjunct to this new structure, the DOE Assistant Secretary for EM requested the BRWM to form a Committee on Environmental Management Technologies (CEMT) to provide independent reviews of OST programs and to give recommendations on technology development and use. As part of this effort, a Subcommittee on Mixed Waste Characterization, Treatment, and Disposal was formed under the CEMT. The mixed waste subcommittee, as well as the other 5 OST was previously named the Office of Technology Development. 6 Focus areas 3 and 4 were combined and renamed Subsurface Contaminants. 7 First renamed Decontamination and Decommissioning, and more recently Deactivation and Decommissioning (DOE, 1998d).
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--> CEMT subcommittees, were reorganized as independent committees under the BRWM in 1997. A CEMT report that summarized findings from all five subcommittees was published in 1996 (NRC, 1996c). Role of the Waste Form in DOE Mixed Waste Management Mixed waste can be disposed of either in the form in which it originally was generated or, in most cases, after treatment to reduce volume and render the waste suitable for disposal. Knowledge of the inventory and characteristics of the waste itself is necessary to determine the subsequent steps for its treatment and disposal. For mixed wastes within the responsibility of EM, treatments are being identified and developed by the MWFA that are designed to reduce waste volume, destroy hazardous organic materials, remove or stabilize toxic chemicals, and produce waste forms that meet disposal criteria (DOE, 1997a, 1996a). A waste form is considered to be a solid material that is the product of one or more treatment processes. During interim storage and after disposal, the waste form should constitute an important barrier against dispersion of hazardous and radioactive components. For disposal of waste in a near-surface facility,8 protection of the environment can be achieved by a complete disposal system that includes the following in its design: the waste form; additional engineered barriers such as synthetic and clay liners that are intended to retard migration of contaminants from the repository; site geology, which affects the contaminant migration beyond the engineered barriers; ground water movement, which affects the time and concentration at which contaminants become a concern; and 8 Near-surface disposal is a standard practice for hazardous or low-level radioactive waste. The specially designed and constructed facility is typically located at least 3 m, but no more than about 30 m, below the surface of the surrounding land, and usually above the water table.
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--> climatic conditions. The waste form should not be considered a separate entity; rather, it is an integral part of the entire treatment and disposal system. The primary function of a waste form is to retain the hazardous and radioactive constituents of the waste, or to retard their release.9 In addition, the physical stability of the waste form facilitates its handling, storage, and transportation. For disposal in a near-surface facility, the mechanical strength and stability of the waste form will also help to prevent subsidence and assist in maintaining the integrity of the cover. In case of inadvertent intrusion into the repository, a stable, high-integrity waste form can limit potential exposure by discouraging excavation or drilling, and by reducing the dispersal of the waste if the repository is breached (Berry, 1994). On the other hand, waste forms that degrade rapidly through mechanical, chemical, biochemical, radiological, or other mechanisms may result in untimely release of contaminants and in some cases may actually facilitate migration of contaminants from the disposal facility. For example, poor quality waste forms can contribute to subsidence or cover failure which, in turn, increases the potential for release of contaminants into ground or surface water. To determine whether a waste form is appropriate for disposal, waste form performance criteria must be identified. Compliance with the criteria can be demonstrated by characterization of the chemical and physical properties of the waste form, and testing its performance under laboratory and realistic disposal conditions. Reliable tests and testing protocols must be established by technology developers and regulators to provide a basis for waste form development and to judge if a proposed waste form is acceptable for a given waste and disposal condition. 9 The useful life expectancy of presently available waste forms ranges from a few hundred to several thousand years. The waste form and engineered barriers can be expected to effectively retain radionuclides with half-lives of up to 30 years, e.g., 90Sr and 137Cs. In instances where the waste form and engineered barriers may degrade more rapidly than the contaminants in the waste, the entire disposal system is relied upon. Methods to measure durability are described in Chapter 5 .
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--> Mathematical models, known as performance assessment models, are used to describe the performance of the integrated waste disposal system over very long time periods. These PA models quantify the importance of individual components as well as the multiple components of the repository system including the stabilized waste form, natural and engineered containment systems associated with the disposal facility, and the disposal site geology to assure future protection of humans and the environment. In reviewing and evaluating the state of development of waste forms, this report will present information gathered by the mixed waste committee and the committee's findings and recommendations in the context of mixed waste management. This includes the characteristics and inventory of EM's mixed wastes, regulatory controls, technology availability, characterization of waste forms, and use of PA models.
Representative terms from entire chapter: