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 includes both mixed low-level wastes (MLLW) and mixed transuranic (MTRU) wastes.¹ Site cleanup and decommissioning activities during the next several years are expected to nearly double this inventory, and 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 close former DOE production sites (DOE, 1998c).

Within EM, the Office of Science and Technology (OST, EM-50) is charged with assuring that safe, cost-effective technologies are available for the entire closure program. To address mixed waste technology needs, OST established a special program and management team, the Mixed Waste Characterization, Treatment, and Disposal Focus Area (MWFA), in 1994. At the request of OST, the National Research Council (NRC) convened a Committee on Mixed Waste³ to assess specific technical issues being addressed by the MWFA. A review of all OST technology development activities was completed in 1995 (NRC, 1996c).

For the present task, the mixed waste committee was requested to review and evaluate the state of development of the final forms for disposal of mixed wastes as they arise from current and emerging treatment technologies.⁴ The review was also to identify the technology development options DOE might consider in order to achieve waste forms that are cost-effective and safe for disposal.⁵

In carrying out the review, the committee received formal presentations from DOE staff and other individuals, and examined documents and data provided by DOE and other sources. Some committee members visited waste contractors and DOE sites to gather additional information. The committee assessed the state of development of waste forms within the context of DOE's site closure program, technical approaches, and constraints (DOE, 1998c). The following subjects are discussed and commented upon in this report:

• the current and expected inventory of DOE mixed waste;
• laws and regulations that control mixed waste management;

• technologies for treating mixed wastes and producing the waste forms;
• tests to characterize the chemical and physical durability of waste forms; and
• methods to demonstrate the long-term performance of the disposal system.

The committee's general finding is that currently available waste forms are adequate (sufficiently developed) to meet regulatory requirements for disposal of DOE's known and expected mixed waste inventory. The general classes of waste forms that are available to the MWFA include grout, glass, ceramics, polymers, and compacted waste. Many of these waste forms have resulted from the intensive worldwide efforts and experience in developing waste forms for high- and low-level radioactive waste. No single form is appropriate for all wastes, but collectively the variety of available waste forms and well-established waste form production technologies make it unlikely that any totally new class of waste forms will be necessary to complete EM's planned cleanup program.

There are a number of caveats to this finding:

1. Analysis by DOE concludes that nearly all of EM MLLW can be safely disposed at Hanford, Washington and Envirocare in Utah due to their dry climates. However, it is likely that political and economic considerations will establish a need for other disposal facilities. Uncertainties regarding where these facilities will be located and future waste acceptance criteria introduce significant risk in judging the adequacy of EM's planned mixed waste treatment and stabilization processes.
2. Optimization of existing technologies to allow higher concentrations of waste in the product waste forms and provide less expensive production methods is possible, and potential cost savings justify continued effort toward process optimization. In particular, fabricating waste forms that contain higher concentrations of ash and salt would reduce the volume of waste to be disposed and the concomitant disposal costs. Methods to stabilize mercury need optimization.

3. There are no generally accepted tests for evaluating the integrity of any waste form over the required 1,000- to 10,000-year time frame. Consequently, performance assessments take little or no credit beyond a few hundred years for the waste form, and rely on other features of the overall disposal system to demonstrate compliance with long-term release requirements.⁶

1. Waste form development should no longer be a primary focus of MWFA. Future research and development should emphasize integration, demonstration, and optimization of performance and reliability of mixed waste treatment processes, cost reduction, and provision of a more quantitative description of EM's mixed waste inventory.
2. MWFA should continue its practice of defining, identifying, and responding to technology deficiencies. The MWFA has established a rational and systematic program that identifies and prioritizes deficiencies. The committee compliments MWFA on this effort and encourages continued updates of the Technical Baseline Report that documents the state of its technology development activities.
3. MWFA should broaden its use of a systems approach in its efforts to assist EM in its closure program. This approach includes determination of the characteristics of the raw waste, definition of the required performance of a proposed technology, and design of the technology to attain the required performance. An important aspect of a reliable system design is flexibility to accommodate new information, experience, and reasonable changes in the performance requirements.
4. EM should work with the U.S. Environmental Protection Agency and the U.S. Nuclear Regulatory Commission to agree on clear guidelines that describe acceptable waste forms for disposal of mixed waste in future, near-surface disposal facilities. This should be done as soon as possible to reduce the risk that EM will deploy technologies that are later judged to be inadequate.