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Glass as a Waste Form and Vitrification Technology: Summary of an International Workshop Executive Summary The National Research Council's Board on Radioactive Waste Management convened an International Workshop on Glass as a Waste Form and Vitrification Technology in Washington, D.C., on May 13-15, 1995, to review the current state of knowledge of glass as a waste form for immobilization of radioactive wastes. This report summarizes some of the key issues and findings arising from the presentations and discussions at the workshop. One of the important contexts in which glass technology must be evaluated is that for waste disposal within a permanent underground repository. Although the workshop was not organized to address the repository environment, the behavior of glass on geological time scales is highlighted. Glass is a demonstrated and appropriate waste form for the disposal of a large proportion of the liquid high-level nuclear waste in the U.S. Department of Energy (DOE) defense waste complex (see abstract by S. P. Cowan in Appendix E). However, the very large volume of radioactive wastes, including hundreds of thousands of cubic meters of high-level waste, and their chemical complexity provide a daunting challenge to the technologies required for vitrification of these wastes and to the science of waste form design. Additionally, new glass compositions and/or alternative waste forms may be required for special waste stream compositions or for the containment of exceptionally long-lived radionuclides. The difficulty in assessing the status and needs of future research and development for glass as a waste form is that there may be very different, even conflicting, criteria imposed by (1) industrial-scale technologies required to process often large volumes of highly radioactive material, (2) geological disposal strategies that may place a wide range of requirements on the glass (e.g., the glass may be viewed only as a medium for transportation and short-term storage rather than as a long-term barrier to radionuclide release from the repository), and (3) regulatory requirements that will, but have not yet, set standards in the United States for repository materials. Successful geological containment does not necessarily require a durable glass, but, conversely, a failure of the geological containment will require a durable waste form. Short-term tests of product consistency may have little relation to long-term durability. To distinguish among the technology, scientific, and regulatory issues, this summary report identifies three ''futures'' for glass as a waste form: (1) glass as the only barrier to long-term release of radionuclides, (2) glass as an effective but not primary barrier to long-term release of radionuclides, and (3) glass as an ineffective barrier to long-term release and used primarily for transportation and short-term storage. For each future, key scientific and technological issues should be addressed. In reality the purpose of the glass waste form is not found in any single "future." There are always multiple purposes. Detailed observations and conclusions based on an analysis and summary of the workshop presentations and discussions are made by the steering committee within the context of the three envisioned futures. Several general conclusions are the following: There is broad agreement on a phenomenological model that describes glass behavior in a repository environment; however, the models are not sufficiently developed to allow extrapolation of the behavior of glass over long periods of time. Waste form performance assessments for the U.S. waste repository are generally made with little regard to scientific understanding of performance and properties of the glass waste form. Performance assessments must be designed to take advantage of the improved
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Glass as a Waste Form and Vitrification Technology: Summary of an International Workshop knowledge and understanding of the corrosion of waste form glasses that result from future research and development. In the United States there is almost no connection between waste acceptance criteria (essentially a determination of product consistency) and the scientific basis for understanding the long-term behavior of glass waste forms. There is considerable worldwide experience in the vitrification of high level waste, providing a strong basis for proceeding with vitrification technologies; however, a single technology will not solve every waste problem. The properties of any solid (glass, crystalline, or glass-ceramic) depend, to a considerable extent, on composition as well as on the solid's thermal and processing history. The diverse waste streams found in the DOE complex may require (a) extensive pretreatment to develop more narrowly defined compositions for vitrification, (b) the development of alternative waste glass compositions, and/or (c) the development of alternative waste forms. In summary, the present knowledge of glass properties, particularly corrosion behavior, is considerable, and industrial-scale experience with the vitrification of radioactive waste is impressive. This provides a firm basis for present DOE programs. However, based on what its members heard at the workshop, the steering committee believes that continued research and development to improve waste form performance, as discussed in the body of this report, may lead to substantially reduced risks to the public, reduced costs to the taxpayers, and reduced uncertainty in the regulatory analysis of compliance.
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