types are under construction, although there appear to be potential problems with flat bottom crucibles because of the possibility of sludge buildup. However, there is no compelling reason to believe that this wide variation in approaches to vitrification is bad or that it is likely to lead to a glass waste form that is unacceptable for the multiple barriers application. In fact, the Russian experience with phosphate-based glasses suggests that this role may still require several different glass formulations.

The glass waste form may be seen as a means for immobilization of liquid waste for short-term storage and transportation to a repository. Under this future, solid-state immobilization enhances handling and transportation but is not of consequence for disposal. This places a minimum requirement on waste form performance in the repository.

Such an assumption may be made in safety assessments, particularly for longer periods of time, but in this case one must rely on other technological barriers emplaced in the near field and, above all, on the geological barrier (i.e., the far field) to play the dominant role in isolating nuclear waste. However, it is almost impossible to improve by engineering means the far-field capability of a repository's geological barrier to prevent transport and dispersion of long-lived radionuclides. Therefore, however low the probability that the nuclear waste glass will actually play a role as a barrier to long-term release of radionuclides, some would argue that the disposal concept (with its system of multiple barriers) must be developed as if the glass will actually play no role as a barrier to the long-term release of radionuclides.

The present level of knowledge of glass properties and performance appears adequate to meet this role.

If no credit is to be taken for the waste form for long-term containment, the primary constraints on vitrification of the high-level waste are those related to the costs and operation of the vitrification plant. Waste glass formulations may be chosen to simplify plant operation, reduce capital costs of construction and operation, and ensure ease and cost effectiveness for the dismantling and disposal of the vitrification plant. However, research may have important impacts on the technology, such as in the development of longer-lived melters and improved operating conditions.

Front Matter (R1-R10)

Executive Summary (1-2)

1 Introduction (3-7)

2 Three Futures for Glass (8-18)

3 Special Applications of Glass (19-19)

4 Conclusions (20-21)

References (22-24)

Appendix A Biographical Sketches of Committee Members (25-26)

Appendix B Workshop Program (27-30)

Appendix C List of Workshop Participants (31-38)

Appendix D Identification and Summary of Characterization of Materials Potentially Requiring Vitrification (39-52)

Appendix E Abstracts of Invited Workshop and Poster Presentations (53-162)