Glass as a Waste Form and Vitrification Technology: Summary of an International Workshop (1996)

Based on workshop discussions, the committee concludes that glass is a demonstrated and appropriate waste form for the disposal of a large proportion of high-level nuclear waste in the U.S. Department of Energy (DOE) complex. However, for special waste streams or exceptionally long-lived radionuclides, pretreatment and/or alternative waste forms might need to be considered.

Long-term interim storage (up to 100 years) prior to final disposal is a common feature of many national programs. Therefore, there is time to develop and improve the understanding of present glass formulations and to develop new and better glasses.

There was agreement among the workshop participants that phenomenological models broadly describe glass behavior in repository environments. However, the models are not sufficiently developed to allow the extrapolation of the behavior of glass over long periods of time. The present models are useful for:

1. planning future experiments,

2. interpreting experimental data,

3. interpolating data,

4. formulating future research and development needs, and

5. providing a framework for technical discussions.

Improved models can reduce the uncertainty in the source term used in performance assessment. Present models suggest that under certain conditions the expected lifetime of the glass may be extended; however, there can be wide variations in the extrapolated behavior depending on the model used.

At present, there appears to be poor communication between those who conduct the performance assessments of waste form behavior and those who study glass corrosion. Future performance assessments need to reflect and take advantage of the improved knowledge and understanding of corrosion mechanisms of the glass. Total system performance assessments in present practice do not provide for an evaluation of the glass waste form as an independent barrier. Sensitivity analyses on each barrier of the multibarrier system would help identify the key parameters that affect the corrosion of glass as it is extrapolated by models over long periods of time.

At the workshop, participants noted that there appeared to be a very limited connection between the waste acceptance criteria (particularly product consistency) and the scientific basis for understanding the long-term behavior of the glass waste form. Demonstrable relationships between short-term leach tests and long-term behavior have yet to be established.

Because glass is generally accepted internationally as a waste form and in the United States is a ''qualified'' (by DOE) waste form, much can be gained by exploiting glass as a waste form for diverse applications (low-level waste, nonradioactive waste, etc.).

Vitrification of HLW is a major step in reducing the risks to the public of the tank wastes at Savannah River and West Valley. The Hanford tank wastes offer the opportunity to build on this success, as well as to improve the present technology and consider new approaches (e.g., new glass compositions, sintered glass, glass ceramics, crystalline ceramics, cold-wall crucible technology). A phased approach using multiple technologies is probably most appropriate to the complexity of the Hanford tank wastes.

A single technology will not solve every waste problem. The compositions of the wastes under DOE management (particularly the high-level waste at Hanford) are too complex or poorly defined to expect that a single vitrification technology or glass type will be an appropriate solution for all waste compositions. Three approaches may be envisioned: (1) more extensive pretreatment to develop waste streams of more narrow and defined compositions, (2) use of alternative glass compositions (including phosphate or aluminosilicate glasses), or (3) use of alternative waste forms (e.g., glass ceramics, crystalline ceramics, low-temperature hydroxylated ceramics).

The NRC workshop focused on vitrification technologies and glass as a form for radioactive waste. The steering committee made a conscious decision to exclude oral presentations, discussion, or comparisons of glass to other waste forms. However, it should not be assumed that there are no other technologies or waste forms. For some waste forms, considerable work has already been completed (e.g., Synroc, a titanate-based crystalline ceramic assemblage developed in Australia). Such waste forms may find special applications in the immobilization of long-lived radionuclides, such as excess plutonium from dismantled nuclear weapons.