United States and most other countries. While the plutonium disposition mission is different in some important respects, it appears desirable (to minimize costs, delays, difficulties of gaining approvals, and the like) for the plutonium disposition mission to make use of existing processes, approaches, and facilities to the extent practical, a logic that focuses attention on the borosilicate glasses already scheduled to be produced. (A brief discussion of a few of the alternative waste forms that have been proposed for this mission is also provided.) As discussed in this chapter, plutonium could also be incorporated in glass without fission products, but we do not believe this would provide a large enough barrier to reuse in weapons to be satisfactory as a final disposition option.

Much engineering development work has been done over several decades on vitrification of radioactive materials, both in the United States and in other countries. Based on this work, the general features of the technology are well known: several vitrification facilities have been built and operated around the world using different glasses, different radioactive species, and different throughputs. While glasses containing substantial quantities of plutonium have never been produced on a large scale, the key engineering parameters that would govern a large-scale WPu vitrification operation are believed to be understood. In this sense, the technical feasibility of vitrifying WPu has been adequately demonstrated.

Several important technical issues must be resolved before vitrification of WPu could move from a technical possibility to an operational reality, however. Some of these issues stem from the fact that plutonium has never been vitrified on a large scale before. In addition, further work is required to determine the best mix of plutonium, glass, and fission products for this purpose. The principal objective of vitrifying WPu is to deter its potential reextraction for weapons use. To make this deterrent most effective, it would be desirable to have: (1) small amounts of plutonium in each "log" of glass; (2) large and heavy logs that would be difficult to steal; and (3) large quantities of fission products and other contaminants in the logs to make reextraction difficult. But to minimize cost, take maximum advantage of existing vitrification programs, and meet other criteria, there may also be reasons to increase the amount of plutonium in each log, decrease the amount of fission products in each log, or make the logs smaller. Hence the selection of how much WPu to put in logs of what size, with what composition of other contaminants, requires a systematic exploration of the parameter space, taking into account engineering, handling, and cost aspects. For WPu vitrification, such a comprehensive evaluation has not yet been done.

Some of the technical issues associated with this approach are discussed below. In general the technical uncertainties associated with this approach are somewhat greater than are those surrounding the use of mixed-oxide fuel (MOX) in light-water reactors (LWRs). Nevertheless, the panel believes that WPu vitrification represents a feasible technology that could meet the "spent fuel standard," could be available in the relatively near future (within about a

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