Development of a ceramic form for immobilization of excess plutonium

Between 8 and 50 metric tonnes of excess plutonium are currently planned to be immobilized in a glass or ceramic waste form in the US. The immobilized Pu would then be encased in HLW glass (the can-in-canister alternative), which would provide a radiation barrier to enhance the proliferation resistance of the material. Associated with the plutonium are about 15 metric tonnes of uranium primarily {sup 238}U and a variety of other impurities (primarily Ga, Mo, Al, Mg, Si, and Cl) totaling about 1 metric tonne or less. Immobilization of this material is complicated by the fact that the uranium content in the various feed streams varies widely, from 0 to about 95%. The proposed ceramic form is composed of about 90% zirconolite (CaZrTi{sub 2}O{sub 7}) and/or pyrochlore (CaPuTi{sub 2}O{sub 7}) with about 10% other phases, typically hollandite (BaAl{sub 2}Ti{sub 6}O{sub 16}) and rutile (TiO{sub 2}). The form is a variation of Synroc-C, which contains nominally 30% zirconolite, 30% perovskite, 30% hollandite, and 10% rutile and noble metal alloys. Zirconolite and perovskite are the actinide host phases in Synroc-C with zirconolite being the more durable phase. The pyrochlore structure is closely related to zirconolite and forms at higher actinide loadings. Thus, this mineral is of interest for plutonium deposition in ceramic. Pyrochlore has the advantage that it is cubic rather the monoclinic like zirconolite. Cubic mineral swell isotropically when radiation damaged. As a result, differential strain in the microstructure will be minimal, leading to significantly less microcracking of the form after thousands of years in a repository. Zirconolites and pyrochlores containing uranium and.or thorium exist in nature and have demonstrated actinide immobilizations for periods exceeding 100 million years