New Mexico Mineral Symposium — Abstracts

Mineralogy has had only a limited impact on present nuclear waste disposal strategies. This is due to the emphasis in performance and safety assessments on "geologic isolation" or the near-field engineered barriers (e.g., canister and backfill), rather than any significant reliance on "immobilization" or "containment" by durable materials, specifically designed for particular waste streams and defined geologic environments. However, highly durable ceramics can find unique applications in the immobilization and disposal of long-lived actinides, particularly the ²³⁹Pu that results from the dismantling of nuclear materials. Radionuclides that are geochemically mobile, toxic, and fissile require special materials of high chemical and mechanical durability.

I will review the areas in which mineralogy can contribute to the long-term isolation of actinide elements. The presentation will specifically discuss the considerations that are required for the design of materials for the immobilization of different waste streams. Critical to the design of the waste form are: (1) waste stream composition, (2) processing technology, (3) long-term chemical and mechanical durability, (4) stability of alteration products, (5) radiation-solid interactions. Principal minerals to be considered include: silicates (zircon and titanite); phosphates (monazite and apatite); oxides (zirconolite, pyrochlore, and perovskite); and the UO₂ in spent nuclear fuel. Minerals can be used to evaluate long-term durability.

The immobilization of excess weapons plutonium in zircon and monazite will be discussed as a specific example of a solution for the disposal of excess weapons plutonium. A powerful aspect of this approach is the use of minerals to "confirm" projections of long-term behavior in a variety of geochemical environments.