FUNDAMENTAL THERMODYNAMICS OF ACTINIDE-BEARING MINERAL WASTE FORMS

Abstract

In September 2000, the US and Russia reached an agreement to jointly disposition roughly 68 metric tons of weapons usable plutonium. (Agreement 2000) In Russia, 34 metric tonnes of weapons-grade plutonium will be dispositioned by burning the plutonium as mixed oxide (MOX) fuel. In the US, 25 metric tons of plutonium recovered from pits and clean metal will likewise be dispositioned by burning as mixed oxide (MOX) fuel and about 9 metric tonnes of plutonium stored throughout the DOE complex will be dispositioned by immobilization in a ceramic which will then be encapsulated in high-level waste (HLW) glass. In all cases, the plutonium will be made equal to or less attractive for reuse in nuclear weapons than the much larger and growing inventory of plutonium in spent nuclear fuel. This threshold of unattractiveness is commonly referred to as the ''spent fuel standard.'' In the US, the final products from plutonium disposition, irradiated fuel and ceramic encapsulated HLW, will be emplaced in the Federal Waste Repository, which is assumed to be Yucca Mountain. The ceramic form selected for the disposition of plutonium is composed of a series of titanate-based phases which are generally referred to as SYNROC (short for Synthetic Rock). The particular formulation that was selected is composed of about 80 vol % pyrochlore, about 15 vol % brannerite, and about 5 vol % rutile. If impurities are present in the PuO{sub 2} feed material, about a half a dozen other phases can also form. The most common of these are zirconolite and a silicate glass. A screening process conducted in 1995, resulted in the selection of borosilicate glasses and titanate-based ceramics (e.g. SYNROCs) as the best available options for immobilization of plutonium. In 1998, a pyrochlore-rich ceramic form was selected in preference to a boro-silicate glass form. More information about the development and selection of the ceramic formulation can be found in the Plutonium Immobilization Program's Baseline Formulation report.