EQ6 Calculations for Chemical Degradation of Pu-Ceramic Waste Packages

In this study, the long-term geochemical behavior of waste package (WP), containing Pu-ceramic, was modeled. The ceramic under consideration contains Ti, U, Pu, Gd and Hf in a pyrochlore structure; the Gd and Hf stabilize the mineral structure, but are also intended to provide criticality control. The specific study objectives were to determine:
1) the extent to which criticality control material, suggested for this WP design, will remain in the WP after corrosion/dissolution of the initial package configuration (such that it can be effective in preventing criticality), and
2) the extent to which fissile plutonium and uranium will be carried out of the degraded WP by infiltrating water (such that internal criticality is no longer possible, but the possibility of external criticality may be enhanced), and
3) the nominal chemical composition for the criticality evaluations of the WP design, and to suggest the range of parametric variations for additional evaluations.
For this purpose, the chemical compositions (and consequent criticality evaluations) are modeled for time periods up to 106 years. This longer time frame is consistent with the one million years time horizon recently recommended by the National Academy of Sciences to the Environmental Protection Agency for performance assessment related to a nuclear repository (Ref. 1).
The calculation included elements with high neutron-absorption cross-sections, notably Gd and Hf, as well as the fissile materials. In the calculations, the thermodynamic/geochemical behavior of Hf was represented by Zr; this substitution was necessary, due to the lack of thermodynamic data for Hf, and is justified by the extreme chemical similarity of the two elements (Assumption 3.16). The results of this analysis will be used to ensure that the type and amount of criticality control material used in the WP design will prevent criticality.