Disposal criticality analysis for immobilized plutonium: Internal configurations

The analysis for immobilized Pu follows the disposal criticality analysis methodology. In this study the focus is on determining the range of chemical compositions of the configurations which can occur following the aqueous degradation processes, particularly with respect to the concentrations of uranium, plutonium, and the principal neutron absorber, gadolinium. The principal analysis tool is a mass balance program that computes the amounts of plutonium, uranium, gadolinium, and chromium in solution as a function of time with inputs from a range of possible waste form dissolution rates, stainless steel corrosion rates, and compound solubilities for the neutronically significant elements. For the waste forms and degradation modes considered here, it is possible to preclude the possibility of criticality by maintaining a plutonium loading limit. Since the presence of hafnium is shown to increase this loading limit, the defense-in-depth policy would suggest the maximization of the amount of Hf as a backup criticality control material. At the end of 1997, after this study was completed, the ceramic waste form was downselected and a new formulation was developed, with the amount of Hf increased to the point where internal criticality may no longer be possible. In addition, recent calculations indicate that GdPO{sub 4} is insoluble over a much broader range of pH than is Gd{sub 2}O{sub 3}, so that its use as the Gd carrier in the waste form would provide an extra margin of defense-in-depth