PNNL

Abstract

ABSTRACT Single-pass flow-through experiments were conducted on a set of non-radioactive TiO2-based ceramics at 90°C and pH = 2 to 12. The specimens contained 27.9 to 35.8 mass% CeO2 as a surrogate for UO2 and PuO2. Pyrochlore and Hf-rutile and pyrochlore, zirconolite, and Hf-rutile constituted the major phases in the “pure phase” pyrochlore and pyrochlore-baseline ceramics, respectively. Results from dissolution experiments between pH = 2 to 12 indicate a shallow pH-dependence with an ill-defined minimum. Element release rates determined from experiments over a range of sample surface areas (S) and flow rates (q) indicate that dissolution rates become independent of q/S values at 10-8 to 10-7 m•s-1. Dissolution rates dropped sharply at lower values of q/S, indicating rates that are subject to solution saturation effects as dissolved constituents become concentrated. Forward dissolution rates were 1.25•10-3 (±0.299) and 5.54•10-3 (±1.26) (esd) g/(m2•d) for “pure phase” pyrochlore and pyrochlore-baseline ceramics, respectively. Dissolution rates obtained in other laboratories compare well to the findings of this study, once the rate data are placed in the context of solution saturation state. These results make progress towards an evaluation of CeO2 as a surrogate for UO2 and PuO2 as well as establishing a baseline for comparison with radiation-damaged specimens.