Solubility of thorium from Th3UO8 and ZrTh3UO10 ceramics

The solubility of thorium from Th3UO8 and ZrTh3UO10 ceramics was studied to evaluate the solubility constants for the materials. Zirconium was added in order to investigate potential additional stabilization of the thorium matrix from ZrO2. Studies were performed from pH 4 to pH 9 under both 100% Argon and 10% CO2/90% Ar at 25degreesC in 0.1M NaClO4. Reaction kinetics and equilibrium concentrations were evaluated for each condition. Rate constants increased with pH and varied from 6 x 10(-4) hr(-1) to 4 x 10(-2) hr(-1). The surface area normalized dissolution rate decreased with increasing pH, the highest being 5 x 10(-7) gm(-2) d(-1) at pH 4. The solubility product constants calculated from the equilibrium concentrations for the two ceramics were equivalent, with K-spTh3UO8 = -48.3+/-0.4 and K-spZrTh3UO8 = -47.9+/-0.9. The data was used to calculate the Th solution phase concentration. The result was compared to the dissolution of UO2 from pH 5-9 in 0.1 M NaClO4 under a 1% CO2 atmosphere at 25 degreesC. It was found that the U from the UO2 had a higher solution phase concentration than Th at high pH and was highly dependent upon the solution Eh of the evaluated conditions.