Influence of Microstructure and Aqueous Chemistry on the Dissolution Behavior of CeO2 Under Repository-Relevant Conditions

Cerium dioxide (CeO2) is used as a non-radioactive analog for selected studies of fluorite-type nuclear fuel materials. Ce release from commercial CeO2 powder and sol–gel-derived CeO2 pellets was investigated in batch experiments conducted for approximately 100 days under an argon atmosphere with an oxygen concentration below 1 ppm. Experiments were performed in ultrapure water, a Volvic aqueous solution containing approximately 1 mM bicarbonate, and a synthetic bicarbonate-rich solution containing 20 mM NaHCO3 and 10 mM NaCl. The solids were characterized by X-ray diffraction, scanning electron microscopy, gas adsorption and laser diffraction. The powder had a BET surface area of 5.93 m2·g−1, whereas the pellets had a BET surface area of approximately 0.70 m2·g−1 and a relative density of 80.6 ± 3.5% of the theoretical density. Ce concentrations increased during the initial stage of the experiments and changed more slowly thereafter. The powder released substantially more Ce than the pellets in ultrapure water, whereas this difference was reduced or reversed in the bicarbonate-containing media. Across aqueous media with increasing bicarbonate concentrations, accompanied by changes in pH and ionic strength, Ce release did not increase proportionally with bicarbonate concentration. Differences between filtered and ultrafiltered samples indicated an operationally defined ultrafiltration-retained fraction, but direct evidence for colloid formation was not obtained. BET-normalized apparent release rates indicated that the measured surface area alone could not account for the differences between powder and pellets. PHREEQC calculations indicated undersaturation with respect to the Ce-bearing phases considered, subject to the assumptions inherent in the thermodynamic database and the imposed redox conditions. The results support the use of CeO2 for investigating selected microstructural and interfacial processes in tetravalent fluorite-type oxides, while highlighting the limitations of extrapolating these results to redox-sensitive UO2 alteration.