Contribution of energetically reactive surface features to the dissolution of CeO2 and ThO2  analogues for spent nuclear fuel microstructures

Corkhill, Claire L. and Myllykyla, Emmi and Bailey, Daniel J. and Thornber, Stephanie M. and Qi, Jiahui and Maldonado, Pablo and Stennett, Martin C. and Hamilton, Andrea and Hyatt, Neil C. (2014) Contribution of energetically reactive surface features to the dissolution of CeO2 and ThO2  analogues for spent nuclear fuel microstructures. ACS Applied Materials and Interfaces, 6 (15). 12279–12289. ISSN 1944-8244

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    In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthesised CeO2 and ThO2, spent nuclear fuel analogues which approximate as closely as possible the mineral structure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) were investigated during dissolution. The effects of surface polishing on dissolution rate was also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called "instant release fraction" of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude.