Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures

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 synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure 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) was investigated during dissolution. The effects of surface polishing on dissolution rate were 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

Dissolution of ThO2: study of dissolution process with initial 229Th spike

The sparingly soluble ThO2 dissolution behaviour was studied under conditions that are expected to prevail in bedrock after the closure of the spent nuclear fuel disposal facility. The objective was to investigate the characteristics of initial dissolution of crystalline 232ThO2 by adding 229Th tracer to the aqueous phase in the beginning of the 534 days experiment.The evolution of 232Th concentration and 229Th/232Th ratio were followed by sector field ICP-MS (inductively coupled plasma mass spectrometer). Selected samples were measured also by alpha spectrometry to compare the results from the used analysis techniques and they gave comparable results. In the early stage of dissolution there was no significant control by chemical equilibrium and the dissolution process seemed to be controlled by the stability of surfaces. </p