Claire L. Corkhill, et al., report that the release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2‑x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration.
Upon dissolution in nitric acid medium at 90°C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2‑x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce3+ influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.
We present an investigation of the role of energetically reactive surface sites on the initial dissolution rate of CeO2 to ascertain whether these features may result in overestimation of dissolution rates in the laboratory. Reactive surface sites and “sharp edges” were simulated by crushing samples to different size fractions and through high temperature annealing. Oxygen vacancy defects in CeO2‑x were generated by high temperature annealing in oxygen-free conditions. Surface area was used as a
measurement of “sharp edges” and intrinsic defects, and the total Ce3+ fraction measured by XPS was used as a measurement of the oxygen vacancy concentration. The relative dissolution rates of these materials showed that there was a quantifiable relationship between energetically reactive surface sites and dissolution rate, which was dependent upon reactive surface site concentration and the solubilities of Ce3+ and Ce4+; the greater the concentration of reactive surface sites, the higher the dissolution rate. The elimination of oxygen vacancies in CeO2‑x was found to significantly influence the dissolution rate; this was associated with changes in the lattice volume and
lattice strain, manifested as grain boundary decohesion and increased surface area. Thus, it is clear that reactive surface sites, including artifacts of sample preparation, intrinsic material defects, and grain boundaries, have the potential to significantly enhance the dissolution of spent oxide fuel. We show that such effects can be quantified, thus it is possible to reduce the uncertainty associated with laboratory measurements of UO2 dissolution, albeit in a simplified analogue system. It will be essential to develop further careful studies designed to interrogate the effect of reactive surface features on the durability and dissolution of materials such as UO2 and MOx, as distinct from solubility, surface area, and radiation effects.
Original working URL: https://pubs.acs.org/doi/pdfplus/10.1021/acsami.5b11323
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