Oxidative dissolution of Cr-doped UO₂ nuclear fuel

Alternative UO₂ nuclear fuels, incorporating Cr as a dopant, are currently in use in light-water reactors. Dissolution experiments using Cr-doped UO₂, performed as a function of Cr content in a simplified groundwater solution and under oxic conditions, established that the addition of Cr to the UO₂ matrix systematically reduced the normalised dissolution rate of U at 25 and 40 °C. This effect was most notable under dilute solution conditions, and is the result of galvanic coupling between Cr and U, resulting from the presence of Cr²⁺ in the UO₂ matrix, as corroborated by activation energy determination. Under conditions of solution saturation, where schoepite ((UO₂)₈O₂(OH)₁₂·(H₂O)₁₂) and Na₂U₂O₇·6H₂O were identified as secondary phases, the rate of U dissolution was invariant with Cr content. Moreover, at 60 °C, the trend was reversed and the rate of U dissolution increased with increasing Cr content. Under these conditions, other factors, including U solubility or bicarbonate-surface interactions, exert a stronger influence on the U dissolution kinetics than Cr. Increased grain size, a feature of Cr-doped UO₂ fuel, was also found to reduce the normalised dissolution rate of U. In establishing the mechanisms by which Cr dopants influence UO₂ fuel dissolution, it can be concluded that, overall, Cr-doped UO₂ nuclear fuel possesses similar dissolution kinetics to undoped UO₂ fuel, giving confidence for its eventual disposal in a geological facility.