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.

Treatment of radioactive wastes: an X-ray absorption spectroscopy study of the reaction of technetium with green rust.

Technetium is a long-lived product of nuclear fission that readily forms the soluble pertechnetate anion [TcO(4)](-). Green rusts (layered hydrous oxides containing both Fe(II) and Fe(III) and with interlayer sulfate or carbonate anions) concentrate >99.8% of 99Tc, present as [TcO(4)](-), from aqueous solution, even in the presence of high concentrations of NaNO(3), a common constituent of radioactive waste streams. The mechanism of removal from solution is apparently reduction and formation of strong Tc(IV) surface complexes. X-ray absorption spectroscopy shows that [TcO(4)](-) is indeed reduced by reaction with both sulfate- and carbonate-form green rusts and is found in a TcO(2)-like environment. On contact with air, the green rusts oxidize to poorly crystalline goethite but the Tc environment is unchanged. There is no increase in Tc solubility associated with oxidation of the host green rust. This behavior suggests that green rusts may be useful in the treatment of Tc-containing waste streams, in groundwater cleanup, and in restricting Tc migration from repositories.