The electrochemistry of SIMFUEL in dilute alkaline hydrogen peroxide solutions

The work described in this thesis is a study of the electrochemistry of SIMFUEL (SIMulated nuclear FUEL) in dilute, alkaline hydrogen peroxide solutions.;In the second series of experiments, the electrochemical reduction of hydrogen peroxide on SIMFUEL was studied using the steady-state polarization technique. Kinetic parameters for the reaction, such as Tafel slopes and reaction orders, were determined. The results were interpreted in terms of a chemical-electrochemical mechanism involving UIV/UV donor-acceptor reduction sites. The large values of the Tafel slopes and the fractional reaction orders with respect to H2O2 can be understood in terms of the potential-dependent surface coverage of active sites, similar to that observed in the reduction of hydrogen peroxide on oxidized copper surfaces. The effects of pH over the range 10-13 were also investigated. The H2O 2 reduction currents were nearly independent of pH in the range 10-11, but were slowed at more alkaline values. The change in pH dependence appears to be related to the acid-base properties of the H2O2 molecule.;The next set of experiments focused on the effects of dissolved bicarbonate and carbonate ions on the rates and mechanism of H2O2 reduction on SIMFUEL. The reduction kinetics were found to be influenced by the dissolved carbonate ions. The formation of hydrated UVI species on the electrode surface is avoided in carbonate solutions, allowing H2O2 reduction to proceed at more positive potentials than in carbonate-free solutions. At more negative potentials, the adsorption of carbonate ions on the active reduction sites inhibits the H2O 2 reduction reaction. Over a narrow potential region, the reduction of peroxide is catalyzed by coadsorption of H2O2 and HCO3-/CO32-. The pH dependence of the H2O2 reduction reaction is more complex in carbonate solutions than in solutions that do not contain carbonate. This can be attributed to the displacement of inhibiting CO32-/HCO 3- adsorbed ions by OH-. Under natural corrosion conditions, steady-state corrosion potentials were found to be very similar to those measured in carbonate-free electrolytes. XPS evidence shows that the electrode surface contains a much lower UVI contribution at high corrosion potentials, confirming that the formation of hydrated U VI species is prevented.;The influence of carbonate on the anodic dissolution mechanism was also examined in a limited number of experiments. The onset of dissolution occurs at more negative potentials as the carbonate concentration is increased, indicating an acceleration of the SIMFUEL dissolution reaction. EIS evidence suggests that adsorbed carbonate and bicarbonate species are involved in the dissolution mechanism.;In the first set of experiments, the reaction of H2O 2 on SIMFUEL electrodes was studied electrochemically and under open circuit conditions in 0.1 mol L-1 NaCl solutions at pH 9.8. The composition of the oxidized UO2 surface was determined by X-ray photoelectron spectroscopy. Hydrogen peroxide reduction was found to be catalyzed by the formation of a mixed UIV/UV (UO 2+x) surface layer, but to be blocked by the accumulation of UVI species (UO3· yH2O or adsorbed (UO2)2+) on the electrode surface. The formation of this UVI layer blocks both H2O2 reduction and oxidation, thereby inhibiting the potentially rapid H2O2 decomposition reaction to H2O and O2. Decomposition is found to proceed at a rate controlled by the desorption of the adsorbed (UO2)2+ or reduction of adsorbed O2 species. Reduction of (O2) ads is coupled to the slow oxidative dissolution of UO2 and formation of a corrosion product deposit of UO3· yH2O.;Lastly, the influence of the dopant elements in SIMFUEL (which chemically simulate fission products in spent fuel) on the rate of hydrogen peroxide reduction was studied. Using steady-state polarization, the reaction rate was found to increase with the doping level of the SIMFUEL, under transport-compensated conditions. The noble metal ϵ-particles in SIMFUELs appear to be responsible for this enhancement of the H2O2 reduction currents.;Keywords. Uranium dioxide, hydrogen peroxide, reduction mechanism, corrosion potentials, electrocatalysis, active sites, metal oxide, nuclear waste disposal.