The effects of hydrogen and temperature on the electrochemistry and corrosion of uranium dioxide.

摘要

The research in this thesis describes a study on the influence of hydrogen on the electrochemistry and corrosion of uranium dioxide (UO2) inside a failed nuclear waste container under the conditions anticipated in a deep geological repository.;Since studies involving H2 were conducted at 60°C, it was necessary to characterize the influence of temperature on UO2 behaviour in order to interpret the results. The anodic dissolution of U IVO2 was studied at 60°C in 0.1 mol·L -1 KCl using a range of electrochemical methods and X-ray photoelectron spectroscopy (XPS). The results were compared to results previously obtained at 22°C. This comparison shows that the threshold for the onset of anodic dissolution (-400 mV vs. SCE) is not noticeably changed by this increase in temperature. However, both the oxidation of the surface (to UIV/VO 2+x) and the rate of anodic dissolution (as UVIO 22+) leading to the formation of a UVIO 3·yH2O deposit are accelerated at the higher temperature. The XPS analysis shows that the conversion of UV to U VI occurs at lower potentials at 60°C. Consequently, once the surface becomes blocked by the presence of a UVIO3·yH 2O deposit, rapid dissolution, coupled to uranyl ion hydrolysis, causes the development of locally acidified sites within the fuel surface at lower potentials at 60°C than observed at 22°C.;The influences of oxic (O2-purged), anoxic (Ar-purged) and potentially reducing (5% H2/95% Ar-purged) conditions on the corrosion of UO2 (nuclear fuel) have been studied on SIMFUEL specimens in 0.1 mol·L-1 KCl (pH ∼ 9.5) solutions at 60°C using corrosion potential measurements and XPS. A number of SIMFUEL specimens, doped to simulate various degrees of in-reactor burn-up, were used. The doping yielded specimens containing REIII (rare-earth) ions at U IV lattice sites within the UO2 matrix and noble metal (epsilon) particles interspersed throughout the solid. Under oxic and anoxic conditions, the corrosion potential and surface composition did not vary significantly with the degree of simulated burn-up. However, when H2 was present both the corrosion potential and the extent of surface oxidation decreased as the degree of simulated burn-up increased. This was attributed to the increased number of noble metal particles on which H2 oxidation is possible. Since these particles are galvanically-coupled to the rare-earth doped UO 2 matrix, H2 oxidation suppresses the corrosion potential of the matrix, thereby preventing its oxidation.;Keywords. Uranium dioxide, SIMFUEL, hydrogen, corrosion potential, nuclear waste disposal, hydrogen peroxide, epsilon particles, X-ray photoelectron spectroscopy. (Abstract shortened by UMI.);A number of electrochemical experiments were employed to investigate the effects of hydrogen on UO2 corrosion. A combination of corrosion potential (ECORR) measurements and cyclic voltammetry indicated that dissolved hydrogen can polarize the UO2 surface to reducing potentials; i.e., to ECORR values more negative than those observed under anoxic (argon-purged) conditions. A comparison of the behaviours of SIMFUEL specimens with and without incorporated noble metal epsilon-particles indicates that these particles act as catalytic electrodes for H2 oxidation, H2 ↔ 2e- + 2H+. It is the galvanic coupling of these particles to the rare earth-doped UO 2 matrix which suppresses the fuel corrosion potential.