Doctor of Philosophy
Shoesmith, David W.
Noël, James J.
The safety assessment of spent nuclear fuel under permanent disposal conditions requires examination of the corrosion of the spent fuel waste form (UO2) inside a failed waste container. The objective of this research project was to develop a detailed mechanism of the UO2 corrosion process when exposed to groundwater. The redox conditions within a failed container in a deep geologic repository will be complex. The oxidant, H2O2, produced by the α-radiolysis of groundwater, will be the main driving force for fuel corrosion. However, the efficiency of fuel dissolution will be determined by the competition between UO2 corrosion and H2O2 decomposition to the much less reactive O2. As a consequence, the corrosion of the UO2 will be determined by the relative importance of 3 reactions, the anodic oxidation of UO2 and H2O2 both of which will be coupled to the cathodic reduction of H2O2 under corrosion conditions.
The relative importance of the two anodic reactions was studied electrochemically on SIMFUEL (simulated spent fuel) in HCO3-/CO32- solutions. It was found that both reactions were suppressed by the formation of UVI surface films at low HCO3-/CO32- concentrations. When the formation of these films was prevented at higher HCO3-/CO32- concentrations both reactions occurred readily on the sublayer of UIV1-2xUV2xO2+x. At high potentials H2O2 was directly oxidized on the noble metal (ε) particles in the SIMFUEL which were rendered catalytic by preoxidation (e.g., Pd to PdII).
The reduction of H2O2 has been studied on a range of UO2 electrodes such as RE(III)-doped and non-stoichiometric (UO2+x) electrodes and SIMFUEL. It was found that reduction on a UO2 surface proceeded through a two-step reaction sequence, the chemical oxidation of UIV to UV followed by the electrochemical reduction of the surface back to UIV. The rate of H2O2 reduction decreased in the order UO2.002 ~ UO2.5 ~ SIMFUEL > Gd-UO2 ~ Dy-UO2 > UO2.1. The low reduction rate on RE(III)-doped electrodes was attributed to the stabilized UO2 matrix by the formation of RE(III)-OV clusters. The reduction rate may be catalyzed by ɛ-particles in SIMFUEL electrodes.
The coupling of these anodic and cathodic reactions was also studied under corrosion conditions. H2O2 was found to decompose to O2 and H2Oboth homogeneously and heterogeneously accompanied by a minimal amount of UO2 corrosion. Homogeneous decomposition proceeded via a peroxycarbonate (CO42-) intermediate while heterogeneous decomposition was catalyzed by the reversible redox transformation in a thin surface layer. The rate of the heterogeneous decomposition reaction depended on whether UVI surface species were allowed to accumulate on the surface blocking access of H2O2 to the catalytic surface layer.
A series of computational analyses were performed using a model previously developed to describe fuel corrosion inside a failed container. The influences on fuel corrosion of fuel defect geometry, ɛ-particle distribution and H2O2 decomposition on UO2 corrosion rate were investigated. The defect geometries, in the form of pores and fractures, was found to exert only a minor influence on the rate of fuel corrosion rate. Similarly, changes in the number of ε-particles exerted only a minimal effect. Decomposition of H2O2 caused a significant decrease in fuel corrosion rate since the slowly reacting O2 was dominantly lost by transport out of the defects.
Zhu, Ziyan, "The Electrochemical Study of Simulated Spent Nuclear Fuel (SIMFUEL) Corrosion under Permanent Disposal Conditions" (2018). Electronic Thesis and Dissertation Repository. 5787.