The Electrochemical Study of Simulated Spent Nuclear Fuel (SIMFUEL) Corrosion under Permanent Disposal Conditions

Author

Ziyan Zhu, The University of Western OntarioFollow

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Shoesmith, David W.

2nd Supervisor

Noël, James J.

Co-Supervisor

Abstract

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^-/CO₃^2- solutions. It was found that both reactions were suppressed by the formation of U^VI surface films at low HCO₃^-/CO₃^2- concentrations. When the formation of these films was prevented at higher HCO₃^-/CO₃^2- concentrations both reactions occurred readily on the sublayer of U^IV_1-2xU^V_2xO_2+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 Pd^II).

The reduction of H₂O₂ has been studied on a range of UO₂ electrodes such as RE(III)-doped and non-stoichiometric (UO_2+x) electrodes and SIMFUEL. It was found that reduction on a UO₂ surface proceeded through a two-step reaction sequence, the chemical oxidation of U^IV to U^V followed by the electrochemical reduction of the surface back to U^IV. The rate of H₂O₂ reduction decreased in the order UO_2.002 ~ UO_2.5 ~ SIMFUEL > Gd-UO₂ ~ Dy-UO₂ > UO_2.1. The low reduction rate on RE(III)-doped electrodes was attributed to the stabilized UO₂ matrix by the formation of RE(III)-O_V 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 (CO₄^2-) 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 U^VI 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.

Recommended Citation

Zhu, Ziyan, "The Electrochemical Study of Simulated Spent Nuclear Fuel (SIMFUEL) Corrosion under Permanent Disposal Conditions" (2018). Electronic Thesis and Dissertation Repository. 5787.
https://ir.lib.uwo.ca/etd/5787