Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Noël, James J.

Abstract

This thesis describes the studies performed on a number of natural uranium dioxide (UO2) specimens to determine a range of reactivities. This work will support the safety case for the long-term disposal of spent nuclear fuel in Canada. Under relevant long-term disposal conditions, water radiolysis could produce oxidizing conditions at the surface of the fuel, potentially leading to fuel corrosion and the release of radionuclides into the groundwater. The rate of such a process would be determined by the radiation dose rate from the fuel and the reactivity of the fuel, which will be determined by its composition, in particular by the degree of non-stoichiometry and the concentration of fission products.

For undoped UO2 (i.e., in the absence of fission products), the reactivity has been shown to be very dependent on the degree of non-stoichiometry (x in UO2+x). The conductivity of natural UO2 specimens fabricated between 1965 and 2017 was characterized using several surface analytical and electrochemical methods. The reactivity of the characterized specimens in H2O2-containing environments was subsequently found to be only loosely related to the measured resistivities (conductivity). However, most of the H2O2 consumed on the surface of the UO2 electrode was consumed by H2O2 decomposition rather than surface oxidation.

In the absence of H2, the peroxide oxidized the surface to UIV1-2xUV2xO2+x with x varying with H2O2 concentration and eventually achieving a composition of UIV0.34UV0.66O2.33. At this surface composition, the surface becomes unstable with respect to dissolution, but the dominant reaction is H2O2 decomposition. In the presence of H2, the initial oxidation of the UIVO2 when H2O2 was added was reversed by the ability of H2 to scavenge the OH• with the H• radicals formed. This led to a reduction of the oxidized surface. The efficiency of this process is determined by the relative concentrations of H2O2 and H2.

Summary for Lay Audience

Uranium dioxide (UO2) is an extremely energy-dense material that produces reliable, low-carbon-emitting power. The waste produced from this power generation is a stable ceramic material but is highly radioactive, requiring hundreds of thousands of years to return to its natural radiation levels. The internationally accepted solution for the long-term disposal of this waste is to contain and isolate the UO2 in corrosion-resistant containers within a deep geological repository. While there is extensive research to support the durability of these containers until the UO2 returns to its natural radiation levels, it is necessary to evaluate the potential consequences of a failed container filled with groundwater.

A series of UO2 pellets fabricated between 1965 and 2017 were characterized to determine a range of reactivities under potential long-term disposal conditions. The primary oxidant of concern is hydrogen peroxide (H2O2), produced naturally as water reacts with alpha radiation from the spent fuel. However, despite initial suggestions of a large range of reactivities, this work found that most of the H2O2 that reacts on the surface of the spent fuel results in the decomposition of H2O2 rather than the oxidation of the spent fuel, regardless of the suggested reactivity.

Further studies in the presence of H2, which has been shown to scavenge H2O2 on simulated nuclear fuel samples, suggested that even on natural UO2 samples, H2 can suppress corrosion under expected disposal conditions and, in some cases, reverse surface oxidation.

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