Corrosion of UO₂ and Thermodynamic Properties of Solid Solutions in the Zircon Group.

Abstract

Corrosion resistance and the ability to incorporate and retain impurities are important properties in determining the usefulness of a material for applications as varied as designing a nuclear waste form or calculating the temperature or time at which a particular rock formed. This thesis applies both experimental and computational techniques in order to gain insight into the corrosion and substitution behavior of actinide oxides and orthosilicates. Experimental studies of uraninite corrosion in miniature waste packages show that oxidation of UO2, a proxy for spent nuclear fuel (SNF), is minimal in the presence of a relatively large amount of corroding steel over laboratory time-scales. The oxidative dissolution of spent nuclear fuel in an oxidizing repository with a steel canister may be broken into three phases: (1) before breach, when no water is available and the corrosion rate is negligible, (2) immediately after breach, when the steel begins to corrode, conditions inside of the canister are more reducing, and SNF does not corrode significantly, and (3) long after breach, SNF is exposed to the open air and oxidized. The common accessory mineral zircon (ZrSiO4) is considered a host phase for Pu and other elements using a combination of quantum-mechanical and Monte-Carlo simulations. Results show that while hafnon (HfSiO4) and zircon form a nearly ideal solid solution, the maximum amount of Pu, U, Th, and Ce that can be incorporated at thermodynamic equilibrium is extremely limited and can be ranked in the following order: (Zr,Hf)SiO4 > (Th,U)SiO4 > (Zr,Ce)SiO4 > (Zr,Pu)SiO4 > (Hf,Pu)SiO4 > (Zr,U)SiO4 > (Zr,Th)SiO4. Calculations on Ti incorporation in zircon for use as a geothermometer confirm that Ti prefers to occupy the Si site at pressures below 1 GPa. However, at higher pressures, particularly above 3.5 GPa, substitution into the Zr site is preferable. Results suggest a pressure correction of 100 °C/GPa at 750 °C, twice as large as a previous estimate. The Ti-in-zircon geothermometer must be recalibrated if it is to be used on ultra-high pressure samples.