Dissertations, Theses, and Capstone Projects

Date of Degree

2-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Chemistry

Advisor

Donna McGregor

Committee Members

Lynn C. Francesconi

Benjamin P. Burton-Pye

Steven Greenbaum

Subject Categories

Analytical Chemistry | Inorganic Chemistry | Radiochemistry

Abstract

Spent nuclear fuel is reprocessed by utilizing several large-scale solvent extraction processes. One of the most prevalent of these processes is the plutonium uranium reduction extraction (PUREX) process. This process involves an organic phase composed of tributyl phosphate (TBP) in kerosene and an aqueous phase comprised of concentrated nitric acid. Both uranium (U) and plutonium (Pu) are extracted into the organic phase by nitrate and TBP complexation while the various fission products remain in the initial aqueous phase. Pu is then chemically reduced to its trivalent state and back-extracted into a second aqueous phase, thereby separating the two fissile materials. However, it is known that several other elements present in the spent nuclear fuel complicate this process by following U and Pu into the initial organic phase.

This thesis focuses on the high-yield U-235 fission product technetium (Tc), particularly Tc-99, which has a half-life of 211,000 years. Tc exhibits a wide range of achievable oxidation states (-1 to +7) which results in complicated redox chemistry that is induced by the radiolysis of the high radiation fields of spent nuclear fuel. This redox behavior is sensitive to the reductants and oxidants used to modify the oxidation states and extraction behavior of U and Pu. These factors, as well as the presence of dibutyl phosphate (HDBP), the radiolysis-induced degradation product of TBP, result in Tc being extracted into the organic phase in Tc-TBP and Tc-HDBP complexes that are not yet well-determined. Changes in the nitric acid concentration of the aqueous phase and the presence of other high-yield fission products, such as Zr, additionally influence the Tc-phosphate complexation. These investigations focus on cataloging the Tc-phosphate and Tc-metal-phosphate complexes for Tc in its most common oxidation states, +7 and +4. This research aims to build new knowledge in fundamental Tc chemistry that is critical for understanding the various aspects of the nuclear waste stream, especially fuel reprocessing and remediation of nuclear waste sites such as Hanford

Download

Share

COinS