Award Date

May 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Committee Member

Frederic Poineau

Second Committee Member

Kenneth Czerwinski

Third Committee Member

Alfred Sattelberger

Fourth Committee Member

Gary Cerefice

Fifth Committee Member

Michael Pravica

Number of Pages

152

Abstract

From 1943 to 1987, the Hanford site was the primary location for the production of Pu for military applications in the United States. Decades of Pu production have been accompanied by generation of large amounts of liquid and solid radioactive waste. One of radioelements present in the Hanford wastes is Tc, principally the isotope 99Tc (1500 kg). In the waste, it is primarily present in the heptavalent state as TcO4 - . One option for the management of Tc at the Hanford site is its vitrification into a borosilicate glass. The behavior of Tc during vitrification is problematic, as it volatilizes and only a fraction is retained in the glass. Minimization of Tc volatility is a challenge for the development of safe vitrification processes. The nature of the volatile Tc species has been discussed and HTcO4, Tc2O7 and alkali pertechnetate salts have been proposed. Until now, no characterization of the volatile Tc species has been performed.

In this context, the vitrification of Tc in simulated borosilicate glass was performed and the volatile Tc species were characterized by X-ray absorption spectroscopy. The EXAFS results indicated the presence of TcO3(OH)(H2O)2 in the volatile species. The mechanisms of formation of volatile species were proposed and involved the oxidation of TcO2 to Tc2O7 followed by the hydrolysis of Tc2O7 to HTcO4.

In order to better understand the mechanism of formation of volatile Tc species, the gas-phase and solid-state chemistry of Tc2O7 was revisited. Here, Tc2O7 was synthesized from the oxidation of TcO2 with O2 and its crystallographic structure was determined at 100 K and the data compared with the previous structure elucidated in 1969. The electron impact-mass spectrum of Tc2O7 consisted of both mononuclear and dinuclear species. The main dinuclear species in the gas-phase were Tc2O7 and Tc2O5, while the main mononuclear species were TcO3 and TcO2. The optimized structure of the Tc2O7 molecule was in good agreement with the experimental one. Energetic calculations confirmed the linear structure to be the most favorable in the solid-state, while the bent geometry is the most stable in the gas-phase. Stability with respect to compression and high cohesive energy per molecule are possibly the reason why the linear structure is observed in the solid-state. Finally, because water can be present during the vitrification process, studying the effect of water on the nature of the volatile species is of importance. The effect of water on the nature of the volatile species formed during oxidation of TcO2 was studied. The oxidation of TcO2 with water and O2 was performed at 250 °C; after the reaction a red product was observed and characterized by mass spectrometry.

Keywords

Glass; Nuclear Waste; Radiochemistry; Red; Synthesis; Technetium

Disciplines

Chemistry

File Format

pdf

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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