Date of Award

5-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Robert M. Counce

Committee Members

David J. Keffer, Barry B. Spencer, Claudia J. Rawn, Tom T. Meek, Jack S. Watson

Abstract

Various facets of the voloxidation process and processes that have been derived from the voloxidation process have been investigated since its development over four decades ago. Despite the numerous studies performed, gaps remain in understanding of particular fundamental aspects of the reaction processes. In this work, several of these specific aspects of the oxidation processes for standard voloxidation and NO2 [nitrogen dioxide] voloxidation are studied experimentally and modeled.

In the case of standard voloxidation, the oxidation rates of simulant UO­2 [uranium dioxide] pressurized water reactor pellets in oxygen-rich environments were studied with an emphasis on the controlling phenomena for the reaction and the influence of cladding on these phenomena. Parametric isolation experiments for the oxidation of UO2 pellets using thermogravimetric analysis were employed in which oxidant concentration, temperature, gas flow rate, and effect of cladding were studied. To supplement the thermogravimetric experiments, the reaction interface was characterized using neutron diffraction to validate assumptions for model development. From these experiments, a model approach is derived for the oxidation of clad UO2 pellets during voloxidation. This work provides needed insight into the influence of various parameters on oxidation rate and reveals the potential controlling phenomena and their parameter dependencies to allow for improved process design.

Advanced NO2 voloxidation, unlike standard voloxidation, is a novel process only recently proposed and thus there is much to investigate. The NO2 voloxidation experiments and reaction models presented focus on the oxidation process of U3O8 [triuranium octoxide] to UO3 [uranium trioxide]. A structure for the ε [epsilon]-UO3 polymorph is proposed and employed for in situ X-ray diffraction studies for quantitative analysis to determine reaction rates and reaction mechanism. The data collected were modeled using a phenomena-based approach to propose the controlling mechanism for reaction. From the findings of the research presented, a better understanding of the oxidation process of U3O8 to UO3 by NO2 was achieved.

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