Studies of Reaction Processes for Voloxidation Methods

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 UO_­2 [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.