Date of Award

May 2020

Document Type


Degree Name

Doctor of Philosophy (PhD)


Environmental Engineering and Earth Science

Committee Member

Brian A Powell

Committee Member

Lindsay Shuller-Nickles

Committee Member

Nicole Martinez

Committee Member

Daniel Kaplan


Management of legacy nuclear waste, along with proposed geologic disposal of commercial spent nuclear fuel, necessitates prediction of environmental fate and transport of actinide solid phases, which comprise a majority of long-lived waste products. Actinide dioxides are key for performance assessment due to their ubiquity throughout the nuclear fuel cycle. Neptunium-237, produced in burn-up of commercial nuclear fuel and various other nuclear processes, is often found as an oxide (NpO2(s)) and has the potential to become highly mobile in near-surface geochemical conditions due to oxidation processes. In this dissertation, the oxidation and dissolution of NpO2(s) was studied through both field and laboratory scale work, confirming conceptual models across large spatial scales. Overall, the objective of this dissertation was to determine the dissolution mechanisms of NpO2(s) in environmentally relevant conditions. Initial field scale studies of NpO2(s) dissolution and transport in field lysimeters over 0.8 to 3 years identified mobility of Np in the vadose zone from both aqueous and colloidal Np. Subsequent laboratory scale work confirmed and developed a conceptual model of grain-boundary facilitated dissolution through dissolution experiments and advanced characterization of the of NpO2(s) solid phase. Finally, the effect of grain boundaries on NpO2(s) dissolution was further probed by controlling grain size through synthesis parameters and dissolving various nanograined NpO2(s) materials.



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