Chemistry Departmental Seminar – Prof. Brian A. Powell
About the event
Speaker: Prof. Brian A. Powell, Fjeld Professor in Nuclear Environmental Engineering and Science, Clemson University
Title: Understanding tetravalent actinide oxide formation, stability, and dissolution under far field environmental conditions
Abstract: Comprehensive thermodynamic understanding of nuclear materials is paramount for long-term management of legacy nuclear waste and commercial spent nuclear fuel. Actinide oxides (AnO2(s)) are ubiquitous materials throughout the nuclear fuel cycle, yet existing thermodynamic data has noticeable discrepancies, creating uncertainty in environmental prediction of the fate of nuclear materials. In this work, we have deployed a variety of neptunium and plutonium solid phases in a series of field lysimeter experiments at the Department of Energy Savannah River Site. The Radionuclide Field Lysimeter Experiment (RadFLEx) testbed is equipped to deploy highly characterized radionuclide source terms into natural settings for periods from months to years by deploying the sources at the center of 10 cm x 50 cm soil columns which are subject to rainfall infiltration and natural atmospheric and temperature fluctuations. Thus, RadFLEx allows us to examine how these solid phases may age in the environment. All plutonium sources are transforming to disordered PuO2+x type phases regardless of the initial oxidation state and the extent of transformation to disordered PuO2+x phases appears to be accelerated in the field lysimeter sources relative to the archived sources. Comparison of the downward migration of each source indicates some notable differences between lysimeters with different initial Pu and Np sources. These differences were further studied with laboratory studies examining the influences of the grain boundary microstructure on the oxidative dissolution of NpO2(s). This work demonstrated the sizable differences in dissolution of NpO2(s) based on process conditions, such as the temperature at which the material is calcined and resultant particle size, and the need for more thorough evaluation of the microstructure of solid phases used to generate thermodynamic data of actinides and understand environmental fate. The current working hypothesis to explain these differences is that colloidal PuO2+x and NpO2 phases are forming during the chemical/physical transformations of the source materials and downward migration is enhanced by these colloidal phases. Thus, the extent of transport appears to be somewhat dependent on the initial chemical/physical state of the source.