Doctoral Preliminary Oral Exam – Natalie Yaw
About the event
Speaker: Natalie Yaw
Group: Professor Guo
Title: The effect of cation substation and valency on formation energetics of brannerite ceramics for nuclear waste applications
Abstract:
Titanate brannerites (AB2O6, prototypically UTi2O6) have emerged as attractive nuclear ceramic wasteforms due to their thermodynamic stability, chemical inertness, and high loading of actinides.1,2 Both A and B sites are open to substitutions if the average valency of the combined sites is +4, thus brannerite ceramics have the advantage of incorporating neutron absorbers, actinides from mixed oxide fuels, and charge-balancing elements.3–5 In this work, we propose to study the correlation between composition and formation energetics by systematically examining cationic (Ce, Th, U) solid-solution brannerites relevant to nuclear waste applications. We hypothesize that solid solutions of the endmember brannerites will impact and (de)stabilize the brannerite structure due to the local arrangement of the mixing elements/valence states, whether they are randomly distributed or show short-range ordering (SRO). These structural features translate to both enthalpic and entropic impacts which have not yet been studied. End-member and solid solutions of Ce/U/Th brannerites have already been synthesized. Their local and long-range structures will be studied by XRD combined with XAFS, with valence determined by XPS, XANES, and reducing TGA. For thermodynamic stability, I will use high temperature oxide melt solution calorimetry to interrogate their formation enthalpy (ΔH). In tandem, leaching experiments will be done to understand their dissolution behaviors and chemical durability, further validating their applicability as nuclear wasteforms. Tracking compositional changes onto corresponding changes in entropic and enthalpic properties in this way, we will build out our understanding of cation mixing, critical stability constants, and valence-driven (de)stabilization in these important mixed actinide ceramics.