Prelim Defense – Matthew Heany
About the event
Speaker: Matthew Heany
Group: Dr. Guo Xiaofeng
Title: In Situ Characterization of Uranium Molten Salt Systems
Abstract: Many models exist to describe how liquids undergo phase transitions to the solid state, from simplistic classical nucleation models to more complex non-classical models. In binary and higher-order systems, the formation of heterogeneous or homogeneous structures is possible as a result of nucleation and subsequent growth of solids. These structures, whether they be crystalline or amorphous, heterogeneous or homogeneous, can have myriad properties. Of particular importance for this solidification is molten salt reactors (MSR), where the uranium (U) fuel is liquid and mixed with base salts to form low temperature eutectic melt. Despite its significance, little to no research has been conducted on the solidification processes of these molten salts, especially regarding the behavior of the U species. This research specifically aims to investigate the structure, local environment, thermodynamics, and electronic properties of U species as molten salts cool and solidify and outlining the methods to do so.
To keep these molten salt mixtures in a liquid state, and an MSR in operation, salts must be kept at high temperatures. This poses the potential issue of what would happen should the reactor cool down intentionally or unexpectedly making the salts prone to solidification. Furthermore, what concerns might there be for the fissile fuel present in the mixture. Would it remain evenly dispersed, or would it induce phase segregation and form pockets of fissile material? How big might those pockets be, and could they pose new challenges to restarting or repairing the reactor? While these are critical questions they have been fundamentally overlooked in literature. Additionally, there exists evidence that small ionic clusters or polymer-like complexes may occur inside molten salts that could affect MSR performance and should be considered in engineering and operation. This all requires studies involving both X-ray and thermodynamic characterization.
X-ray characterization techniques are some of the most common means employed for characterizing structures, electronic properties, and local environments. However, most X-ray characterization techniques possess a more advanced form in which the X-ray source energy is varied in addition to the other varied parameter, such as detector angle for X-ray diffraction. These X-ray techniques have the adjective “anomalous” or “resonant.” The benefit of using an anomalous X-ray technique is due to the competitive relationship between the scattering and absorption of photons. The fact that the energy of those photons determines which process dominates, which is intrinsically linked to the identity of that element we can identify element specific contributions to data that is collected. Once characterized, thermodynamic properties of these structures are important to identify under what conditions certain structures of U might form. Since even small changes in a still molten state MSR can greatly change how the neutronics of the reactor behave, let alone what will occur as the molten salt cools and undergoes solidification, all of which will inform how the solid salt should be dealt with.
In order to conduct such in-depth analysis specialized instrumentation and environments need to be designed to handle the extreme temperatures and corrosive properties of molten salts while also allowing for in situ characterization. The necessity of in situ measurements arise from the transient nature of nucleation and potentially rapid changes in molten salt environments. This particular work aims to look into the fundamental properties of U molten salts, and how they would behave in the conditions expected for an MSR in off-normal conditions.