Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Nuclear Engineering

First Advisor

Theodore M. Besmann

Abstract

As part of Accident tolerant fuel initiative, the uranium-silicide compound, U3Si2, is under consideration as a potential replacement for conventional uranium dioxide fuel. It is of interest as its higher uranium density of 11.3 g(U)/cm3 compared to 9.7 g(U)/cm3 for UO2 may allow use of more robust, but less neutronically economical fuel cladding. The improved uranium content would not only accommodate the neutronic penalty inherent to certain accident tolerant cladding concepts but also facilitate improved reactor performance with the potential for longer fuel cycles.

The U-Si system has been the subject of various studies that mainly focused on thermophysical properties, environment stability, fabrication methodologies, irradiation testing, electronic and mechanical properties, and fuel-cladding compatibility. Despite the large number of studies on the uranium-silicide system, there is concern regarding the accuracy and completeness of our understanding of the 40-66 at.% silicon region of phase diagram. Phase equilibria form the foundation upon which to explore fuel fabrication and fuel performance related properties. These include thermal properties, radiation damage effects, and fission product behavior. Therefore, a comprehensive understanding of U-Si phase equilibria and thermochemical behavior will be necessary to support licensing efforts should U3Si2 continue to be considered as alternative fuel form.

The current U-Si phase diagram is characterized by seven intermetallic compounds (U3Si, U3Si2, USi, U3Si5, USi1.88, USi2, and USi3) of which only the U3Si compound is well understood. In this work, experimental techniques for thermal an compositional analysis, and crystal structure determination were coupled with computational predictions for the characterization of the six intermetallic compounds U3Si2, USi, U3Si5, USi1.88, USi2, USi3, and additional compounds between U3Si2, and USi2. Information such as phase transitions, homogeneity ranges, and crystal structures were used, along with critically assessed literature data, to construct a thermodynamic database describing the U-Si system utilizing the CALPHAD method. Some of the gaps in the understanding of the U-Si system that were filled based on the results of this work include the homogeneity range for the U3Si2 and U3Si5 compounds; the phase stability of U5Si4 and U2Si3; the crystal structure of the monosilicide, USi; and finally, the nature of the 450°C phase transition observed in the U3Si5 phase.

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