Towards Improving the Properties and Furthering Acceptance of Advanced Technology Nuclear Fuels
Authors: Watkins Jennifer K. 
Published: September 2022

To avoid detrimental environmental impacts from climate change, the world community needs to push for the use of clean energy technologies.
Development of proposed advanced technology nuclear fuels supports efforts to ensure nuclear energy is included as a non-carbon emitting source
of electricity generation. Advanced technology nuclear fuels, also referred to as accident tolerant fuels (ATFs), have received renewed interest
for use in the current nuclear reactor fleet as well as in advanced reactor technologies due to their high uranium loading, desirable thermophysical
properties, and performance under irradiation as compared to the benchmark oxide fuel. A limiting consideration for the implementation of these ATFs
is their poor performance in oxidative and corrosion conditions, as well as challenges associated with synthesis and fabrication. As a full understanding
of these ATFs has not been achieved, this work aims to advance the state of knowledge related to these fuels and their composites in corrosion conditions,
their grain growth mechanisms, and includes efforts to improve thermal conductivity in the benchmark oxide fuel using these ATFs. Chapter Two presents a
study of uranium mononitride (UN) and UN composites with uranium dioxide (UO2) under hydrothermal corrosion conditions to assess the mechanism
of degradation at elevated temperatures, identified as secondary phase formation at the grain boundaries leading to pellet collapse. Chapter Three combines
experimental and theoretical studies of composite systems, UN-Zr and UN-Y, for the purposes of improving the corrosion resistance of monolithic UN.
The results indicate the formation of undesirable secondary phases in the sintered materials and provided insight to the atomic level structural changes
which occurred due to the addition of the metallic constituents. An extensive review (included as Appendices A, B, and C) of the state of the literature
for oxidation performance of UN, triuranium disilicide (U3Si2), uranium carbide (UC), and uranium diboride (UB2), was performed to identify the challenges and opportunities to alloyed and composite architectures of these ATF candidates to mitigate corrosion behavior.

In addition, an understanding of the microstructural evolution during the fabrication of various fuel forms, such as grain growth, is important in
predicting its performance under irradiation (e.g., fracture, creep, fission gas release, thermal conductivity, etc.). Accordingly, it is important to
understand the driving force behind grain growth and the factors which influence it. Chapter Four presents a fundamental study on grain growth in conventionally
sintered UN. The study identified the most likely mechanism and proposed an activation energy for grain growth with a discussion on the factors that influenced it,
as well as the lack of expected texture present in the sintered samples. Chapter Five describes work on successful incorporation of uranium diboride (UB2, another
ATF candidate) to a UO2 matrix via conventional fabrication and sintering methods, for the purposes of improving overall thermal conductivity
of the bulk composite. Presented together, this work provides foundational inquiry and analysis which can be used to further research on ATF candidates
and assist in acceleration of qualifying these fuels for use in the current and future nuclear reactor fleets.