Dr. Amani Cheniour
Reactor & Nuclear Systems Division
Oak Ridge National Laboratory

Study of Microstructure Behavior in U₃Si₂ under LWR Conditions Using the Phase Field Method

ABSTRACT: U₃Si₂ is considered as a potential accident tolerant fuel for Light Water Reactors (LWRs) alternatively to UO₂, for its better thermal conductivity and higher uranium density. In order to fully assess the behavior of U₃Si₂ in LWR conditions, it is necessary to develop predictive computational models that account for the observed mechanisms under these conditions. Here we focus on two phenomena: grain growth and grain subdivision.

Grain growth is thermally activated and can affect fracture behavior and fission gas release. To model grain growth using the phase field method, it is necessary to obtain the grain boundary energy and mobility. The U₃Si₂ grain boundary energy has already been calculated at different temperatures and misorientation angles using Molecular Dynamics (MD). We used experimental grain growth data, the MD grain boundary energy, and phase field simulations to determine the grain growth parameters for a 3-D model.

The High Burnup Structure (HBS) has been observed in UO₂ at the rim of the fuel pellet at high burnup, characterized by nano-sized grains and high porosity. While the mechanism behind HBS formation is debatable, a comparison between the HBS energy and the energy of a highly irradiated structure with large grain sizes can establish the HBS temperature and burnup thresholds. A phase field radiation damage model coupled with binary collision Monte Carlo was developed and initially applied to UO₂ for validation. Using recent irradiation data on U₃Si₂ and the phase field model, conclusions on the HBS burnup range were reached and a hypothesis on the mechanism behind pore formation and growth was proposed. Simulation results for both UO2 and U₃Si₂ will be discussed.

BIOGRAPHY: Amani Cheniour recently joined the Nuclear Fuel Materials group at Oak Ridge National Laboratory (ORNL) as a R&D Assistant in modeling composite materials in nuclear systems. She earned a Ph.D. in Materials Science and Engineering from the University of Florida (UF) in 2020. She has a master’s degree in Nuclear Engineering from Grenoble Institute of Technology in France. Before joining ORNL, she was a graduate assistant at the Tonks Research Group at UF, where she worked on developing phase field models to study grain growth and grain subdivision conditions in U₃Si₂.