Excessive Dislocation Loop Growth in Uranium Mononitride Under High Temperature Proton Irradiation

We investigate dislocation loop evolution in uranium mononitride under 2 MeV proton-irradiation at doses of 0.1 and 1 dpa, and temperatures of 25 °C and 800 °C. Transmission electron microscopy reveals strong dependencies of loop density and size on irradiation conditions. At room temperature, small loops dominate, while at 800 °C, loop density decreases but loop size increases significantly, with perfect loops exceeding 150 nm. This behavior contrasts with other fuels and surrogates (e.g., uranium dioxide, thorium dioxide, cerium dioxide, and zirconium carbide), where loops remain smaller under similar conditions. Rate theory modeling attributes this to lower defect migration barriers and higher uranium interstitial mobility in uranium mononitride. Molecular dynamics simulations suggest rapid unfaulting and enhanced mobility of perfect loops promote coalescence, forming fewer but much larger loops. These findings offer new insights into the distinct kinetics of faulted and perfect loop evolution in uranium mononitride, with implications for its thermal transport properties.