Anshul Kamboj

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 irradiation 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.

The formation of solute clusters in irradiated low-alloy steels, such as reactor pressure vessel steels, is a critical cause of radiation hardening and embrittlement. However, the chemical interactions among solute elements and excess vacancies remain to be fully understood. This study employs density functional theory, cluster expansion, and lattice-based Monte Carlo simulations to examine the stability and morphology of nano-size coherent solute clusters with excess vacancies. The findings reveal that excess vacancies are crucial in stabilizing and promoting the growth of Mn-Ni-Si-vacancy clusters. A minimum of seven vacancies is required for stable nucleation and growth of these clusters. These Mn-Ni-Si clusters act as defect sinks, effectively trapping and absorbing mobile vacancies generated under irradiation. Furthermore, phosphorus (P) preferentially dissolves in Mn-Ni-Si clusters due to chemical coupling with vacancies. This study offers new insights into solute-vacancy interactions, enhancing our understanding of solute-defect cluster formation and embrittlement in low-alloy steels.