Irradiation Damage in Uranium Mononitride with C and O Impurities: A Molecular Dynamics Study

Uranium mononitride (UN) is a promising advanced nuclear fuel, but radiation-induced defects compromise its performance and lifespan. Molecular dynamics simulations are indispensable for capturing these atomic-scale behaviors, yet their application to UN has long been limited by the insufficient accuracy of classical empirical potentials. In this study, we employ a high-fidelity neuroevolution potential for the UN system containing C and O impurities to systematically evaluate irradiation damage in both pristine and impurity-bearing UN fuels. Our results show that the defect evolution behavior induced by cascade collisions along different primary knock-on atom (PKA) directions exhibits little difference. The PKA energy is roughly proportional to the residual defect number; however, at the same PKA energy and temperature, the residual defect number in UN is significantly smaller than that in UO2. Over the temperature range investigated, the peak defect number tends to increase with rising temperature, while the residual defect number decreases slightly, corresponding to an increase in annihilation efficiency with temperature. Vacancies are more prone to form clusters than interstitials, and both the cluster number and size in the UN system are significantly smaller than those in UO2. For the impurity-containing systems, increasing impurity concentration slightly reduces the residual defect number, while the variation in annihilation efficiency does not exhibit an obvious concentration dependence. This work provides a systematic atomic-scale reference for the radiation damage behavior of UN and highlights the importance of impurity effects in evaluating its irradiation performance.