# Atomistic Investigation of Plastic Deformation and Dislocation Motion in Uranium Mononitride
## 5 Conclusions  
In this study, we employed MD simulations to investigate specific aspects of the mechanical behavior of UN. First, we analyzed the deformation behavior in nanometer-sized UN single-crystals. The Kocevski potential predicted the principal slip system as \(\frac{1}{2}\langle 110\rangle\{110\}\), aligning with experimental data, while the Tseplyaev potential predicted slip on \(\frac{1}{2}\langle 110\rangle\{111\}\). The study demonstrates that MD simulations of stress-strain curves in single crystals with free surfaces yield accurate estimates of the nanoindentation hardness of UN. Nanoindentation hardness was accurately predicted by the Kocevski potential, though it struggled to model dynamic plasticity. This methodology is particularly advantageous at elevated temperatures, where oxidation hinders direct measurements of UN's nanoindentation hardness. Further exploration involved the determination of the Peierls stress for the \(\frac{1}{2}\langle 110\rangle\) edge dislocation along the \(\{110\}\), \(\{111\}\), and \(\{112\}\) slip planes and for the \(\frac{1}{2}\langle 110\rangle\) screw dislocation along \(\{110\}\). The \(\{111\}\) plane displayed the smallest value (2.98 GPa), agreeing with the results of the stress-strain behavior predicted by the Tseplyaev potential. For the screw dislocation, the Peierls stress is determined to be about 790 MPa using the Kocevski potential. Both edge and screw dislocations are found to move via the kink-pair nucleation mechanism before displaying a linear variation in their velocity with stress. Based on these results, complete dislocation mobility functions were fitted and the linear mobility was found to be 817 Pa\({}^{-1}\cdot\)s\({}^{-1}\) for the edge dislocation using the Tseplyaev potential, and 4546 Pa\({}^{-1}\cdot\)s\({}^{-1}\) for the screw dislocation using the Kocevski potential. The threshold Schmid stress was estimated to be between 179 and 197 MPa, corresponding to an upper-limit uniaxial yield stress of 548-603 MPa for polycrystalline UN. These values show the expected orders of magnitude and can be used as inputs for plasticity or dislocation dynamics models. Finally, we observed that, at intermediate stresses, the subsonic steady-state motion of the edge dislocation in UN can be interrupted by jumps that have a maximum velocity equal to the average sound velocity. To the best of our knowledge, this study is the first computational investigation of the stress-strain behavior and dislocation properties in UN, and the first that attempts to fit complete mobility functions for dislocations in a nuclear fuel. We believe this work provides a baseline for the computational study of dislocations in nuclear fuels that can be expanded and built upon by, e.g., conducting the analysis at higher temperatures, which are more relevant for the operational conditions of nuclear reactors. Conceptualization, M.A., B.B. and A.C.; methodology, M.A. ; formal analysis, M.A. ; writing--original draft preparation, M.A. ; writing--review and editing, M.A., B.B. and A.C.; visualization, M.A. ; project administration, B.B. and A.C.; funding acquisition, A.C. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Westinghouse Electric Company. This research made use of the resources of the High-Performance Computing Center at Idaho National Laboratory, which is supported by the Office of Nuclear Energy of the U.S. Department of Energy and the Nuclear Science User Facilities under Contract No. DE-AC07-05 ID14517. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request. **Acknowledgments:** The authors would like to thank [PERSON], [PERSON], [PERSON], and [PERSON] for the fruitful discussions and useful suggestions. [PERSON] dedicates this work to the memory of [PERSON].