# Atomistic Investigation of Plastic Deformation and Dislocation Motion in Uranium Mononitride
## 1 Introduction  
Uranium mononitride (UN) is a promising advanced nuclear fuel due to its many attractive characteristics including a high uranium density, high thermal conductivity, good chemical compatibility with most potential cladding materials, and long fuel residence time [1, 2, 3]. However, it suffers from some drawbacks including complicated fabrication processes, high fuel cost owing to the need for \({}^{15}\)N enrichment, and low resistance to oxidation by high-temperature steam [1, 2, 3]. Many aspects of the behavior of UN under high temperatures and/or irradiation are lacking in both consistent experimental data and detailed mechanistic understanding. The mechanical properties of nuclear fuels are crucial for a better understanding of the pellet-cladding mechanical interaction (PCMI) that occurs during reactor operation. The mechanical aspects relevant to PCMI in UN include deformation behavior (stress-strain) and dislocation properties. These mechanical propertiessignificantly influence its behavior under high temperatures and high radiation intensity. Specifically, the deformation behavior affects the material's ability to accommodate thermal expansion and fission gas swelling, impacting the integrity of the fuel-cladding gap and influencing PCMI [4; 5]. Additionally, dislocation interactions with radiation-induced defects can alter the material's creep behavior and stress relaxation mechanisms at elevated temperatures, potentially leading to enhanced deformation or cracking under operational conditions [4; 5]. Therefore, a detailed understanding of the mechanical properties of UN is essential for predicting its performance and ensuring its reliability as a nuclear fuel. Molecular dynamics (MD) simulations of deformation behavior have been conducted for many materials including silicon (Si) and silicon carbide (SiC) [6], aluminum [7], and copper [8]. Due to the size limitation of MD simulations, most of these studies are of a descriptive nature, except for those studies that specifically target nanomaterial applications and for which the MD size is representative. To the best of our knowledge, no computational studies of the stress-strain behavior of UN exist in the open literature. The only experimental study found in the literature on the deformation behavior in UN is by [PERSON] and [PERSON] [9]. They observed that the majority of slip traces on UN samples exist within the \(\{110\}\) planes, which implies that \(\frac{1}{2}\langle 110\rangle\{110\}\) (with angle brackets referring to the slip direction and curly brackets referring to the slip plane) are the primary slip planes in UN. They also report that most of the dislocations observed are nearly pure screw dislocations. A secondary slip system along the \(\{112\}\) planes has been observed by the authors, who hypothesized a process for its occurrence that is based on the reaction of two screw dislocations to form an edge dislocation confined to the \((112)\) plane. However, the mechanism by which this edge dislocation slips remains unclear. Another study was conducted on dislocations in UN by [PERSON] et al. [10], who investigated dislocation loops in a proton-irradiated uranium-nitrogen-oxygen system. They stated that only \(\frac{1}{2}\langle 110\rangle\) dislocation loops can be observed in the UN phase. They also conducted 0 K molecular statics using the Tseplyaev and Starikov angular-dependent potential [11] to determine the formation energies of \(\frac{1}{2}\langle 110\rangle\) and \(\frac{1}{3}\langle 111\rangle\) dislocation loops at various loop sizes. However, as shown previously [12] and verified in this study, 0 K calculations using the Tseplyaev and Starikov angular-dependent potential [11] of defective UN structures can be trapped at metastable states and should be treated cautiously. Nucleation of dislocations is a primary product of radiation damage in nuclear materials. Knowledge of dislocation mobility is essential to model glide-controlled creep accurately [13].