Abstract: Irradiation-induced microstructural defects play a decisive role in the mechanical degradation of structural materials operating in advanced nuclear energy systems. For Ni-based alloys proposed for high-temperature gas-cooled reactors, the accumulation of defect clusters during irradiation may substantially alter dislocation motion, giving rise to irradiation hardening and a loss of deformation tolerance. Among these defects, Frank dislocation loops are of particular importance because of their sessile nature and strong interaction with mobile dislocations. A mechanistic understanding of dislocation loop interactions is therefore essential for clarifying the atomistic origin of irradiation hardening and for establishing physically informed constitutive descriptions of irradiated Ni-based alloys. This study aims to elucidate the interaction mechanisms between a dissociated edge dislocation and a Frank dislocation loop in face-centered cubic (FCC) Ni, with particular attention to the effects of dislocation loop size and temperature on obstacle strength. The work further seeks to quantify the critical resolved shear stress (CRSS) and to develop a hardening equation suitable for bridging atomistic simulations and higher-scale mechanical models. Molecular dynamics (MD) simulations were performed using the LAMMPS. The atomic interactions were described by an embedded atom method potential for Ni. A simulation cell containing a dissociated 1/2〈110〉 edge dislocation and a 1/3〈111〉 Frank dislocation loop was constructed. Frank dislocation loops with diameters from 1 nm to 5 nm were placed on the dislocation glide plane to represent irradiation-induced obstacles of different strengths. Simulations were conducted at temperatures ranging from 300 K to 1 200 K, covering conditions from near room temperature to elevated temperatures relevant to high-temperature nuclear applications. The CRSS was obtained from the shear stress required for the gliding edge dislocation to overcome the Frank dislocation loop. Atomic configurations and dislocation structures were analyzed to identify the evolution of the interaction process and the associated deformation mechanisms. The simulations reveal a well-defined sequence of dislocation loop interaction. The mobile edge dislocation first glides freely toward the Frank dislocation loop, followed by elastic attraction between the two defects. Subsequent interaction leads to pinning of the leading partial dislocation, pinning of the trailing partial dislocation, and eventual unpinning of the entire dislocation. Two characteristic mechanisms are identified. In the shear mechanism, the gliding dislocation cuts through the Frank dislocation loop and temporarily reacts with it, producing a transient full-dislocation segment before the dislocation re-dissociates into partials. The Frank dislocation loop remains largely preserved after this process. In the absorption mechanism, the Frank dislocation loop is incorporated into the moving dislocation, leading to the formation of a super-jog structure and a substantial reconstruction of the local dislocation configuration. The prevailing interaction mechanism is governed by the combined effects of temperature and loop size. Increasing temperature markedly reduces the CRSS, indicating that thermal activation facilitates dislocation unpinning and weakens the effective obstacle strength of Frank dislocation loops. Elevated temperature also promotes the absorption mechanism, particularly for small dislocation loops, because thermally assisted atomic rearrangement lowers the barrier for loop incorporation into the gliding dislocation. By contrast, larger Frank dislocation loops exhibit stronger pinning ability. The CRSS increases with dislocation loop diameter, reflecting the longer effective pinning length and the greater resistance imposed on the dislocation line. Based on the MD results, a hardening model was formulated within the Bacon-Kocks-Scattergood (BKS) framework. The CRSS data were fitted by introducing a temperature-dependent parameter, Δ, which characterizes the effect of thermal activation on dislocation loop interactions. The resulting equation captures the dependence of irradiation hardening on both Frank dislocation loop size and temperature, providing a quantitative link between nanoscale obstacle characteristics and macroscopic hardening behavior. In conclusion, this study clarifies the atomistic mechanisms by which Frank dislocation loops impede dislocation glide in FCC-Ni. The results demonstrate that irradiation hardening is controlled not only by obstacle size but also by thermally activated changes in the dislocation loop interaction pathway. The proposed BKS-based hardening equation offers a mechanistic basis for predicting the strengthening effect of irradiation-induced Frank dislocation loops. These findings provide useful input for crystal plasticity and continuum constitutive models and contribute to the performance assessment and microstructure-informed design of Ni-based alloys for next-generation nuclear energy systems.