###### Abstract  
Fission gas plays a significant role in fuel rod performance following accidents. The amount of fission gas increases dramatically under accidental conditions. This leads to a subsequent rise in the fuel rod internal pressure and temperature due to aggravated gap conductance between the fuel pellet and cladding. As a result, fuel rod performance degrades. Therefore, studying fission gas behavior is crucial for accident assessment and evaluating fuel rod performance. Minimizing the Impact of fission gas on fuel rods is essential for maintaining their integrity and safety within nuclear reactors. One important aspect of ensuring safety is predicting fission gas release (FGR). In this study, we presented an extended model to be used in light water reactors (LWRs). The FGR can be modeled using COMSOL Multiphysics with the finite element method. This modeling approach considers both normal and abnormal conditions, with the latter categorized as Class-II type incidents. The model assumes that the gas diffusion inside a spherical grain varies over time. By examining perfect sinks with gas production, perfect sinks without gas production, and imperfect sinks under steady-state conditions, different initial and boundary conditions are set. To validate the accuracy and universality of expressions used in the model, input parameters from other models and experiments are utilized. By comparing the model's results with these inputs, the accuracy and applicability of the expressions can be confirmed. This validation process ensures that the model provides reliable predictions for fission gas behavior in fuel rods under both normal and abnormal operating conditions. Based on our findings, it is evident that the FGR fraction displays an upward trend as diffusion coefficients and temperatures rise. Conversely, larger grain sizes and higher linear heat generation rates are associated with a reduction in the FGR fraction. Notably, enhanced resolution leads to a postponed onset of FGR. Furthermore, the influence of the diffusion coefficient on the FGR fraction primarily stems from the interconnected effects of temperature and linear heat generation rate. fission gas release (FGR); fission gas diffusion; fission gas behavior; light water rector; accidental condition +
Footnote †: journal: selected  
0000-0002-4000-3300][PERSON]  
0000-0002-0002-3300][PERSON]  
0000-0002-3300-3000-3000-3000-3000-3000-3000-3000-3000-3000-3000-30000-30000-3000-30000-30000-30000-30000-30000-30000-30 opening grain boundary [6]. The FGR is subjected to gas sweeping on the grain boundary in the PHWR [7]. The simulation results show that fission products raise with grain diameter in the VVER [8]. After the Fukushima accident, the international community recognized the need for advanced fuel designs that offer substantially improved performance under severe accidents, leading to the U.S. Department of Energy's Nuclear Energy Office (DOE-NE) launching the Accident Tolerant Fuel Campaign (ATF Campaign). The safety margin of nuclear power plants (NPPs) can be enhanced by improving the oxidation-resistant performance of fuel cladding. There are primarily three types of advanced fuel cladding. The first is Cr-coated Zr-based cladding, which possesses several merits. It can improve the peak cladding temperature limit during the design of basic LOCA and extend the time of before post-quench ductility failure occurs [9]. Meanwhile, Chromium coated on ATF is viewed as one of the most promising forms of ATF cladding. In terms of its resistance to high-temperature oxidation and deformation, the retention of fission products, and heat transfer between the coolant and cladding, the performance of ATF cladding is better than conventional Zr-based cladding [10]. The second type of advanced fuel cladding is FeCrAl alloys that can resist oxidation under high temperature and steam conditions.