Improvement of clad deformation model in MARS-KS with physical creep characteristics for mechanistic rod ballooning analysis

MARS-KS, a regulatory confirmatory code of the Republic of Korea, models fuel rods using simplified heat structures, which limits its capability to represent clad deformation and associated flow blockage under LOCA(Loss-of-Coolant Accident) conditions. To address this limitation, a new clad deformation model was developed to replace its original model, which evaluated effective deformation based on burst correlation. The new model was designed to implement elastoplastic deformation of fuel clad, including thermal deformation, by evaluating true stresses with thick-wall formula, and model time- and temperature-dependent plastic deformation through the Norton-Bailey creep relation to mechanistically describe ballooning behavior. The performance of the developed model was evaluated against DIMAT out-of-pile rod ballooning test, and the results were compared with those of the original model to confirm both improvements and remaining limitations. The results clearly revealed that the original model in MARS-KS was not capable of reproducing continuous creep deformation observed in the experiment, instead predicting discontinuous deformation governed by the burst correlation. In contrast, the new model reproduced creep-dominated ballooning with good fidelity in the anisotropic -phase region when true stress was used, whereas the use of engineering stress, as in the original model, led to inadequate strain behavior. However, significant discrepancies remained in the + transition region due to the assumed linear temperature dependence of creep parameters, which prevented accurate representation of the inherently nonlinear creep characteristics in this regime. In addition, both the original and new models systematically overpredicted burst strain owing to the conservatism of the employed NUREG-0630 model. These results indicate the limitations of the present models and, thereby, the need for further improvements to refine both creep properties in the transition region and burst criteria for best-estimate LOCA and fuel performance analyses.