Development of a coupled fuel (GIFT) and thermal (COBRA-SFS) analysis code for dry storage analysis and its application for the spent fuel safety analyses

This study introduces an integrated analysis code (GIFT/COBRA-SFS) for spent fuel analysis during dry storage. The integrated code performs dry storage simulations including dry storage cask thermal analysis and fuel analysis based on steady-state operation history. Assuming a reference assembly, dry storage cask, and identical assembly loading, integrated analyses were performed by varying discharge burnups ranging from 50 to 70 MWd/kgU and wet storage periods to ensure PCT remain below 400 degrees C. Within the range of discharge burnup and PCT, hoop stress remained below 90 MPa, mitigating the degradation due to hydride reorientation. Cladding hoop strain results also remained below 3 %, suggesting no failure due to creep rupture. Considering results, impact of discharge burnup and wet storage duration on spent fuel integrity was analyzed. An increase in discharge burnup results in elevated pressure and decay heat due to fission products, increasing cladding deformation and hoop stress during the dry storage of spent nuclear fuel. Reducing heat load by increasing wet storage time effectively decreases temperature and hinders creep deformation. However, fuel pellet swelling during wet storage decreases internal void volume, thereby increasing rod internal pressure. As a result, despite the temperature difference, change of hoop stress is limited with increasing wet storage time. The simulated conditions of this study confirm that there is no apparent threat to dry storage from a regulatory perspective up to discharge burnup of 70 MWd/kgU. Moreover, under the simulated discharge burnups, enhancing dry storage safety is feasible with additional months or years of wet storage, which can alleviate cladding hoop strain.