The objective of this study is to conduct a sensitivity analysis on multiple fuel assembly misloading within the spent nuclear fuel (SNF) cask to evaluate the reactivity change when underburned fuel assemblies are loaded. As the fuel composition changes during burnup, burnup credit has been considered involving several underburned values. The calculations have been performed on the GBC-32 Westinghouse 17 × 17 fuel design involving 1–16 misloaded assemblies with various values of discharge burnups and several different loading patterns. The isotopic depletion and decay analysis have been evaluated using ORIGEN-ARP, while the criticality calculations have been performed using KENO VI Monte Carlo criticality code. The results show that different loading patterns have significant effects on k _eff for each misloaded assembly. The maximum variation of k _eff is 0.12040 for six misloaded assemblies, which have 40 different loading configurations. It was also found that one misloaded assembly is acceptable for all values of underburned fuel from 0% to 70% of the minimum value required for loading, while 0% underburned cases are acceptable to be loaded for 1–16 misloaded assemblies.