In this study, an optimization of the canister loading for used nuclear fuels is performed, solely based on their decay heat values. By taking into account the individual irradiation history for each fuel assembly, as well as with a number of assumptions on the number of used fuels to be disposed, this study indicates that a canister filling fraction of 94% is achievable for both PWR and BWR fuel, thus lowering the number of required canisters compared to a previous study. This work emphasized the need for a precise fuel accounting, including the reprocessed ones, as well as considering the individual irradiation history for all fuel assemblies. Finally, it is demonstrated that a higher filling fraction can be reached by delaying the transfer of used nuclear fuels from storage casks to disposal canisters.