MCNPX computer code is used to model the general PWR cask which contain 32 typical PWR spent fuel assemblies. For Safe storage and transportation of the cask , factors that affect the criticality were studied using the concept of burn up credit. Several parameters such as initial fuel enrichment , fuel burnup history , cooling time , and axial burnup profile were analysed. The analysis was performed in two different steps , first burn the fuel assembly at different burnup and storage (cooling ) conditions , secondly, incorporate the details of the assemblies into the cask (canister ) model and perform a criticality calculations for the cask. Several cases of unnormal storage conditions are considered in the case of UO2- PWR only]. In this research high discharged fuel assemblies burn up include Standard UO2 - PWR and next generation MOX fuel. The present results are compared with similar GBC-32 benchmark and satisfactory agreements were found..