Abstract: Based on the design characteristics, operational strategies, as well as the complicated fuel assembly designs, factors leading to a harder neutron spectrum are considered and a combination of operational parameters is identified to obtain the conservative isotopic inventories. Criticality analysis models are built, and factors such as the end effect and the credible accident conditions are studied in detail. Finally, a primary（criticality） design of the high density spent fuel storage rack is developed which meets the most updated（criticality） criteria and can be used in NPP projects.