Abstract: Exampled by the dry storage container CASTOR 1000/19 loading Tianwan Nuclear Power Plant hexagonal spent fuel assembly, the criticality safety of dry storage of hexagonal spent fuel assembly was studied. Based on fresh fuel assumption, the k_eff of the CASTOR 1000/19 loading hexagonal spent fuel assembly fully was calculated at different conditions by using the MONK9A code. The results show that k_eff is far less than the critical safety limit under normal condition, i.e. it is safe. When the ²³⁵U enrichment is greater than 3.15%, the system has critical safety risk at accident conditions. And the loading capacity had to be reduced to ensure the criticality safety. Considering burnup credit (BUC), a reference loading curve was proposed based on the same calculation model, which could ensure criticality safety. The availability of the storage container increases by adopting BUC. The achievements could be reference for spent fuel dry storage scheme in Tianwan Nuclear Power Plant.