Abstract Dry cask storage systems (DCSS) is a method of storing high-level radioactive nuclear spent fuel. Due to the decay heat from fission products, effective cooling of the spent fuel is one of the key roles of dry cask storage systems. This study proposes a method to improve the cooling performance of DCSS by adding a small amount of high-density inert gas into helium backfill gas and verifies the method by numerical analysis. First, the candidate group of additive gases that can improve heat transfer in the DCSS environment is selected through figure-of-merit (FOM) analysis based on natural convection heat transfer models. Then, the candidate gases are evaluated using detailed computational fluid dynamics (CFD) modeling without the porous media assumption. From the analysis, it is found that Xe and Kr can reduce the peak cladding temperature (PCT) inside the cask at pressure greater than 1.5 atm. The optimal composition of the additive gas (Xe and Kr) is estimated to be approximately 0.8 in mass fraction. The effect of the additive gases becomes more significant as the pressure increases due to the effect of increased density.