High level radioactive waste generated from reprocessing of spent fuel from nuclear reactors are encased in canisters after vitrification. They have high heat generation rate and need interim storage under surveillance and are to be cooled continuously until major portion of the heat is dissipated. Natural circulation air cooling (using suitable stack dimensions) has been considered to cool the overpacks containing canisters. Thermal analysis has been carried out for a reduced scale model of such a facility. Theoretical and experimental results have been compared.