A method of storing high level radioactive waste comprising: a) positioning a metal canister containing high level radioactive waste in a storage cavity of a ventilated system comprising a cask body , a cask lid positioned atop the cask body, at least one outlet duct extending from a top of the storage cavity to an ambient atmosphere, and a plurality of inlet ducts , the plurality of inlet ducts having a combined cross-sectional area that is a sum of a cross-sectional area of each of the inlet ducts of the plurality of inlet ducts; b) convectively flowing ventilation air from the ambient atmosphere into a bottom of the storage cavity through the inlet ducts , the ventilation air being heated at a heating rate and rising from the bottom of the storage cavity to the top of the storage cavity, the ventilation air exiting the storage cavity through the at least one outlet duct ; and c) progressively reducing the combined cross-sectional area of the plurality of inlet ducts over time to reduce an amount of the ventilation air that enters the storage cavity and maintain the heating rate of the ventilation air above a predetermined threshold; characterised in that step c) comprises: c-1) positioning a first air flow adjustment member in at least one of the plurality of inlet ducts , the first air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a first percentage; and c-2) replacing the first air flow adjustment member with a second air flow adjustment member , the second air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a second percentage that is greater than the first percentage. a) positioning a metal canister containing high level radioactive waste in a storage cavity of a ventilated system comprising a cask body , a cask lid positioned atop the cask body, at least one outlet duct extending from a top of the storage cavity to an ambient atmosphere, and a plurality of inlet ducts , the plurality of inlet ducts having a combined cross-sectional area that is a sum of a cross-sectional area of each of the inlet ducts of the plurality of inlet ducts; b) convectively flowing ventilation air from the ambient atmosphere into a bottom of the storage cavity through the inlet ducts , the ventilation air being heated at a heating rate and rising from the bottom of the storage cavity to the top of the storage cavity, the ventilation air exiting the storage cavity through the at least one outlet duct ; and c) progressively reducing the combined cross-sectional area of the plurality of inlet ducts over time to reduce an amount of the ventilation air that enters the storage cavity and maintain the heating rate of the ventilation air above a predetermined threshold; characterised in that step c) comprises: c-1) positioning a first air flow adjustment member in at least one of the plurality of inlet ducts , the first air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a first percentage; and c-2) replacing the first air flow adjustment member with a second air flow adjustment member , the second air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a second percentage that is greater than the first percentage. c-1) positioning a first air flow adjustment member in at least one of the plurality of inlet ducts , the first air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a first percentage; and c-2) replacing the first air flow adjustment member with a second air flow adjustment member , the second air flow adjustment member reducing the cross-sectional area of the at least one of the plurality of inlet ducts by a second percentage that is greater than the first percentage. <EOS>