Cooling by natural convection
Spent fuel and high-level waste are hot, though their heat diminishes considerably over time. When this waste is in storage, the heat it releases has to be removed. Over very long periods of time and with limited monitoring, it would not be possible to rely on cooling in a pool. Dry cooling, by natural convection, would have to be used. The packages would be stacked in shafts. The air would circulate passively due to the stack effect.