A nuclear fuel storage pool has to be designed to ensure subcriticality under any conditions. Within this framework, a peculiar moderation phenomenon that yields supercritical states from accidental (though very unlikely) circumstances is analyzed. Namely, in an overmoderated spent fuel pool, a reduction in the water density can lead to an increase in reactivity. Equally, a dry storage might be accidentally filled with water mist or foam, leading to a critical state. A numerical assessment is presented to point out the phenomenon and to clarify it. The dependence of results upon calculation methodologies and assumptions is also analyzed. The conclusion is reached that current methods with a slightly large number of energy groups should be used in this task. It is also found that poisoning the storage with a strong neutron absorber should avoid the aforementioned super-criticality.