In a spent fuel pond exists tons and tons of volatile elements that without a moderator present i.e. exposure to air, heat, shock-waves, vacuums, etc. I’m not sure all the possible scenarios have been explored yet.
Water is one of the main moderators. Spent fuel rods have had about 1/3 of the usable uranium expended. Without reprocessing, it's just being wasted. Regardless of the conditions, it cannot explode like a nuclear bomb. There is almost nothing in the field of science more thoroughly studied than radiation. Look for
Before It's Too Late, A Scientist's Case FOR Nuclear Energy by
Bernard Cohen. Also look for things by Petr Beckmann, such as
THIS.