Putting the fuel storage pool on the fifth floor of the reactor does seem questionable, especially given the violence of the explosion at reactor 3.
Another "unknown bug" exposed by these events will probably motivate mitigating the risk of hydrogen explosions from hydrogen generated in a partial meltdown. As you can see in the second picture in this link,
the force of the explosion crushed the concrete structure of the adjacent reactor building, and launched enough material into the air that it created holes 60 feet in diameter in the roof of the turbine building when it came down. This was not anticipated; if it were the reactors would have been constructed further apart.
The spent fuel rods must be actively cooled for many months after they are removed from the reactor due to heat-generating radioactive daughter products. Since the pools holding them have either boiled away due to the absence of active cooling or drained into the basement due to damage, it is likely these rods overheated. Japanese authorities have mentioned the possibility of "re-criticality", which would be a bad thing, especially in a fuel pond with no containment whatsoever.
There will probably be many lessons gained from these events that will be used to reduce risks in other operating reactors and future designs.
Another "unknown bug" exposed by these events will probably motivate mitigating the risk of hydrogen explosions from hydrogen generated in a partial meltdown. As you can see in the second picture in this link,
http://news.ycombinator.com/item?id=2399676
the force of the explosion crushed the concrete structure of the adjacent reactor building, and launched enough material into the air that it created holes 60 feet in diameter in the roof of the turbine building when it came down. This was not anticipated; if it were the reactors would have been constructed further apart.
The spent fuel rods must be actively cooled for many months after they are removed from the reactor due to heat-generating radioactive daughter products. Since the pools holding them have either boiled away due to the absence of active cooling or drained into the basement due to damage, it is likely these rods overheated. Japanese authorities have mentioned the possibility of "re-criticality", which would be a bad thing, especially in a fuel pond with no containment whatsoever.
There will probably be many lessons gained from these events that will be used to reduce risks in other operating reactors and future designs.