As another poster pointed out, this MIT site covers all the bases and updates events although very clinical in descriptions still easy to understand...
Answers to your question about spent fuel rods probably found under the heading: ‘On worst case scenarios
Posted on March 17, 2011 3:55 pm UTC by mitnse’
I also have heard nuke experts wondering why the spent fuel rods are stored on top of the reactors, cost savings for the company has been the mostly likely answer.
Lack of room at the site, is what I have read.
I think it's just an old design. At the link I posted above it explains that they flood the area between the spent fuel pool and the reactor core with water to move fuel into and out of the core. So proximity is needed during this phase.
Newer designs may have below ground level storage for spent fuel.
I've also seen speculation that the pools that are heating up may have small water leaks causing the loss of cooling water.
Likely because the reactors are loaded and unloaded from the top.
So the top is where they are stored. For example reactor 4 was unloaded for maintenance, all the rods were in the storage pool. Having to move the rods a long distance to store them elsewhere would likely greatly increase the possibility of an accident while moving them.
That said, on top still seems dangerous... A melt down in the pool melts down to the reactor below. Though on the other hand, a melt down in the pool is probably worse than a meltdown in the reactor... There’s more fuel in the pool...
Those reactor cube buildings are about 150’ wide (or half a football field) to give some perspective... That’s pretty darn large...
spent fuel rods stored above reactor-
I read that storing them above made it very easy for the robotic system to remove the rods from the reactor and then place them into the pool.
They are now paying DEARLY for that convenience.