FBaggins writes:"Much of it is thought to have boiled off... not leaked out" Re: out of the spent fuel pools.
Yes, boiled off. The pools overheated and boiled off the water. Then when the water level went below the tops of the rods the rods reacted with the air and began burning. Releasing radiation.
Remember the small crane they had outside the fuel pool spraying water on the pool to fill the pool back up to cover the rods? That proves your point that the pools overheated and boiled off the water. It was so bad they had to bring in that pump.