Imagine a huge pool supported in the air, the bottom of the pool 50 feet off the ground. And in that pool are stored spent fuel rods from a nuclear reactor. There are many tons of material sitting in that pool. The reason these rods are in that pool is to keep the rods from releasing their heat to the atmosphere.
If the rods were not in water the reaction with the atmosphere would cause fire and explosions. The simple fact is: water puts out fires.
In these pools the water is supposed to circulate. Rod heated water is run through heat exchangers and cooled down and pumped back into the pools.
What happened at Fukushima is that the pipes circulating the water broke due to the earthquake. Also the backup electricity failed and the pumps stopped working. Too, the pumps were inundated by water from the tsunami.
All in all a complete failure. What we do know is that the same systems which keep pools from overheating also failed when it came to the cores and three cores full of these hot-rods have melted down.
What we don't know is the past conditions of the 4 spent fuel pools. There is speculation that the spent fuel pools did lose water like the cores did, did overheat, and did release radiation to the atmosphere.
The big fear now is that the pools of the four reactors may fall to the ground dumping the rods all over the place.
If they could just pick the rods out of the pools with a crane and place them in a pool on the ground, they would, but they can't. That is why the rods are still in the pools 50 feet off the ground, just hanging there.