...even as the risk of meltdown is falling? New Scientist has the scoop and it proves yet again that it's not the problem you expect that gets you - it's the one that you didn't realize was coming. Or the one caused by a complex series of unlikely events.
The pools are housed on the top floors of the reactor buildings. Spent fuel rods are transferred to them as soon as they come out of the reactor itself, and are kept under water to cool them down and trap the radioactive material within them. Once they have cooled down enough, the rods are then transferred to outdoor pools for long-term storage.
It is the pools inside the reactor buildings that are causing the problem. Two of the reactor buildings - 1 and 3 - have lost parts of their roofs, thanks to the hydrogen explosions that have taken place over the last few days. While these explosions apparently did not damage the reactors within, they have left the pools exposed to the outside air.