When they are removed from the reactor core, spent fuel rods are too hot and too radioactive to be disposed of, so they are stored in pools of water, which keep the radioactive material cool and prevents human exposure, while constantly circulating the water to cool things down.
One problem at the Fukushima Daiichi plant is that a number of the cooling systems are malfunctioning, allowing the temperature in several spent fuel pools to rise, and the water in at least one of the pools to come to a boil. This is a concern because it’s difficult and dangerous for workers to replenish the boiling water, the spent fuel rods can catch fire, as happened on March 15.
