Spent fuel rods from nuclear reactors are stil very hot and are cool after use in storage ponds. At Japans Fukushima Daiichi plant there are two sets of ponds, the primary ponds situated above each reactor are used to initially cool the spent rods and usually sit at around 50C. Secondary ponds are located outside and are used to store the cooler rods more long term.
The primary ponds at Fukushima have now reach over 80C and may possibly boil over and expose the spend rods. Since several explosions at the plant have blown the roofs off the reactor buildings, this means the ponds and in turn the spent rods if water levels get low enough, are exposed to the atomosphere presenting another source of contamination to the environment.
With the continuing drama of these power plants over the last few days, i think it had brought new light on the negatives of nuclear power which in many circles is often referred to as green power. Now if the ingeneous Japanese cannot make a nuclear power plant safe then who can?