Fuel for nuclear reactors is mostly made of uranium oxide containing a few percent of the uranium-235 isotope. It's made into rods which make it easy to slide into and out of the reactor. When most of this uranium has been used, the fuel no longer heats up enough to be useful. The fuel is 'spent' and is pulled out of the reactor. This is where the problems start. The fuel rods are still very hot and remain that way for some time, so they have to be cooled in special cooling ponds. The heat is created by all the radioactive isotopes which form in the fuel rods during the nuclear fission process inside the reactor. These isotopes are unstable and as they break down into more stable elements, they release heat and enormous amounts of radiation. So the spent fuel rods have to be kept well away from all living things. The cooling ponds are made of thick concrete and the spent fuel is stored deep under cool water. Both concrete and water absorb radiation.

But what happens next? The spent fuel stores can't just be left. The waste needs to be put somewhere safe. One option seems to be to bury it deep underground in such a way that it will be kept out of the environment for thousands of years. At present, nobody can agree where these waste repositories should be built... so the stores of hot fuel rods continue to grow. But there are now other options which include 'burning' up the dangerous waste in special reactors.