Japan's nuclear disaster highlights a contentious and still unresolved issue: how best to manage and dispose of highly radioactive used fuel from reactors that generate electricity.
The explosions, fires and radiation leaks at the Fukushima plant not only involve over-heating in some of the reactor cores. Most of the radioactive material at the site is uranium fuel that has been "burned" in reactors, then removed and put in temporary storage ponds to cool.
In Japan and the nearly 40 other countries in Asia, North America and Europe that operate nuclear power reactors, the amount of spent fuel has been mounting since the first commercial plants started in the 1950s. There are now about 270,000 tons of used fuel in storage, much of it at reactor sites where critics say safety and security needs to be improved.