Spent nuclear fuel is used fuel from commercial nuclear power facilities, reactors on nuclear submarines and ships, and university and government research reactors. Most nuclear reactors use solid ceramic pellets of enriched uranium for fuel. The pellets are sealed in strong metal tubes, which are bundled together to form a nuclear fuel assembly.

Depending on the type of reactor, fuel assemblies can be as long as 16 feet and weigh up to 1,200 pounds. After three to five years in a reactor, the fuel is no longer efficient for producing electricity and the assembly is removed. After removal, the assembly (now called "spent nuclear fuel") is highly radioactive and, therefore, requires heavy shielding and remote handling. Some elements in spent nuclear fuel remain radioactive for many thousands of years.
 

Fuel Assembly