Nuclear reactor fuel (primarily uranium, but also thorium) is recovered from suitable mineral deposits. Processing then occurs to produce usable fuel products. These are then “burnt” in a reactor. The irradiated fuel (also called spent fuel) can then be disposed of directly, or recycled to extract fresh fuel material.
The term “cycle” is used because, in principle, the fuel can be recycled: usable uranium and plutonium are recovered from spent fuel and fabricated into new fuel. In practice, many countries operate an open fuel cycle (also called a once-through cycle) where spent fuel is stored and eventually disposed of without reprocessing.