The fuel, uranium oxide, is compressed into small pellets which are stacked inside zirconium metal fuel rods. The rods are then bundled together into assemblies.
Spent fuel assemblies appear outwardly identical to new ones, but the composition of the uranium pellets has changed. Spent fuel removed from the reactor emits high levels of radiation. Over time, uranium, associated fission products, and transuranium elements gradually decay into other elements and finally into non-radioactive elements. Some elements only take a few seconds to decay, while others need billions of years.
Most of the radioactive fission products generated in the fuel are short-lived. The radioactivity of the fuel decreases to approximately one hundredth in one year and to a thousandth in forty years. The nature of the radiation also changes as time passes. To begin with, the most significant form of radiation is penetrating radiation. Over the long term, radiation emitted by heavy elements, such as uranium, which is not penetrating in nature, takes over. The radioactive elements remaining at this point are only toxic to humans if ingested or inhaled.