High-level radioactive waste contains mixtures of radioactive materials from fission reactors.

Many fission byproducts are much more radioactive, with shorter half-lives, than leftover U-235.

Storing spent fuel rods on-site is a temporary solution and can suffer from a catastrophic event such as the tsunami that disabled the Fukushima Daiichi plant in 2011.

A central storage such as one proposed for Yucca Mountain in Nevada faces political and logistical hurdles.