Why can’t we dump nuclear waste in the same place we mine nuclear fuel?
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Not all uranium is the same. Spent uranium fuel is “spent” because all the fissile uranium has decayed already. Fissile uranium is U^238 which means there are 92 protons and 146 neutrons in this isotope of uranium. This is the most common form of uranium [*in* Earth about 99.3%](https://www.radioactivity.eu.com/site/pages/Uranium_238_235.htm#:~:text=The%20half%2Dlife%20of%20uranium,and%20only%200.70%25%20uranium%20235). Enriching uranium gets you to a higher ratio of U-235 to U-238.
The fissile kind of uranium is U^235 which means 92 protons and 143 neutrons in the nucleus. [This isotope is fissile because when it is hit by a neutron the now U^236 fissions into barium-141 and krypton-92](https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_%28Petrucci_et_al.%29/25%3A_Nuclear_Chemistry/25.08%3A_Nuclear_Fission).
As the fissile uranium is used in fission less of U-235 exists in the fuel and eventually there’s a point of diminishing returns and at this point most of the fuel is U-238. So at this point the spent material no longer has sufficient U-235 to be useful for fission and is instead used in [military applications as armor piercing rounds, tank armor](https://www.epa.gov/radtown/depleted-uranium).
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