system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If uranium is radioactive for millions of years, why do nuclear reactors go through spent rods so quickly?
gpt
think of it like burning wood vs burning cordite(gunpowder). One is a raw material which releases its chemical energy slowly, the other is a well refined, chemically similar product (carbon) which burns very quickly.

Uranium decaying naturally releases its energy very slowly. Refined into nuclear fuel it releases its energy much faster (though still in a controllable way, unlike in a nuclear weapon)

Whilst theoretically possible, there is a very low chance of finding naturally-occuring uranium which wouldn't need refinement. Similar to the unlikelihood of finding naturally occurring cordite.

TL:DR, We make the Uranium get "burnt" much faster than would happen naturally.