Answer:Uranium
80 years
To make that nuclear reaction that makes that heat, those uranium pellets are the fuel. And just like any fuel, it gets used up eventually. Your 12-foot-long fuel rod full of those uranium pellet, lasts about six years in a reactor, until the fission process uses that uranium fuel up.
Transuranic wastes, sometimes called TRU, account for most of the radioactive hazard remaining in high-level waste after 1,000 years. Radioactive isotopes eventually decay, or disintegrate, to harmless materials. Some isotopes decay in hours or even minutes, but others decay very slowly.
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