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If you want an immutable mapping, why not use an enum?
This precious fuel is so full of energy that it gets very hot ("decay heat").
Fresh reactor fuel is even more full of potential energy but it doesn't get hot because uranium 235 and 238 have very long half lives. Fuel that has been used in a reactor gets hot primarily due to fission products (lighter elements formed when fuel atoms split apart) that undergo faster radioactive decay. There's also some decay heat from the production of transuranic elements (elements heavier than uranium, generated by neutron capture). But the fission product decay heat dwarfs the transuranic element contribution until several decades have passed.
It makes sense to be more afraid of the super duper radioactive material from spent fuel than the slightly radioactive material in brand new fuel. The radiotoxicity is vastly higher, the heat generation complicates handling/storage, and the chemical composition has gained dozens of elements scattered around the periodic table. In terms of usefulness, a fresh fuel rod is like a clean cardboard box and a used rod is more like a cardboard box that held a hot pizza. It's so dirty that it costs more to recycle it into something usable than to just sequester it and start with fresh material.
Except for metal whiskering: https://en.wikipedia.org/wiki/Whisker_(metallurgy)
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