[dkbrk]

That's not what "hot" means in this context. "Hot" means "highly radioactive", i.e. high number of decay events per second, high concentration of short half-life isotopes, high power/volume resulting from radioactive decay.

Nuclear reactors do not work off radioactive decay. U-235, for example has a half life of 704 million years. Radioisotope thermal electric generators [0] by contrast do run off radioactive decay, an isotopes used for that application have short half-lives, such as Pu-238 with 87.7 years.

Commercial nuclear reactors use unenriched or minimally enriched fuel. This means that, within a fairly short period of time, the percentage of fissile material in the fuel drops to the point where continuing to use it is no longer economical. At that point the fuel is a mixture of extremely hot fission products, transuranics, unreacted fuel, and non-fissile (but fertile) isotopes such as U-238.

It's not practical to use the decay energy from the fission products for power. What would make much more sense would be to remove the fission products and recycle the fuel that remains into new fuel (for a reactor that's designed to use it). This would be a much more efficient use of mined nuclear fuel (allowing nuclear power to be used for thousands of years), it would vastly reduce the volume of nuclear waste, and it would mean nuclear waste would only be hazardous for decades to centuries.

The US was on the path to this with the Integral Fast Reactor and Pyroprocessing [1] developed by the Argonne National Laboratory. This was killed [2] in 1994 by the Clinton administration. Not for any technical reason, but because it was a "threat to nuclear non-proliferation". How that makes sense when, to the best of my knowledge the process developed by Argonne couldn't be used to produce weapons-grade material, and even if it could the US already had nuclear weapons so it wouldn't be proliferating it to a non-nuclear country, I don't know. But, apparently, since some other forms of nuclear waste reprocessing can be used to generate weapons-grade material (by extracting Pu-239), it was a bad symbol so it had to go.

[0]: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

[1]: https://en.wikipedia.org/w/index.php?title=Integral_fast_rea...

[2]: https://en.wikipedia.org/w/index.php?title=Integral_fast_rea...