> If it can generate enough energy to be dangerous then it probably has an economic use if enough of it can be gathered in one place
This is the basis of the radiothermal generator (RTG); but generally, the spent fuel is deemed spent in the first place because it's no longer emitting enough heat/neutrons to be worth keeping in the reactor. It's already got to the point of "it's no longer worth the hassle of handling this and dealing with all those neutrons/gamma radiation in exchange for a mediocre amount of warmth".
> spent fuel is deemed spent in the first place because it's no longer emitting enough heat/neutrons to be worth keeping in the reactor.
It's in a fission reactor and those isotopes aren't fissile, and aren't a huge proportion of the "spent fuel" to begin with. To be useful it has to be separated.
Short-lived highly radioactive substances are commercially valuable as radiation sources. Medium-lived radioactive substances are useful in RTGs (not fission reactors). Long-lived radioactive substances are often fissile and therefore useful as reactor fuel.
But none of them are useful when they're all mixed together, because what they're each useful for is a different thing. So separate them.
This is the basis of the radiothermal generator (RTG); but generally, the spent fuel is deemed spent in the first place because it's no longer emitting enough heat/neutrons to be worth keeping in the reactor. It's already got to the point of "it's no longer worth the hassle of handling this and dealing with all those neutrons/gamma radiation in exchange for a mediocre amount of warmth".