If your nuclear waste is that "hot" (highly radioactive), then you're not draining as much energy out of the material.
Ideally, the closer to lead you are, the more energy is drained out. And that's what I'd hope to see from the mining required for uranium. Instead, we seem to run it once and then go "whoop have to stop".
(1) the radiation levels increase, making it more dangerous for workers to do maintenance and safety operations (so overall reactor design would need to take this into account),
(2) some fission products and transuranics (plutonium etc.) act as neutron poisons (absorbing neutrons rather than generating energy-productive fission events), resulting in reduced reactor output and instability issues as the accumulate in the fuel rods.
(3) since you are actually transmuting elements, physical and chemical changes take place resulting in fuel rod corrosion and degradation, swelling and structural changes, etc. which could lead to catastrophic failure by clogging the reactor core and preventing coolant circulation.
[edit] note that reprocessing the spent fuel (plutonium recovery) for use in MOX reactors is possible, but since this is also how you run a nuclear weapons program, there are additional concerns and the costs are really high:
https://world-nuclear.org/information-library/nuclear-fuel-c...
In spent nuclear fuel 96% of the mass is the remaining uranium is still usable. Spend nuclear fuel (=LEU) used in these reactors have almost all potential energy left.
You can use them if you close the nuclear fuel cycle with fast neutron reactors. The downside of it is that they FNR's are also good for making fuel for nuclear weapons.
Reprocessing can recover some plutonium, which can be reused (once more) in MOX fuel. Spent MOX fuel cannot be reprocessed further for use in today's thermal reactors, though.
Legality might depend which country you live in, and the stores I've seen sell only tiny quantities for high prices because they can (only collectors really want it and they will pay), but you can totally just buy some.
What you're referring to is the fact that some reactors (aka breeder reactors, fast or thermal) can fission a much higher percentage of the total atoms loaded into the reactor, including most of the majority isotope (U-238) in addition to the minority (U-235).
I wrote a bit about breeding and recycling here: https://whatisnuclear.com/recycling.html
Well then maybe you should talk about the problem at hand, which is the stuff people are considering "nuclear waste" and wanting to bury.