Since their birth in 1960, fast reactors have been attracting increasing attention around the world because they can provide efficient, safe, and sustainable energy. The closed fuel cycle of fast reactors can support the long-term development of the nuclear power as part of the world's future energy structure and reduce the burden of nuclear waste. Thus, the fast reactor has become one of the development directions of global fourth-generation nuclear power.
I remind readers that Fermi I in Monroe MI was one of these. We had a couple more as well. Japan had one operating, as have a few others. None of those still are operating and yet this technology, which we know works, is a required component of a closed fuel cycle and sane disposal and handling of nuclear fuel.
Why?
Because using either uranium or plutonium (or for that matter Thorium if you breed it; it is fertile, not fissile) produces Actinide byproducts and those are very, very long-lived radioactive nasty things. There is only one sane option for them since trying to bury or otherwise keep them safe (e.g. out of the environment) requires tens of thousands of years of confinement in many cases.
The only sane option is to separate them out of the spent fuel, which fortunately can be done chemically as they're all distinct elements (you don't need centrifuges and turning them into a gas first, as you do with Uranium) and if you put them back into a fuel pin and stick that back in a reactor along with more active fuel they will be burned up by neutron bombardment and turned into less-dangerous radioactive elements over time.
They also release a little more energy but you don't do it for that reason, you do it because the only sane place for something so dangerous is where what's in there is so dangerous anyway that it doesn't make it worse. A fuel pin, in an operating reactor, is already full of ridiculously dangerous stuff and in addition its behind a lot of shielding and physical protection from various physical insults (e.g. terrorist attack, etc.)
Each cycle of said fuel, as you reprocess it, results in more and more of that material being turned into lighter, shorter-lived radioactive elements. The shorter the better; when you get to things that have half-lives measured in single and double-digit years now we're talking about reasonable confinement requirements that we know how to handle.
The other part of it is that in a commercial, large-scale power reactor (either PWR or BWR) only 5% of the fuel pin contents are fissile. This is both a safety thing and a requirement thing -- fissile material is expensive and putting more of it into there than you actually can burn up between refueling cycles is stupid. The rest of the space has to be consumed with something and your choices are U-238 (the most-abundant natural isotope of uranium) or spent fuel material that has been separated out from usable plutonium and is extremely dangerous. The latter is the obvious wise choice if you have a surplus of it (and you always do when using nuclear fission for power) because you have to do something with it. In addition in that spent fuel there is both U-238 that did not transmute and Plutonium in a few isotopes that did. Its very hard (and very dangerous) to separate out the Plutonium isotopes if you want to make bombs because only one of them is usable for that -- the others actually poison the bomb in that they make it "fizz" rather than "boom." But for power purposes you don't care because you don't want it to go boom anyway so for power purposes chemical separation, which again is much easier and cheaper, is just fine.
If you recycle the fuel like this you can reach nearly 100% utilization although that's probably stupidly-expensive. Reasonably-economic projections are around 60%. Contrast this with 5% or less with a "once through and done" system which is what we're doing now and I think you can figure out which is the wiser choice.
We stopped this progress in the United States because Jimmy Carter issued an E/O banning reprocessing. Those firms who had invested in it lost their money, and that Reagan reversed the order was irrelevant to them: The government had demonstrated that it would screw them out of their money and they had no interest in that happening again, quite-obviously, so they didn't do it again.
Can we fix this? Of course. Should/must we fix this? Absolutely.
Would I prefer that we head toward Thorium-based fuel? Yep. Why? Because it is in coal and a high-temperature reactor (e.g. LFTR), using thorium as a fuel, brings both the capacity to generate electricity and turn the coal into synfuel which solves two problems at once because the cause of lung cancer from coal use comes from the thorium that naturally occurs within the coal and it is trivially able to be separated out as it is both metallic and much heavier than the carbon.
Nonetheless that China has decided to pursue closing the fuel-cycle for nuclear fission and we are not will turn into a significant disparity in energy policy and outcomes -- and its a disparity we cannot afford to let them have, particularly when we have more than 50 years of time under our belt in knowing how to do so ourselves.