That said, there might be some other way to make MOX, or some other fuel formulation based on molten salts, metal fuel, nitrides, carbides, etc.
As for economics a big problem is that the LWR works at low temperatures and requires a huge steam turbine and huge heat exchangers. This is apart from the problem that large LWR construction projects frequently go over schedule and budget by a large margin. There is some hope that a smaller reactor like China's ACP-100 can find some simplification at small scale that makes up for the diseconomies of small scale and avoids the project management problems but it will take some kind of water-free reactor that operates at higher temperatures for nuclear to be competitive with natural gas.
MOX can be fed back only once. After that too much Pu-240 accumulates, and that's a non-fissile element that absorbs a lot of neutrons, so it's a neutron poison (see [1]). Pu-240 can't be separated from Pu-239, except using the same methods to separate U-235 from U-238 (but more difficult, since the relative atomic mass difference is much smaller). If you get into the business of isotope separation, you might as well enrich Uranium.
Source? I was aware it was using MOX, but did not see any discussion over whether it was weapons derived, PWR SNF, from the breeding blanket, or twice used MOX
https://nmisite.org/site-history/
The other thing the article does not even mention is meltdowns of operating plants and attacks on or natural disasters affecting dry cask storage and reprocessing plants. Sure, you have to eat or breath the material for it to be dangerous after 600 years, but if a plant melts down or an idiot blows up the spent fuel then it gets into water, food, soil and the air and everyone ingests it, for a very long time thereafter, BEFORE the 600 years is up. Also, keeping something safe for 600 years is something nobody has ever done. Most societies don't even last 600 years. Look at what a mess we made of it just 50 years ago:
https://www.latimes.com/environment/story/2020-07-01/us-says...
And here is a list of how well we've handled nuclear materials so far:
"11 Nuclear meltdowns and Disasters"
https://www.cnbc.com/2011/03/16/11-Nuclear-Meltdowns-and-Dis...
The Zaporizhzhia nuclear plant in Ukraine is currently being shelled. Again. So it seems world peace is a prerequisite, I'd like to see the plan for that. After we've achieved world peace then the 600 year containment experiments can start and we can figure out how to get our engineering to 99.99999% reliability. Smaller, less dangerous, and inherently safer reactor designs are being tried, maybe one of those will work well enough to be able to ramp up nuclear power, but the technology in current use is not good enough.