> Unlike France, the US did not use a closed-loop multi-grade fuel cycle for economic reason

Those are the existing reactors. The premise is building new ones of a different design.

> It is more complex, as running on low-grade fuel is not the same as running on spent-fuel.

It has to be accounted for but it's not some kind of impossible sorcery.

> The US shuttered their own facility a long time back, but it is unclear why the research was effectively abandoned. There probably was a legitimate reason, but who knows for sure.

There is a lot of politics involved in energy in general and nuclear in particular.