I guess I assumed that there must have been a large gap between being useful and being inert

It’s a matter of money and access.

If you can get nuclear fuel, it’s cheaper/easier to buy new.

But it’s not like we can’t just not use money as the sole deciding factor on whether we recycle or bury in a mountain.

But like, say you have 100% pure fuel and use it till it’s 50%, it’s not like you use it from the top down, it’s on an atom by atom basis throughout the fuel. So the more you use it before you refine again, the harder it is for it to be cost effective.

That’s why while we sell the “used” fuel from military ships, the stuff in an civilian reactor gets thrown under a mountain. The military want to keep theirs “topped off” in case new fuel becomes inaccessible.

We could easily change the pipeline to:

Military use > civilian use > refinement > military use

And just keep adding more fissible material as needed.

It might not be “cost effective” but it completely elimates the nuclear waste issue. It just all comes down to the price our leaders put on the environment.

Quick edit:

Obviously refinement isn’t as easy as popping it into a microwave for five minutes, and comes with it’s own energy needs and other things that would effect if we should do this, nothing is a perfect solution.

But if we’re just talking about eliminating nuclear waste, this is a valid path.