Enrichment handling of non-U235/U238 material - product and tails should work the same way

[nuclearkatie]

Currently, enrichment strips all material that is not U-235 or U-238 and sends it to the tails buffer.

I understand that enrichment is not set up to handle fluorine (maybe it should, though?), and for this reason it makes sense to me that the fluorine is stripped from the product. I think that the tails should act the same way, where the tails are purely U-235/8, and any other material should go to an additional "waste" or "other" buffer that doesn't currently exist.

Right now, the product gets the enriched material, the tails gets the depleted material, and then the tails also gets any other material that enrichment doesn't know how to handle. So if you give Enrichment UF6, you end up with product of pure uranium and tails of UFx, with the total amount of fluorine from the feed (i.e. x>6 in an unphysical way).

I expect this hasn't been an issue for anyone who hasn't used the tails in their fuel cycle. It's not too hard to mitigate as-is, can be resolved by placing the tails in a Separation stream to get pure U and then re-mixing with the correct amount of fluorine.

But I think treating the tails like the product stream would be more consistent and would be nicer on anyone trying to use their tails for anything besides waste.

[gonuke]

Since UF6 is so common in enrichment, I think we should probably handle that better, but definitely consistently!