> The brine came from the ocean.
Sure, and enriched uranium comes from the ground, but that doesn't mean it's safe to dump it back in after the enrichment process!
> So just dilute it back to close to ambient salinity using municipal waste water…
Wouldn't it generally be easier to process that municipal waste water, as is already fairly common?
The analogy would be if you "un-enrich" it. Then it's safe. Or at least no worse than when you took it out of the ground.
Enriched uranium is perfectly safe to dump but it would be stupid to do so. Fission products are nasty but uranium itself is not, comparatively.
> Sure, and enriched uranium comes from the ground
Uranium can also come from the ocean water (there is, apparently, quite a lot of it in there, relatively speaking). Japan experimented with the technology in the nineties, but it really was much cheaper to just mine it from the ground, so they abandoned it.