> cannot be easily retrieved

This is I guess where the analysis gets fuzzy, where honest assessments may vary widely.

My gut instinct is that enriched uranium blown up underground (assuming it was even there) would take at most around a year to recover, by turning the site into an open-pit uranium mine. The product wouldn't need to be re-enriched from scratch, as simpler mechanical filtering could probably isolate much of the already-enriched uranium.

But perhaps it would be much harder.