Easier just to do a conventional detonation in place. Let the 60% uranium flow together and reach at or near critical mass. A Chernobyl-like event 500 feet underground would leave even the unreacted enriched Uranium unrecoverable for - one hell of a long time. There have been mishaps like this on a smaller scale in Plutonium manufacture, where through accident or stupidity plutonium nitrate solutions have been allowed to form a critical shape. The cleanup is long, expensive, and the mixed reaction products cheaper to just bury for a few millennia and start over rather than even attempt a hazardous separation process. We don’t actually need their enriched Uranium if we can just see that they don’t get it for a few thousand years.