Second, they are using air-burst at about 1500'. Overpressure and thermal will wipe the surface clean and create a glass plain. Now, the Oil is deep. I'm not discounting some surface contamination, but the general field should be safe.
Any "Radioactive" oil can be used in reactors where it would get exposure to Gammma anyway...so long as the contamination was below (X), and not of a corrosive/damaging particulate nature. ( I know, I've got contacts that confirm this usage.)
Third, we tend to use Fusion, so the dirty stuff is mostly consumed.
Depends on how recent the nuke is. A Trident is as accurate as any conventional PGM.