Hmm, good point. Maybe small objects not quite close enough to a hydrogen bomb to be vaporized by the flash, then?
(I'm rather amused by the idea that some manager type evidently thought that a hunk of steel might contain a nuclear explosion in what was in essence a very large cannon!)
[1] https://en.wikipedia.org/wiki/Operation_Plumbbob#Missing_ste...
Quotable quote: "However big you think supernovae are, they're bigger than that."
The following paragraph to that quote is a great demonstration of that point.