Nuke missile silo fire went undetected
and I don't find any posting on FR..either....not sure what the exact date was...here is the article date....October 31, 2008.
LOL - a shotgun nuke is a sub-critical mass nuke, where the bit needed to make it bigger than critical mass is blown in by explosives. Effectively, you have a mass of uranium that is too small to go off, and you shoot enough extra at it with a shotgun, to make it go off.
Hence, shotgun nuke.
That is how the did it in WWII. And the yeild was entirely dependent on the mass of stuff involved.
More commonly today, they just tickle it in the right place, and interesting things happen.