You understand that you dont need a nuke to destroy a building right? Even small nukes causes nuclear fallout which spreads much farther then the blast radius of the nuke. Nuclear fallout is not a very nice way to die, as it does horrible things to you before you die. In essence, we would NOT use a nuke of any kind on NK because its just the government thats the bad guys, not all of the people living there who cant afford to feed themselves because Dennis Rodman's Pal is a warmonger. Also the nuke would probably effect SK as well.
California got hit by fallout from the Fukishima incident and now babies are being born here with Thyroid problems. I don't North Korea's neighbors want radioactive fallout to rain down on them. Your better off using Tomahawk missiles to take out communication centers, military command posts, and Kim's bunker. Hell that's what Tomahawks are for.
I'm pretty sure that a small sub-launched nuclear missile is cleaner fallout wise than an open nuclear reactor.
To be honest, if the US does use nukes, which I can see happening, it will use them to take out large military installations, where a nuclear warhead would almost just be more practical than a bombardment with Tomahawks. If I were to guess, if the US does use nukes, it will only use about 4 or 5.
Other than that, as I've stated before, I can't see the US using nukes to hit everything.