USA cannot be nuked. No country in the world can target the US with ICBMs and no country besides the US still has operation nuclear submarines on the US coastline.
As bad as it sounds, if North Korea nuked anybody it would be Japan, simply because it's the heart of US Pacific naval operations and it's the USA's single biggest Asian ally. Even so, China would be given an excuse to annex North Korea and seize their nuclear weapons; China expands it's borders and strengthens it's nuclear arsenal, and they would look like heroes. It'd be a field day for them.
North Korea cannot and will not ever do anything with their nukes that won't result in the total destruction of their nation.