Google "James Woolsey, Michael Hayden & Peter Pry: EMP."  They advised the Congressional Commission on EMP which realized the existential threat North Korea poses to the RoK, Japan, Australia, and the US.  Woolsey & Hayden are former CIA directors.

Clearly, a nuclear bomb could be smuggled in a container.  Only about 5% of them are checked.  

Many experts believe that NK has miniaturized a nuke.  The question is: 'Can it withstand re-entry?'  But that is irrelevant given the other means which I cited.

You are obviously badly informed.  NK has successfully launched missiles from submarines and they could easily do so from a false flag freighter.

"35th in the world?!!?" NK is one of 9 nuclear powers;  the fourth largest army, a massive artillery force, the world's largest diesel electric fleet, and they have one of the most capable cyberwarfare forces;  this potent military is run by a murderous psychopath.

You must enjoy be blissfully unaware of a regime that wants to kill you.