I'm reasonably certain most nations coast guards board and search ships in that condition prior to taking them into harbor. Obviously, there are ways around any security, but it's not quite as easy as "load nuclear weapon on a ship and sail it to the US". A cargo container on some kind of freighter would be the most likely means of delivery, but they'd almost certainly have to pass the container through a few hands (and nations) to get it there with any chance of not being searched. And that process itself increases the odds of such a thing being randomly discovered. I'm sure you're aware that radiation detection devices can be ridiculously sensitive, it's just a matter of whether they're employed (and how). Also realize that the kinds of countermeasures that would be required to conceal such a weapon would also be unusual enough to potentially be picked up (why is this one container opaque to pretty much every frequency of RF?).
Don't even need to be anywhere near a harbor really, lots of populated areas right next to land. Sneaking up on the US would be tricky, but sneaking into range of South Korea? Not difficult at all. Martime officials usually sleep sometime, and night inspections are rare unless they think you are already sneaking something. Would they ignore a north Korea flagged freighter? probably not, but what if it's flying a Japanese flag and has a fresh coat of the finest paint North Korea can buy? Doesn't ever need to exit north Korean control or even be at sea longer than a few hours to get from Pyongyang to somewhere "interesting" at night.
Most radiation detection equipment decreases in sensitivity logrithmically the further you are from the source. We can detect detonation from space easily, or from land anywhere on the planet when the neutron surge happens, but tracking an inert soft ball sized piece of plutonium inside a large lead and carbon box inside a steel cargo freighter at anything
than a mile isn't happeneing. Too many potential background radiation sources.
And even if you avoid detection, how do you set off the bomb? Can't radio detonate it, cause anything capable of preventing the bomb from being detected will also prevent radio waves from penetrating. Any physical trigger (like on the door) runs a risk of detonating prematurely (those containers aren't exactly handled with care). Again, these are surmountable problems, but I'm just pointing out that the kinds of things a terrorist group might try a nation might not precisely because the terrorist group can afford to fail as many times as it takes, while a nation like NK really only gets one shot at doing something like this. I'm not saying that this isn't something to be concerned about, but I just don't see NK going this route anyway. I don't think they want to blow anyone up. They just want people to think they might.
Then again, there's no full accounting for crazy.
Thats an easy one. Physical hardline from the bomb to an external antenna / cell phone. And if you can build the infrastructure to produce a functional nuclear weapon, you can probably figure out a simple remote relay switch. One would hope anyways.
North Korea probably wouldn't try that, and they probably wouldn't try it on us, but against South Korea, who knows. And North Korea is about as zany as they come these days. Who knew Kim Jungle was the stable sane one of the family??? Edited, Mar 12th 2013 11:30pm by Kaolian