Wealthy nations will probably keep pace with defenses. Poor nations will probably just bypass them.
The easiest way for North Korea to nuke Tokyo is to just put the bomb on a legitimate seeming boat, and sail it over.
The easiest way for North Korea to nuke Tokyo is to just put the bomb on a legitimate seeming boat, and sail it over.
Maybe the boat will get through; maybe it won't. The ship might be intercepted before it reaches the target. Or security personnel with geiger counters can find it on the dock. And so on. From North Korea's perspective (or the USSR's during the Cold War, or Russia or China now), the uncertainty of the success of such an attack makes it very, very risky to actually deploy, except maybe in a situation where you're already losing the war (and in that scenario, the odds of a successful detection by the target are obviously that much higher).
A ballistic missile, by contrast, cannot be stopped except with great, great difficulty. That's why North Korea has built missiles for its nukes, and not a fleet of cargo ships and fishing boats.
Finding it at the dock is completely useless. You can set the nuke off while still at sea and still destroy half a city.
There is no mechanism for interception of such an attack at the moment.
You need a neutron detector, rather than a Geiger counter, for nuclear material detection by the way.
But the main thing you want nuke for is as a deterrent. Get in a fight with us and we press the button. It's hard to imagine the sneaky boat trick working when North Korea is under blockade by the entire US and Japanese navies. And even if they run the blockade they're going to have trouble getting close to Tokyo.