As of right now (1500GMT 9 Oct 2006), North Korea's claims to have detonated a nuclear test hasn't been confirmed by outside sources. According to South Korean seismic monitoring, there was indeed a tremblor of magnitude 3.8 Richter at the time mentioned, and that the signature indicated an artificial shock rather than a fault slip; however, the the estimated blast size was in the half-kiloton range.
Now, my question is this: given the technology North Korea is thought to have, is this plausible? As I understand it, even with a pit-implosive design (which is certainly what they would have used - gun-assembly bombs, despite being simpler both to build and understand, were pretty much a dead end development after the late 1940s), there are lower practical size limits as well as upper ones. While it is certain possible to build sub-kiloton fission bombs (the US developed and tested one design, the Davy Crockett tactical system, which had a 'dial-a-yield' setting from 40 to 100 tons), it is my understanding that at the point of technology development they are thought to have reached, it is actually easier to build one in the 15-30 kiloton range (especially if it is a stationary test rather than a deliverable weapon) than one below 5 kilotons. Is this correct?
The half-kiloton size sounds suspiciously close to what could be achieved with conventional explosives - an enormous amount of conventional explosives, but still. After all, larger non-nuclear explosions have occurred, both intentionally (e.g., Ripple Rock) and accidentally (Halifax 1918, Port Chicago 1944*, Texas City 1947, PEPCON 1986, etc.). Indeed, the largest such explosion was the mysterious one in North Korea herself a few years ago, which at the time caused a lot of speculation about nuclear testing. Is NK trying to hoax everyone? If not, does this mean that they are further along than previously assumed? Or am I simply wrong?
* Yes, I know about the Port Chicago nuclear-bomb-test conspiracy theory, and IMAO it's pure bunk. There was less than 300 grams of refined fissile material in the entire world at the time, and they were still working out the details of a practical assembly design. The B-29 bomber, the only plane capable of carrying the 10-ton bombs of the time, was still in it's final testing stages, and could not have been used to deliver a bomb - and a submarine, the other likely delivery method, could not be certain of reaching any target port. The Allies still didn't hold any islands close enough to Japan to stage such an attack, and in any case Germany was still the priority target - thus, any bomb leaving the US would have been going from the East coast, not the West. Given that the Western front forces were still bottled up in Normandy, and it wasn't certain that they wouldn't be driven back into the Channel, chances are that they wouldn't have bother testing a bomb if they had one, but would have used it against Berlin immediately. The same arguments can be made against the similar theories regarding the Dresden firebombing.