France increased its estimate of the blast Monday in North Korea to 1 kiloton or less, but experts still have not confirmed that the explosion was nuclear.And, should you scoff at the Xavier Clement of France's Atomic Energy Commission estimated the blast at "about or less than a kiloton."Frenchyness of my sourcing, Jane's Weekly must be right up your alley:
Although details are tentative, initial and unconfirmed South Korean reports indicate that the test was a fission device with a yield of .55 kT. By comparison the nuclear bomb that was dropped on Hiroshima yielded approximately 12.5 kT. The figure of .55 kT, however, seems too low given the 4.2 register on the Richter scale. This could suggest - depending upon the geological make-up of the test site - a yield of 2-12 kT. If, however, the lower yield is correct, it would suggest that the test had been a "pre- or post-detonation" event (ie a failure), as it had been anticipated that North Korea's first nuclear test would have a significantly higher yield..55 kT? That's a small explosion. There have been non-nuclear explosions as big as 2-5 kilotons, I believe. So, either the North Korean scientists are quite skilled in their enrichment process and lens shaping, or the weapon fizzled. They may have achieved a limited fission event, but if it went off without a hitch they should have hit around 10 kilotons.
If it were a full, to-spec fission explosion, then the amount of nuclear material used would be much lower than is usual for a first attempt. The less fissile material, the tighter a space you have to compress it into in order to achieve a critical mass, and thereby a self-sustaining nuclear reaction. Compressing the fissile material is accomplished by first creating a very homogeneous and precise starting sphere, then compressing it quickly and evenly from all sides with high explosives - the exact kind stolen from the Al Qaqaa weapons facilities in Iraq after the invasion, in fact. There is very little chance the North Koreans are skilled enough to achieve such a feat on their first attempt.