In the days ahead, we'll hear all kinds of reasons why the Nork nuke test was so sucktastic. (Suitcase bomb, anyone?) Dicky Destiny -- a.k.a. Dr. George Smith, of GlobalSecurity.org -- has a plausible early candidate: "'fizzle yield'; that is, the smallest nuclear yield [a] particular device could provide."
"... [T]here is a moment when the [bomb's] fissile material becomes critical (projectile still on its way to its destination [in a gun-type weapon], or only a small part of the material compressed [in an implosion-type weapon]) and the time it reaches its intended state. During this interval, the degree of supercriticality is building up toward its final value. If a chain reaction were initiated by neutrons from some other source during this period, the yield realized would be much smaller --possibly a great deal smaller -- than the nominal yield. Such an event is referred to as preinitiation (or sometimes predetonation).
... "If the [bomb's] assembly velocities (of the projectile or material driven by an implosion) are quite low, the earliest possible preinitiation could lead to an energy release (equivalent weight of high explosive) not many times larger than the weight of the device."
Other parts of the discussion on bomb design obstacles, also presented at the seminar, indicated that yields lower by a factor of ten in crude designs can be indicative of fizzles. What information has been published on the North Korean test falls into this range.
Summarized, there are certain number of things that can go wrong when firing your first atomic bomb, particularly when using a crude design. And one might expect to see them from a weird and crazy hermit nation, like North Korea, endeavoring to enter the nuclear club.
