Call it a weapon of less-than-mass destruction.
At half a kiloton yield, that was a fizzle -- an incomplete implosion of the nuclear core caused by a faulty design.
This shows that the state of NK's nuclear program is actually pretty primative. A low-yield device of this sort could be built at almost any university if they had the plutonium.
I wouldn't sleep very well if I were the head of their nuclear design team.
We won't know what the source of the blast was until a western laboratory "sniffs" some vented radioactive materials from an airborne sample.
One way or the other, that wasn't a very impressive entry into the nuclear club.
This isn't the Cuban Missile Crisis -- the Russians had thousands of H-bombs, some as large as 50 megatonnes - don't let them stampede you into believing that this is an existential crisis, unless the Bush Administration turns it into one.
It's more of a display of bad judgment, a gift from one twisted dynastic son to another.