Hecker estimates that North Korea currently has enough fissile material for eighteen bombs, with the capacity to produce six or seven a year. So far the North Koreans have tested four devices, the first three of which certainly used plutonium. The first test, in October 2006, produced a one kiloton explosion.
More likely, the North Korean bomb was what is known as a “boosted device.” It is initiated by a fission explosion, which causes fusion with the production of very energetic neutrons that cause more fission. (These “fission-fusion-fission†bombs are known as three-stage boosted devices.) This enhances the ratios of the yield to weight and volume of the device. The bombs can be made lighter, which makes them ideal for putting on missiles.
As Albright has noted, the January 6 test occurred about seven hundred to eight hundred meters below a mountain, as opposed to the three hundred fifty meters of the previous test. Thus one might conclude that the North Koreans were expecting a high yield.