 If the North Korean device was significantly short of its predicted yield, it could be classified as a "fizzle" indicating that some aspect of the nuclear weapon design or material production did not function correctly.In a fizzle the warhead blows itself apart too fast for the nuclear reactions to generate a large amount of energy, or fails to form a supercritical mass for some other reason.A fizzle can result from predetonation, insufficient precision in the explosive lenses used to compress the plutonium core, or impurities in the plutonium itself, among other factors.