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. A fizzle can also result from the use of reactor grade plutonium rather than weapons-grade material.