On January 6, the North Korean government announced that it had successfully carried out its first underground test of a hydrogen bomb. Until now, this claim has not been independently verified and many international experts have cast doubt on it. So what was the bomb that was tested? It appears to have produced a yield that was larger than that of any previous North Korean nuclear test. As I write, North Korea has not announced whether this was a plutonium or uranium device, but the information we have about its nuclear program offers some clues.
http://www.nybooks.com/daily/2016/02/04/north-koreas-mystery-bomb/