International scepticism and condemnation have greeted North Korea’s claim to have successfully carried out an underground hydrogen bomb test. If confirmed, it would be North Korea’s fourth nuclear test since 2006 and mark a major upgrade in its capabilities. But nuclear experts have questioned whether the size of the blast was large enough to have been from an H-bomb. UN chief Ban Ki-moon condemned the test “unequivocally”, calling it “profoundly destabilising for regional security”. South Korea called the test a “grave provocation” but said it was difficult to believe it was an H-bomb. Hydrogen bombs are more powerful and technologically advanced than atomic weapons, using fusion - the merging of atoms - to unleash massive amounts of energy. Atomic bombs, like the kind that devastated two Japanese cities in World War Two, use fission, or the splitting of atoms. Bruce Bennett, an analyst with the Rand Corporation, was among those casting doubts on Pyongyang’s test: “The bang they should have gotten would have been 10 times greater than what they’re claiming. “So Kim Jong-un is either lying, saying they did a hydrogen test when they didn’t, they just used a little bit more efficient fission weapon - or the hydrogen part of the test really didn’t work very well or the fission part didn’t work very well.”