Countries working on nuclear weapons usually first develop nuclear weapons that use fission to break large atoms like uranium or plutonium into smaller atoms, creating considerable energy. A subsequent step is to develop fusion weapons, where small atoms like hydrogen are combined to generate immense amounts of energy. Nuclear weapons that primarily use fusion are thus called hydrogen bombs.
The size of the explosion, known as the yield, is usually measured relative to how much TNT would be required to create a comparable explosion. Nuclear weapons based on fission typically have a yield of around 10 kilotons or so, while nuclear weapons employing fusion can have a yield measured in megatons. (A kiloton is 1,000 tons; a megaton is 1,000 kilotons.)
Mastering the technologies and engineering of nuclear weapons is not easy. It takes many years for countries to independently develop fission weapons, and years more (about seven for the United States) between the first test of a fission weapon and the first test of a fusion weapon.
North Korea appears to have had a difficult time mastering even the basics of a fission weapon, starting with a first test of apparently less than one kiloton and achieving only about 10 kilotons by its third nuclear test, seven years later. This suggests that unless North Korea has had help from outside experts, it is unlikely that it has really achieved a hydrogen/fusion bomb since its last nuclear test, just short of three years ago.
With this in mind, it is quite possible that Kim's claim could be untrue, which would come as no surprise to those familiar with the regime's saber rattling. But there is one other possibility. En route to the development of fusion weapons, some countries develop so-called "boosted" weapons, which use a small amount of fusion to boost the fission process, causing more large atoms to fission and thus releasing more energy -- initially, perhaps a weapon of 50 kilotons or so. Because some fusion is involved in such a weapon, Kim may be claiming that he has achieved a hydrogen bomb when in practice he only has a boosted weapon.
What difference would a boosted nuclear weapon make? If North Korea really has a boosted nuclear weapon of perhaps 50 kilotons, it could do significant damage in a city as densely populated as Seoul, South Korea: About 250,000 people could be killed in such a strike, or about 2.5% of the population. This would mark a genuine advance in the level of damage that North Korea is capable of doing. And if North Korea one day produces a true hydrogen bomb of, say, one megaton yield, then it would be deadlier still.
Still, just as interesting as the question of whether North Korea has achieved a true hydrogen bomb (unlikely for now), or a boosted weapon, is why Kim would want to make such a public claim in the first place. Obviously we don't know for sure -- it is difficult to get accurate information from probably the most reclusive state in the world. But we can make some informed guesses.
Kim has been continuing his purges of senior leadership inside North Korea, a potentially destabilizing activity. Just this summer he reportedly had one of his vice premiers killed
, one of what's suspected to be dozens of officials executed since he took power in December 2011. Meanwhile, life in North Korea is still miserable for most of the population
This poses a challenge for Kim in a country where the leadership culture demands a powerful leader, one capable of achieving great accomplishments. So it is not surprising that he needs to periodically demonstrate his power. His claim that he has achieved a major advance in nuclear weaponry could be just such a demonstration, focused significantly on his internal audience. He wants to establish North Korea as a nuclear power, as written now in the North Korean Constitution, and thus a peer of the United States and the other nuclear powers.
But this latest announcement is likely not just for domestic consumption. Kim also wants to demonstrate his power to the outside world, and not just for personal prestige. After all, North Korea consistently violates international norms, from its human rights abuses to its ongoing arms trade. Such moves invite international condemnation and pressure, which further threaten the regime's hold on power. In order to deter military action (something that is actually unlikely, but perhaps not so in the North Korean leader's mind) and other coercive measures like economic sanctions, Kim can therefore be expected to talk up the damage that his regime could inflict.
Whether this latest claim is accurate -- or perhaps an exaggeration -- remains to be seen. But the reason for making it seems very clear.