TOKYO -- North Korea's sixth nuclear test on Sept. 3, which Pyongyang claims involved a hydrogen bomb, showed just how far the country has advanced its weapons technology.
A hydrogen bomb works by detonating a primary fission explosion that causes a nuclear fusion reaction.
From the scale of the ensuing tremor, the Japanese government estimated that the latest explosion was six times more powerful than the previous test a year ago.
Pyongyang conducted its first nuclear test in 2006 under the Kim Jong Il regime. The test involved a plutonium type bomb -- which uses explosives to cause a fission reaction by compressing the plutonium fuel. The country later developed the more powerful boost-type bomb, which explodes through a combination of nuclear fusion and fission.
North Korea also claimed the bomb used in Sunday's test could be mounted on an intercontinental ballistic missile, implying the country has developed the technology to sufficiently miniaturize nuclear bombs for them to be fitted inside warheads.
The latest developments, combined with the increasing range of Pyongyang's missiles and earlier claims it successfully managed to return a warhead into the atmosphere, have raised the threat level to a critical stage.