North Korea has announced it has "successfully tested its first hydrogen bomb".
If confirmed it would be the country's fourth nuclear weapons test and an escalation of the destructive power at the disposal of leader Kim Jong-Un.
Hydrogen bombs - also known as thermonuclear devices - differ from the first generation of atomic weapons such as those developed during World War II, by employing a two-stage detonation process, rather than just one.
A one-stage device uses a nuclear fission reaction to release destructive energy such as the bomb dropped on Hiroshima by the USA which generated 15 kilotons of energy.
A hydrogen bomb uses the fission energy from this initial blast to set off a nuclear fusion reaction which releases much more energy and a far larger blast.
The largest hydrogen bomb in the US arsenal has a yield of 1.2 megatons.
Another advantage of such a weapon is that they can be more destructive yet smaller in size than a one-stage bomb and can therefore be loaded onto long-range missiles.
Doubts have been raised as to whether or not North Korea's bomb is a fusion device as the reported seismic activity recorded at the test site is the same as a previous test on a fission bomb.
Additionally, North Korea is believed to only possess missiles with a range of 6000km so while China, Russia and Alaska are at risk, London is not within range.
Theoretically however, this is the effect the different types of bombs that North Korea has tested would have if they were detonated above London.
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Emil Dall, Research Analyst in the Proliferation and Nuclear Policy programme at RUSI, said: "There is no realistic prospect of that – experts are still sceptical whether the North Korean’s have yet to complete the miniaturization processes needed to place a nuclear device on a missile of any kind, let alone one capable of reaching anything beyond its immediate region.
"What is significant about today’s events: North Korea is sending a clear message both at home and abroad that its nuclear weapons programme is still very much a priority for the country, and that it is (supposedly) making advancements despite international sanctions regimes in place.
"These statements have been condemned by international powers, including China, but will rightfully cause concern with South Korea and Japan, who are North Korea’s strongest, regional opponents.
"For the UK, the nuclear test and North Korea’s statements that followed, North Korea is in clear violation of international obligations."