North Korea test: what is a miniaturised hydrogen bomb?

North Korea is claiming it has successfully detonated a miniaturised hydrogen bomb.
Tremors were picked up by seismologists in the US, South Korea and China at 10am local time (1.30am GMT) at a known nuclear test site at Punggye-ri and confirmed by state television.
What is a thermonuclear bomb?
According to John Carlson, the former head of the Australian Safeguards and Non-Proliferation Office, a thermonuclear bomb – colloquially known as a hydrogen bomb – is essentially a two-stage explosion: one a nuclear fission reaction, the other a nuclear fusion reaction.
The “primary” explosion is the fission reaction – think, a conventional nuclear bomb – that emits x-rays which cause the “secondary” explosion, triggered by the fusion of tritium and deuterium, two hydrogen isotopes.
These two isotopes naturally repel each other, but the x-rays weaken this repellent force, causing the pair to fuse together.
It is the fusion of these two isotopes that triggers the enormous energy release that makes thermonuclear explosions so destructive.
How does this differ from past North Korean nuclear tests?
Past nuclear tests in 2006, 2009 and 2013 – all at the same site in North Hamgyong province, in the country’s east – were purely the primary stage, a nuclear explosion, “and fairly low yield at that”, Carlson says.
What is the significance of a “miniaturised” bomb?
“If it’s true, it means they’ve made something smaller scale, capable of being put onto a missile. I think we can assume the previous tests they’ve carried out have been devices too large to fit onto a missile,” Carlson says.
“There have been questions over whether those past tests were practical weapons at all, if they could even be dropped from aircraft.”
Carlson says the North Koreans would be aiming to develop a weapon “small enough and light enough to put onto a missile, and the usual parameters are something less than one metre in diameter, and less than a tonne in weight”.
“Ideally it would be even smaller, maybe 750mm and half a tonne, but that would probably be beyond their capabilities,” he says.
Admiral Bill Gortney, head of US Northern Command and the North American Aerospace Defense Command, said in October 2015 he believed North Korea had rockets with enough range to hit the continental US.
The secretive state had already developed “miniaturised” nuclear bombs that could be fitted to these rockets, he added.
Why develop this kind of weapon?
The point is to show that North Korea can, that their powers have increased. “It’s the psychological impact,” Carlson says.

Source: The Guardian