NUKE TEST: WHAT WE KNOW

North Korea conducted a sixth nuclear test at the weekend, saying it was a hydrogen bomb that could be fitted on to a missile, prompting global condemnation and calls for further sanctions.

The North says it needs nuclear arms to protect itself, but the US has accused the isolated nation of “begging for war”.

Pyongyang carried out its first nuclear test in 2006. It also has a rocketry programme and in July tested two intercontinental ballistic missiles (ICBMs) that appeared to bring much of the mainland US into range.

Here are some key questions around the latest explosion:

HOW BIG WAS THE BLAST?

Calculating the yield from the blast involves taking into account several different factors, many of which are unknown.

As well as the magnitude of the earthquake — given as 6.3 by the US Geological Survey, although some monitors give lower figures — the depth at which the device was detonated is crucial, as is the type of rock surrounding it.

South Korea’s defence minister put the yield at 50 kilotons. The US-based 38 North website says 100 kilotons or more. Japan yesterday upgraded its estimate to around 160 kilotons.

The numbers are likely to continue to change as more information emerges before a consensus is reached. But all the estimates far exceed the 15 kilotons of the US device that devastated Hiroshima in 1945.

The bigger a bomb is, the less accurately an ICBM carrying it needs to be aimed to ensure a given target is destroyed.

WHAT KIND OF BOMB WAS IT?

Atomic or “A-bombs” work on the principle of nuclear fission, where energy is released by splitting atoms of enriched uranium or plutonium encased in the warhead.

Hydrogen or H-bombs, also known as thermonuclear weapons, work on fusion and are far more powerful, with a nuclear blast taking place first to create the intense temperatures required.

Many experts say Sunday’s blast had the hallmarks of a two-stage hydrogen bomb. But an enhanced fission device, in which fusion fuel is used to boost the yield from an atomic bomb, is also a possibility.

No foreign government has so far confirmed the North’s assertion that it was an H-bomb.

Chinese, South Korean and Japanese monitors have not detected elevated radiation levels or chemical isotopes that could give clues as to what it was, even though a second tremor after the explosion led to suggestions the rock at the test site had collapsed.

Satellite pictures released yesterday showed small landslides at the Punggye-ri test site, but no crater from a cavein, which could have allowed radioactive substances to enter the atmosphere.

WAS ‘THE PEANUT’ REAL?

Hours before the blast, the North released pictures of leader Kim Jong-un at the Nuclear Weapons Institute inspecting an hourglassshaped silver device that looked around a metre long.

The North’s official KCNA news agency said that it was an H-bomb with an “explosive power that can be adjusted from tens to hundreds of kilotons depending on the target”.

All its components were domestically produced, it added.

The device, quickly dubbed the “Peanut” by analysts, was larger than the “Disco Ball”, said to be a miniaturised atomic bomb, that Mr Kim was pictured with in March last year.

Pyongyang previously claimed to have successfully tested an H-bomb after its fourth blast in January last year, but experts doubted the claim at the time due to the relatively weak yield.

Analysts say the Peanut could be a model, rather than the actual device that was detonated on Sunday. But South Korea’s defence minister said Seoul believed the North had succeeded in building a bomb that could fit into an ICBM.