North Korea: What exactly is a H-bomb, and has the reclusive country joined the thermonuclear club?

North Korea: What exactly is a H-bomb, and has the reclusive country joined the thermonuclear club?

North Korea: What exactly is a H-bomb, and has the reclusive country joined the thermonuclear club?

Updated 4 September 2017, 21:20 AEST

It seems Kim Jong-un may be wielding a weapon much more powerful than before, with growing concerns that North Korea has joined the select group of nations in possession of a hydrogen bomb.

There are two types of nuclear weapons, and North Korea now appears to have developed the more potent variety much faster than anyone anticipated.

Key points:

  • Atomic or "A-bombs" work on the principle of nuclear fission, where energy is released by splitting atoms
  • Hydrogen or "H-bombs", also known as thermonuclear weapons, work on fusion and are far more powerful
  • North Korea's latest nuke test was thought to be a H-bomb

Of the six atomic weapons tests the rogue nation has conducted since 2006, the first five most likely used conventional "atom bombs".

But North Korea's effort on Sunday has observers concerned it has joined the thermonuclear club — those nations in possession of a hydrogen bomb.

If so, Kim Jong-un is now wielding a weapon much more powerful than before.

The devastating nuclear bombs dropped on Hiroshima and Nagasaki by the United States during World War II were "fission" weapons, and exploded with a force of between 15 and 20 kilotons.

It was not until 1952 that the US tested its first hydrogen bomb, propelling the world into a new era of thermonuclear peril.

Britain followed suit in developing the more powerful nuclear weapons and carried out tests in Australia from the 1950s to 1963.

It wasn't easy to do and in 1957, after several failures, the British dropped a hydrogen bomb from a Vulcan bomber at Maralinga, recording an explosive yield of 27 kilotons.

If North Korea has developed a thermonuclear device, it has joined a select group.

According to the Union of Concerned Scientists, only the US, Russia, Britain, France and China have nuclear arsenals composed of hydrogen weapons.

Israel, India and Pakistan are generally believed to have nuclear weapons that use only nuclear fission.

Why are they so powerful?

While A-bombs split atoms to release energy, hydrogen bombs use nuclear fusion — in which atoms fuse together — to release far greater amounts of energy.

To make it work, an atomic fission-type explosion is needed to spark the thermonuclear reaction.

The explosive force of a nuclear blast is measured in kilotons, a unit equivalent to a thousand tonnes of TNT.

Until now, North Korea has never tested a weapon greater than 10 to 20 kilotons in size, but experts say Sunday's explosive yield was much greater.

Pyongyang claims it was a 120-kiloton bomb — up to 10 times bigger than previous tests.

It created a magnitude-6.3 tremor, which would indicate an H-bomb.

Can we believe North Korea did it?

It's hard to determine exactly how big North Korea's test was on Sunday.

But based on the tremor the explosion caused, which registered across the globe, it had a yield of between 50 and 120 kilotons.

North Korea claims it has made the jump to thermonuclear technology, but some experts are sceptical.

They suspect the North may have tested a "boosted" atomic bomb and are seeking more information to determine the truth.

Analysts who advise governments on nuclear weapons will firstly study the shockwaves from the blast measured by monitoring stations.

They will also look for clues from traces of nuclear gases that could float into the atmosphere.

Japanese planes with special monitoring equipment have already been taking samples in the region.

Those traces may tell if this test was really a hydrogen bomb, or perhaps something less than a full-scale thermonuclear device.

But it can take weeks for the gases to leak out and be detected.

Why is the test so significant?

While questions remain over whether North Korea has successfully miniaturised a nuclear weapon and whether it has a working H-bomb, there's no doubt among military analysts about the significance of this latest test.

"It shows that they're making progress very fast on their nuclear weapons," said Dr Philip Coyle from the Centre for Arms Control and Non-Proliferation in Washington.

"As we see similarly with their rocket tests — they've done about 84 tests since Kim Jong-un became President, a much higher rate of testing than his father and grandfather — and the same goes for the nuclear weapons tests.

"The most recent just being a year after their fifth test."

Dr Coyle, who was head of weapons testing and evaluation under president Bill Clinton, said Pyongyang's claim that Sunday's test was an H-bomb appears credible.

"Some might quarrel and say the yield was mostly from fission and not from fusion. But technically, this could qualify as an H-bomb either way," he said.

Dr Malcolm Davis, a senior defence capability analyst at the Australian Strategic Policy Institute, cautioned Pyongyang was moving decisively and quickly towards having an effective long-range nuclear weapons capability — and the international community is running out of time to stop it.

He said the regime is also looking at developing a second strike capacity, using sea-based submarine ballistic missiles.

"If we do nothing, if we basically accept them as a nuclear weapons state, that capability will continue to develop and we'll have North Korea with hundreds of warheads and hundreds of missiles," Dr Davis said.

"And then they'll virtually be invulnerable."

"So then at that point they can provoke and threaten as much as they want and we can do nothing about it."