North Korea claimed to have detonated a hydrogen bomb on September 3, which caused widespread alarm. But it will take some time before that claim can be verified.

While North Korea said it has made the jump to thermonuclear technology, some experts are skeptical and point out Pyongyang may have tested a “boosted” atomic bomb and are awaiting more information.

The test created a 6.3 magnitude seismic tremor, which indicates an explosive yield of 50 to 120 kilotons or well in excess of the atomic device dropped on Nagasaki in Japan at the end of World War II.

Beside studying shockwaves from the blast, nuclear weapons analysts  also look for clues from traces of nuclear gases in the atmosphere.

Read: Has North Korea joined the thermonuclear club?

Japanese planes with special monitoring equipment have been taking samples and those traces may indicate if this latest test was a hydrogen bomb, or something less than a full-scale thermonuclear device. But it could take weeks for the gases to leak and be detected.

If North Korea has developed a hydrogen bomb, it has joined a select group. Only five other countries are believed to have this frightening weapon – the US, Russia, Britain, France and China.

Atomic and hydrogen bombs

There are two basic types of nuclear weapons: fission weapons and fusion weapons.

The first was developed during World War II via the US-led Manhattan Project, fission devices or atom bombs create an explosion by splitting the nuclei of heavy atoms. These bombs killed hundreds of thousands of people when dropped on Hiroshima and Nagasaki in Japan.

The core of a fission bomb is made of highly enriched uranium or plutonium that on its own is not explosive. When detonated, the core is compressed with conventional high explosives to create a nuclear chain reaction that releases huge quantities of energy.

The US began work on fusion weapons, or hydrogen bombs, after the Soviet Union also managed to build its own fission devices in the late 1940s.

The explosive power in hydrogen bombs, or thermonuclear weapons, comes from the binding together of atoms as opposed to fission or splitting atoms. Fusion weapons are far more powerful than bombs based on fission.

The most powerful pure-fission device tested by the US was Ivy King, a 500-kiloton bomb 20 times more powerful than the one dropped on Nagasaki (a kiloton is equivalent to 1,000 tons of TNT).

But it was small compared to Castle Bravo, the largest hydrogen bomb tested by the US, which had a yield of 15 megatons. Thermonuclear devices can wreak much more destruction and cause nuclear burns a considerable distance from a blast site.