The H-bomb: What is it? Who has it? Why it matters

If Pyongyang has mastered the technology, it has made a major step forward in its nuclear capabilities.

The plutonium-based atomic weapons it tested up until this point were powerful enough -- the United States dropped such weapons on Japan to end World War II -- but a hydrogen bomb ups the ante many times over.

Atomic bombs use a process called fission. They split plutonium and/or uranium into smaller atoms in a chain reaction that releases massive amounts of energy.

The A-bombs dropped by American forces on Hiroshima and Nagasaki in 1945 killed more than 200,000 people.

H-bombs use fusion, the same process that powers the sun. In a hydrogen (thermonuclear) bomb, "heavy" isotopes of hydrogen are forced together to release a much bigger punch -- hundreds or even thousands of times more powerful than the only nuclear weapons that have been used in warfare.

"What thermonuclear weapons do is increase the potential yield by enormous amounts. The amounts that can be released by thermonuclear weapons are phenomenal," said Martin Navias, senior fellow at the Centre for Defence Studies at King's College, London.

The most powerful nuclear weapon ever detonated was the Tzar Bomba, a hydrogen bomb tested by the Soviet Union in 1961, he said. Its blast of 50,000 kilotons, or 50 megatons, dwarfing the force of the Hiroshima and Nagasaki bombs.

With the development of an H-bomb, reclusive North Korea would be that much more of a threat.

A hydrogen bomb is a complex bit of machinery. It's basically two bombs in one.

While it gets its bang from the fusion reaction, it takes a lot of heat to get the process started -- to get the atoms to smash together and start a nuclear chain reaction. That's why they're called thermonuclear weapons.

What better way to do that with than another much smaller nuclear weapon? An atomic bomb works as the trigger to set off the hydrogen bomb. The two explosions occur in virtually the same instant.

The nuclear arsenals of the United States, Russia, United Kingdom, France and China include these types of weapons.

India and Pakistan have nuclear weapons, but none are believed to be thermonuclear.

Atomic bombs have been only used twice in warfare -- both times by the United States and both times on Japan. The devastation led to Japan's unconditional surrender and brought an end to World War II.

Hydrogen bombs have never been used in war, although there have been times when the world seemed to be on the brink. The Cuban missile crisis of 1962 is the most prominent example.

The major nuclear powers have gradually backed away from that brink over the years. In recent decades the Cold War adversaries have rolled back their nuclear arsenals.

Since dawn of the nuclear age, at least eight nations have conducted a total of more than 2,000 nuclear test explosions.

The United States was first, detonating an atomic bomb in New Mexico on July 16, 1945. In the ensuing years, the U.S. was joined by the Soviet Union, United Kingdom, France, China, India, Pakistan and North Korea. (Israel also is believed to have nuclear weapons, but has never officially said so).

The U.S., UK and USSR agreed to end above-ground testing in 1963. The Soviet Union's last underground test was in 1990, and Russia, which inherited the arsenal after the USSR's in 1991, has not conducted any since. Britain (1991), the U.S. (1992), France and China (both 1996) also have ceased testing their weapons.

India and Pakistan conducted nuclear tests in 1998.

North Korea is the only nation to have conducted any nuclear tests this century, in 2006, 2009, 2013 and this year.

It has not been confirmed that North Korea has successfully tested a miniaturized H-bomb.

Navias, for one, says he does not believe North Korea's claim.

"Their rhetoric tends to run ahead of their actual capabilities," he told CNN.

Wednesday's test yielded a blast of a similar magnitude to a previous North Korean test in 2013, he said.

"One would have expected the yield (from an H-bomb) to have been far greater," he said.

"What they may have done is boosted their fission weapon by introducing various hydrogen isotopes."

Nevertheless, North Korea's advances in nuclear weapons technology are real and cause for concern, he said.

While Pyongyang does not have intercontinental ballistic missile technology that would enable it to strike the West Coast of the United States, it has missiles capable of striking South Korea, Japan and U.S. military bases in the region, he said.

North Korea still needs to make "very challenging" advancements in miniaturizing nuclear warheads to be able to fit them onto missiles, and to improve its missile guidance systems, he said. But it is undoubtedly making efforts to advance in these areas.

"There are people out there that know this stuff," he said.

It also claims to have recently tested a submarine-launched ballistic missile, he said -- a technology that, if mastered, "would be a completely different level of threat."

"I don't think they've reached anywhere with those things, but they're moving steadily in that direction and the outside world has little leverage," he said. "The only people who have any influence over them are the Chinese, and that influence is pretty limited."

"If left unattended, it's not unreasonable to assume that in the long run, all these things will realize themselves."

Navius said it was unlikely that North Korea seeks to wage war on the United States or other foreign power, despite seeing itself as under threat. More likely, the isolated regime is seeking use nuclear capability to gain greater leverage on the international stage.

However, Navius said, nuclear proliferation carries its own inherent risk, especially in such an unstable part of the world.

"The danger is that sometimes when you have this kind of escalation, escalation has its own dynamic," he said. "You may enter into a war even when none of the parties want one."