Here's why a hydrogen bomb test would be a breakthrough for North Korea

North Korea claimed to have tested a hydrogen bomb on Wednesday, shortly after US government monitors detected an earthquake near a known weapons testing site.

Seismic information suggests that North Korea tested a weapon with a comparable explosive yield to the nuclear device the country detonated in 2013 - a 10-kiloton bomb that created a fireball one-fifth of a mile wide.

On January 5th, the White House said it appeared unlikely that North Korea had actually tested a hydrogen bomb.

At the moment, there's little evidence to support Pyongyang's claim. Numerous arms control experts say it's likelier that North Korea tested a more typical fission weapon "boosted" with hydrogen isotopes for increased yield.

Either possibility is worrying, largely for what it suggests about the breadth of North Korea's nuclear efforts. The North Korean nuclear program is alarming partly because it's more accurately thought of as two nuclear programs.

Uranium and plutonium-based weapons require different facilities, fissile material, and bomb designs. North Korea has both types of weapons.

One of the world's poorest and most isolated countries has the heavy water reactor and plutonium reprocessing plant needed to create a plutonium-based implosion device - and the uranium enrichment facilities needed to build a uranium-based device, which is detonated through slamming two centrifuges of bomb-grade material into one another.

The ground-level effects of a hydrogen bomb and simpler plutonium or uranium-based bomb aren't terribly different if the weapons are of identical explosive yield. The more worrying implications are what a successful hydrogen bomb test would reveal about the actual level of North Korean nuclear expertise and development - and what it would suggest about Pyongyang's options moving forward.

A hydrogen bomb, also called a thermonuclear weapon, uses a plutonium or uranium-based reaction to heat hydrogen isotopes, creating a kind of nuclear feedback loop with the potential to dramatically boost the amount of energy that reaction ends up releasing. A thermonuclear weapon is more complicated and more destructive than a typical atomic bomb.

As Alex Wellerstein, a nuclear historian at the Steven Institute of Technology and creator of Nuke Map explained to Business Insider, a country that's mastered thermonuclear weapons design suddenly has a number of possible options open to it.

A country could make "super-big bombs:" monstrous, maximum-yield weapons that could vaporize entire cities but would be too heavy and unwieldy to actually deliver even using a strategic bomber.

It would be possible to build "relatively efficient and compact bombs" with moderate radioactive fallout and a yield in 100s of kilotons - equivalent to 100,000 tons of TNT - that can "fit into something the size of a trash can, albeit a pretty heavy trash can." According to Wellerstein, this design is common with in the US nuclear arsenal, although it "required some fairly tricky technical innovations in the early 1960s." And a country could opt to develop a high-yield weapon with no fallout, a "clean bomb" of a type that the US never built in significant numbers, according to Wellerstein.

It's unlikely that North Korea would dedicate its limited nuclear expertise and resources to building a maximum-yield thermonuclear device - developing a submarine-based second-strike capability is of much greater strategic urgency for Pyongyang. A clean bomb or a miniaturized high-yield thermonuclear device might be beyond North Korea's capabilities as well.

But there's a fourth option. A country with a thermonuclear capability could build "a very thin-cased bomb of low yield [in this case 1-10 kiloton, or 1,000 to 10,000 tons of TNT] that would emit a lot of radiation relative to its blast power."

The so-called "neutron bomb" or "enhanced radiation" weapon isn't all that difficult hard to develop once a country has mastered more basic hydrogen bomb technology.

The biggest issue (as with just about every other nuclear weapon) is miniaturization: "The hard part is getting the hydrogen bomb small enough that you could use it tactically," says Wellerstein.

There's also the related problem of testing data. As Wellerstein notes, the US might have mastered thermonuclear reactions using 1950s technology, but it had to "set off hundreds of test weapons" in the process. North Korea has conducted only four tests, and has only claimed to have used hydrogen bomb technology in one of them.

If North Korea really did test a hydrogen bomb, or even a bomb "boosted" with a hydrogen isotope, it could signal an interest in eventually developing far more destructive weaponry - bombs of horrifically high yield whose development might not remain outside Pyongyang's technological grasp for long.