It’s been more than 60 years since the US successfully tested the first hydrogen bomb. Since then only four other countries—Russia, France, China, and the UK—have been able to make one themselves. This week North Korea claimed it had, but you can disregard Kim Jong-un’s boast for now. (Update Sept. 3: The regime has yet again claimed to have tested a hydrogen bomb.)

A few more countries—India, Pakistan, South Africa, Israel, as well as North Korea—have the know-how to build simpler forms of nuclear weapons: atomic bombs. Still, no other technology in the world has remained out of the hands of so many countries for such a long time. Why?

It may be that the Cold War between the US and Russia deterred states from picking a battle with one of the big guns—but that didn’t stop India and Pakistan. Maybe the non-proliferation lobby after the Cold War convinced states that they don’t need nuclear weapons (South Africa, indeed, gave up its arsenal in 1991)—but Iran and North Korea kept trying.

So the most probable reason, Robert Downes, a nuclear weapons expert at King’s College London told Quartz, is that it’s simply too hard. Let’s start with the basics of building a nuclear weapon to see why.

First make your fuel

Nuclear weapons make use of the “strong nuclear force,” which holds positively charged particles—protons—together in an atom’s nucleus. Though it only acts over very tiny distances, the strong nuclear force is indeed strong—about a hundred trillion trillion trillion (10 to the 38th power) times stronger than gravity—so it easily overcomes the repulsion between the positive charges. (For comparison, think how hard it is to bring together the north poles of two strong magnets.)

There are two types of nuclear weapons, and they make use of the strong nuclear force by either splitting very large atoms apart (nuclear fission in an atomic bomb) or by squeezing very small atoms together (nuclear fusion in a hydrogen bomb, a.k.a. thermonuclear bomb). Both processes release vast amounts of energy. Our sun and most stars are nothing but massive fusion reactors.

The first barrier to building a nuclear weapon is finding nuclear fuel. Very few types of atoms are both the right size and abundant enough to make a nuclear weapon. It’s either uranium or plutonium for fission bombs, or a mixture of deuterium and tritium (both of them rare forms of hydrogen) for nuclear fusion.

To collect weapons-grade uranium is not easy. You need a concentrated (“enriched”) lump of the less stable form, uranium-235, which is only about 1% of naturally occurring uranium. (The other 99%, uranium-238, doesn’t work for an atom bomb because it doesn’t split apart easily enough). Separating these two forms, or isotopes—which are identical in almost every way but differ slightly in weight—is hard, and takes a lot of energy. The plant that enriched uranium for the first atomic bomb covered more than 40 acres (16 ha) of land, with 100 miles (161 km) of piping, and thousands of heaters and compressors to turn the metallic uranium into a gas so the isotopes could be separated.

The problem with tritium—an isotope of hydrogen—is even greater. There is nearly no naturally occurring tritium, so it has to be synthesized. This is done in specially designed reactors, which aren’t easy to build and generate tiny amounts of tritium at a time.

So most countries fail to find enough nuclear fuel to make a bomb. Iran, for instance, struggled to generate enough enriched uranium-235 to get started. And, in the nuclear deal it signed in 2015, Iran had to send to Russia whatever low-enriched uranium it had.

Then create a mini-sun

With enough fuel, you can make a rudimentary nuclear bomb. What you need is to create conditions that can start a nuclear chain reaction.

In a fission weapon, when one atom of uranium-235, for instance, splits apart, it releases two neutrons. If each neutron hits another atom of uranium-235, those too will split, each releasing another two neutrons, and so on. This happens only if there’s enough uranium-235 in one place—the critical mass—for each neutron to have a high chance of hitting another atom.

The Engineer Guy The basic mechanism of an atomic bomb.

Once you’ve made enough uranium-235, though, creating critical mass is relatively easy. You start out with two smaller lumps of uranium and, when it’s time to set the bomb off, bang them together at high speed.

Fusion weapons are more complex. Nuclear fusion requires conditions that exist inside the sun: extremely high temperature and pressure, millions of times of what we have on Earth. And the nuclear fuel needs to be held under those conditions for long enough to kickstart fusion.

Although technical details remain secret, one way to create these sun-like conditions is to first have a nuclear fission explosion. In other words, you need to make an atom bomb that then sets off a hydrogen bomb. But the payoff can be thousands of times more destructive than an atom bomb.

The biggest hydrogen bomb ever tested, Tsar Bomba (1961), was more than 3,000 times bigger than the atomic bomb that was used in Hiroshima. When it was tested in a remote part of Russia, it was predicted that anyone within 100km of the blast would have suffered third-degree burns from the radiation released. After the test, it was observed that the blast wave broke windowpanes 900km away. That is, if the explosion had occurred in Berlin, it would have broken windows in London.

Hitting the target

But there is little point in having a Tsar Bomba-sized hydrogen bomb today. The atomic bomb dropped on Hiroshima in August 1945 weighed 4,400kg (9,700 lb) and Tsar Bomba weighed 27,000kg. These types of bombs can only be moved in specially designed bomber planes. With today’s anti-aircraft technology, such planes would be brought down before the nuclear weapon could be deployed.

So today, if nuclear weapons are to reach the target intended, they need to be small enough to be put on a missile. This makes the design of new nuclear weapons more difficult.

India claims to have tested a thermonuclear device, but the claims remain contested. According to Bhupendra Jasani, a nuclear physicist at King’s College London, instead of working on hydrogen bombs, countries like India and Pakistan are probably working on “boosted” atomic bombs.

A boosted weapon is one that packs more punch by using a higher proportion of its own nuclear fuel; although the Hiroshima bomb caused so much destruction, it used merely 1.4% of the uranium put in it. One way to do this is to put some fusion fuel at the core of an atomic bomb. This mixture of deuterium and tritium is compressed to create a fusion reaction. This produces more neutrons, which then enhance the chain reaction of the fission fuel. In other words, you use an atom bomb to set off a tiny hydrogen bomb which in turn ratchets up the atom bomb.

Jasani thinks, therefore, that North Korea’s claimed “hydrogen bomb” was really an attempt to test a boosted atomic bomb. The seismometer readings suggest that the bomb North Korea tested was in the range of what atomic bombs can yield, rather than what hydrogen bombs usually yield.

So how to explain Kim Jong-un’s claim that it was a hydrogen bomb? You can find a hint in the eye-catching prose of the press release North Korea published after the nuclear test.

The DPRK’s access to H-bomb of justice, standing against the U.S., the chieftain of aggression watching for a chance for attack on it with huge nukes of various types, is the legitimate right of a sovereign state for self-defense and a very just step no one can slander.

All Jong-un wants is to inflate his own ego and tell his people that he is doing all he can to protect them from vicious forces outside. And, although he may not have an H-bomb, the fact that North Korea has conducted four nuclear weapons tests since 2006 should be a cause for worry.