Which countries have nuclear weapons?

There are nine countries that possess nuclear weapons. Five of these (the US, Russia, the UK, France and China) are members of the official owners club, who made their weapons early and had them legitimised in the Nuclear Non-Proliferation Treaty (NPT) signed in 1968, the key piece of international law governing nuclear weapons possession.

NPT has arguably been quite successful. In the 1960s it was widely anticipated that dozens of countries would get the bomb, as it appeared to be the fast track to clout and status on the world stage. But so far there have only been four rogue nuclear weapons states who ignored the NPT and made their own bombs. In order of acquisition, they are Israel, India, Pakistan and North Korea.



Has any country ever given up its nuclear weapons?

More countries have given up nuclear weapons programmes than have kept them, coming to believe they were more of a liability than an asset for national security.

The apartheid regime in South Africa secretly built six warheads, but dismantled the bombs and abandoned the whole programme in 1989 just before the system gave way to democracy.

Even Sweden had an advanced and ambitious plan based on heavy water reactors to build up to a hundred warheads, but gave up the project in the 1960s, preferring to spend defence funds on fighter planes.

The military juntas in both Argentina and Brazil pursued covert weapons programmes, although they stopped short of making a bomb, and the two countries gave up their programmes in the early nineties and joined the NPT.

Taiwan and South Korea began developing plutonium production programmes in the late sixties and early seventies before the US persuaded them to halt in the mid-seventies and rely on Washington for security. Japan is generally considered to have a “bomb in the basement”, in that it has all the materials and know-how to build a warhead quickly if it decided to follow that path and leave the NPT. At present that course seems unlikely.

Three successor countries to the Soviet Union – Ukraine, Kazakhstan and Belarus – inherited nuclear weapons in 1991, and all three agreed to surrender them, in Ukraine’s case in return for sovereignty guarantees from Russia that ultimately proved worthless.

In Iraq, Saddam Hussein dismantled his rudimentary nuclear weapons programme after the first Gulf war in 1991, and Libya’s Muammar Gaddafi handed over his nuclear weapons beginner’s set to the US in 2003. Their ultimate fate offers little incentive for future despots to give up their atomic dreams.

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How do you make a bomb?

It is pretty difficult to make a nuclear weapon. If it was not we most likely would no longer be here. And it is difficult on two levels: making the fissile material and then constructing a device that will detonate it.



Material is fissile when the nucleus of an atom can be split by a neutron that has broken free of another atom, producing large amounts of energy and more neutrons. When those free neutrons go on to split the nuclei of other atoms, there is a chain reaction, causing a nuclear explosion.

Uranium and plutonium are used for nuclear weapons, but only specific atomic configurations, or isotopes, of those elements are fissile. The fissile isotopes used in nuclear warheads are U-235 and Pu-239. The numbers refer to their atomic weights. The biggest single challenge in making a nuclear warhead is producing enough of these isotopes from the elements found in nature.

Following the uranium path to the bomb requires converting refined uranium into a gas and then spinning it at very high speed in centrifuges to separate out the U-235, which makes up less than 1% of naturally occurring uranium. This has to be done repeatedly through “cascades” of centrifuges. Low-enriched uranium, used in civilian nuclear power, is usually 3%-4% U-235. Weapons-grade uranium is 90% enriched or more. Building enough centrifuges, and getting them to spin fast enough in unison, is the greatest technical challenge along the uranium route.

Plutonium Pu-239 is produced in significant quantities by extracting it from irradiated uranium fuel that has been through a reactor. Because it is more fissile, less plutonium is required for a weapon. A sophisticated modern warhead requires as little as 2kg of plutonium, or at least three times that much uranium.

Once you have enough fissile material, you have to make it go bang. And to achieve that you have to force the atoms close enough together to trigger a chain reaction. There are two ways of doing this, and therefore two basic bomb designs.



The most rudimentary is the gun-type warhead, which involves firing one chunk of fissile material into another at high speed with conventional explosives. The Little Boy bomb dropped on Hiroshima was a gun-type device using 64kg of highly enriched uranium (HEU).

A more sophisticated bomb type, which requires less fissile material and allows the use of plutonium (which does not work in a gun-type warhead) is the implosion device, in which a sphere of HEU or plutonium is surrounded by explosives rigged to go off at exactly the same time to violently compress the core. The Fat Man bomb dropped on Nagasaki was an implosion device with about 6kg of plutonium.

What is a hydrogen bomb?

Facebook Twitter Pinterest Demonstators call for a ban on nuclear weapons in 1958. Photograph: Popperfoto/Getty Images

Hydrogen bomb is the colloquial term for a thermonuclear weapon, a second-generation bomb design with vastly more explosive power than a simple fission warhead.

It is a two-stage device – a primary fission bomb which detonates and compresses a secondary bomb filled with two heavy isotopes of hydrogen: deuterium and tritium (hence the name hydrogen bomb). They undergo a process of nuclear fusion, forcing the nuclei of atoms together and multiplying exponentially the amount of energy released by the device. All strategic weapons in modern arsenals are now thermonuclear, or hydrogen, bombs.



Whatever happened to nuclear disarmament?

The bargain at the heart of the NPT was that member states without nuclear weapons agreed not to acquire them, as long as the states with weapons reduced their obscenely large arsenals, capable of destroying the planet many times over. That has indeed happened, to an extent – at first as the result of arms control agreements, and then the collapse of the Soviet bloc and the end of the cold war.



From a peak of 70,000 nuclear weapons in the world at the height of the cold war, in 1985, there are now about 14,000, according to the Federation of American Scientists (FAS), still enough to end life on the planet. Then and now, the overwhelming majority (93% in 2018) of these warheads belong to the US and Russia, with between 6,000 and 7,000 apiece, although only about a quarter of those arsenals are deployed and ready for use. The rest are in reserve stockpiles or in the process of being retired and dismantled.

Of the second-tier nuclear weapons powers, again according to FAS estimates, France has 300 warheads, China 270, the UK 215, Pakistan 130-40, India 120-30, Israel 80, and North Korea between 10 and 20.

The last successful arms control agreement, the New Start treaty, was signed by Barack Obama and Dmitry Medvedev in 2010, limiting the US and Russia to 1,550 deployed strategic warheads each. The hope at the time was that the two nuclear superpowers would pursue a follow-on treaty and at one point Obama suggested he might reduce the US arsenal unilaterally by another third. But that did not happen.

What are the chances of a nuclear weapon falling into the hands of a terrorist group?

The terrorist nuclear weapon is one of the scariest scenarios the world faces. Unlike states, such groups cannot be deterred from using a weapon as the perpetrator could be very hard to identify in the wake of a blast, difficult to find, and ready to accept death as the price of inflicting devastating damage. Terrorist groups would not need expensive missiles to deliver their warheads. They could be sailed into a port in a shipping container or across land borders in the back of a truck.



After the collapse of the Soviet Union, the US spent substantial resources on dismantling many of its weapons and production facilities as well as ensuring that its many nuclear scientists had alternative employment so as not to be tempted to sell their wares and expertise to the highest bidder. But serious concerns about nuclear weapons security remain. Pakistan in particular is a source of anxiety as its military and intelligence services have radicalised elements within them, with links to terror groups.

There are also fears that a cash-strapped or vengeful North Korea could sell one of its warheads for the right price. A more recent emerging threat is that a rogue group could hack into a nuclear power’s command and control computers, triggering a launch, or into an early warning system, giving the impression an enemy attack is imminent.

How likely is accidental nuclear war?

Play Video 2:03 Doomsday Clock moved to two minutes to midnight – video

As the years have passed since the cold war, it has become increasingly clear that we had several lucky escapes from nuclear weapons use during that era as the result of miscalculation or technical glitches. For example, in 1979, when a US watch officer left training tapes in the early warning system when he finished his shift, those in the incoming shift saw their screens light up with the tracks of multiple incoming Soviet missiles. It was only good judgment of the duty officers that avoided a nuclear alert.



In such situations, if the glitch is not identified lower down the chain of command and passed upwards as a seemingly genuine alert, a national leader has only a few minutes to decide whether to launch his or her country’s missiles before the apparent incoming salvo destroys them. Nearly three decades after the cold war, the US and Russia still keep hundreds of missiles on hair-trigger alert, ready to launch within minutes, in anticipation of just an occasion.

In the US system, there is no institutional check or barrier to the president launching those missiles once he has identified himself to the Pentagon war room using his nuclear codes.

What next?

Arms control will be on the agenda when Vladimir Putin and Donald Trump meet in Helsinki on Monday. One option is that the two presidents could extend the New Start treaty by another five years, as allowed for in the agreement. The biggest barrier is Trump’s distaste for any arrangement inherited from Obama. It is more likely he would argue for a more ambitious arms control agreement he could put his own name to. But Putin will be hard to convince, without the US scaling back its missile defence system, and that is unlikely at the moment.

The threat of a conflict with North Korea has receded somewhat since the Singapore summit, but it is increasingly clear that Pyongyang has no intention of disarming any time soon. The big question is what will Trump do once that becomes apparent to him.

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The chances of a nuclear standoff with Iran, meanwhile, are rising. In May, Trump walked out of the 2015 nuclear agreement with Tehran, which curbed Iranian nuclear activities in return for sanctions relief. The US is now piling on sanctions and telling the world to stop buying Iranian oil. Sooner or later it is possible, likely even, that the Iranian government will stop abiding by the agreement and start stepping up its uranium enrichment and other activities. That is likely to raise tensions in the Gulf dramatically and make other regional players rethink whether to acquire nuclear weapons themselves.

Taking all these developments into consideration, the Bulletin of the Atomic Scientists has decided to set its “doomsday clock” to two minutes to midnight, the closest to catastrophe it has been since 1953.

Nuclear weapons in popular culture

The darkest day of the cold war produced some timeless comedy, from the classic movie of accidental apocalypse, Dr Strangelove, to the songs of the mathematician, musician and comedian, Tom Lehrer, with titles like So Long Mom (A song for WWIII), and in the UK, the civil defence sketch by Beyond the Fringe.



There are much darker works in the canon. On the Beach, in 1959, was the first major post-apocalyptic movie, in which survivors gather in Australia, the last continent left habitable. The Day After, in 1983, is even blacker. It starts with a nuclear blast obliterating a column of cars stuck on a highway as panicked people rush to try to evade the attack spreads.

More recent films, since the cold war, have dwelt on the threat of a single nuclear weapon detonated by terrorists or deranged geniuses or both. They include Broken Arrow (1996), The Peacemaker (1997) and The Sum of All Fears (2002), in which – because there is just one bomb involved – the detonation is no longer treated as an exctinction-level event. In that, art is following reality. The use of a nuclear weapon is now more likely than any time since the worst days of the cold war, but the probability of humanity being wiped out entirely by nuclear war is, for the time being, diminished.



