Flattened cities, millions of people burnt to death, and yet more tortured by radioactive fallout. That harrowing future may seem outlandish to some, but only because no nation has detonated a nuclear weapon in conflict since 1945. Countries including the US, Russia, and China wield hefty nuclear arsenals and regularly squabble over how to manage them—only last week, Russia suspended participation in its nuclear arms reduction treaty with the US. Thankfully, nuclear warheads mostly just sit there, motionless and silent, cozy in their silos and underground storage caverns. If someone actually tried to use one, though, would it definitely go off as intended?

“Nobody really knows,” says Alex Wellerstein, a nuclear weapons historian at the Stevens Institute of Technology. The 20th century witnessed more than 2,000 nuclear tests—the vast majority carried out by the US and the Soviet Union. And while these did prove the countries’ nuclear capabilities, they don’t guarantee that a warhead strapped to a missile or some other delivery system would work today.

Surprisingly, as far as we know, the US has only ever tested a live nuclear warhead using a live missile system once, way back in 1962. It was launched from a submarine. The Soviet Union had performed a similar test the previous year, and China followed in 1966. No nation has ever tested a nuclear warhead delivered by an intercontinental ballistic missile. The missile could blow up on the launchpad, explains Wellerstein. No one wants to clean that mess up.

Russia’s invasion of Ukraine has, sadly, brought the specter of nuclear weaponry to the fore once again. In February, Russian President Vladimir Putin claimed new strategic nuclear weapons systems had been placed on combat duty, and he threatened to resume nuclear testing. Russia’s former defense minister, Dmitry Medvedev, has been particularly vocal about his country’s readiness to use nuclear weapons—including against Ukraine

Russia has around 4,500 non-retired nuclear warheads, according to the Federation of American Scientists, a nonprofit that focuses on security. Roughly 2,000 are considered “tactical”—smaller warheads that could be used on, for example, a foreign battlefield. To our knowledge, Russia has not begun “mating” those tactical warheads to delivery systems, such as missiles. Doing so involves certain safety risks, notes Lynn Rusten of the Nuclear Threat Initiative, a think tank: “It would be really worrisome if we saw any indication that they were moving those warheads out of storage.”

If they were brought into operation, multiple things could in theory go wrong with these weapons. For one thing, the delivery systems themselves might not be reliable. Mark Schneider, formerly of the US Department of Defense’s senior executive service, has written about the many problems Russia has faced with its missiles so far during the war with Ukraine. Last spring, US officials said between 20 and 60 percent of Russian missiles were failing, either in terms of not launching or not hitting the intended target. That doesn’t necessarily matter, though, notes Schneider. When firing a nuclear warhead with a big explosive yield, “accuracy is much less relevant,” he says.

Russia certainly has enough missiles to get a nuclear weapon more or less to where it wants—even if it takes more than one attempt. But what about the warheads themselves? Modern thermonuclear devices are complex bits of machinery designed to initiate a specific explosive sequence, sometimes called a fission-fusion-fission reaction, which releases a massive amount of energy.

Wellerstein points out that some warheads designed decades ago are still part of nuclear arsenals. Over time, their parts must be carefully checked for degradation and refurbished or replaced. But certain components can become unavailable due to changes in manufacturing capabilities. Eventually, you might have to find a substitute for some particular out-of-production widget or material in your warhead. Without then testing the whole device, you can only hope it still works. 

In the early 2000s, the US struggled to source a classified material (whose purpose is also classified), codenamed FOGBANK, for its nuclear warheads. Reports suggest the military had to relearn how to make this material. “One can imagine the Russians might have similar sorts of [problems] because the manufacturing has changed,” says Wellerstein. 

Nuclear warheads have also sometimes been built with flawed components. Take the W47, made by the US. These had a wire inside that had to be pulled out in order to arm the weapon. But this wire had a habit of becoming brittle when stored, and it often broke or got stuck at the moment of arming. Metals inside warheads can also become fragile over time, given the intense radiation to which they are exposed. And very expensive components containing tritium, an isotope of hydrogen, have to be quite regularly replaced, since the tritium depletes over time.

So nuclear warheads are actually surprisingly sensitive little things. And wielders of nuclear arsenals have limited options for how to test their kit. At present, most nations don’t test live nuclear warheads due to the Comprehensive Nuclear-Test-Ban Treaty (North Korea, a non-signatory, tested a nuclear device as recently as 2017), though they are able to try out warhead mechanisms. However, a 2020 US government report indicated that Russia and China might both have been testing live but ultra-low-yield nuclear devices. Such efforts are unconfirmed, but this kind of testing would presumably give both nations some idea of how reliable their weapons are.

It is also possible to use lasers and computer simulations to model warhead detonation. And the US has a special X-ray imaging system that can take high-speed pictures, which reveal how a warhead’s primary stage explodes and the likely characteristics of a nuclear detonation. The system uses a substitute for plutonium in these experiments, however, and no actual nuclear explosions occur during the process. “Some of the physics of this stuff you don’t see outside stars,” says Wellerstein, emphasizing the challenges involved. “There’s just an inherent trickiness to even simulating this.”

Provided you make the effort to maintain warheads properly, though, they ought to work. “Russia has a robust nuclear capacity. They refurbish their warheads often,” says Amy Woolf, a US specialist in nuclear weapons policy. Schneider, too, is confident that Russia’s nukes are serviceable. It would be unwise to assume otherwise. 

There is, though, the human factor. The gun might be more or less sure to go off—but will that soldier definitely pull the trigger on your orders? It’s hard to predict what anyone would do in that kind of situation, but experts who spoke to WIRED tend to agree that the Russian military’s chain of command is pretty loyal. Interestingly, to authorize the use of a nuclear weapon, President Putin—unlike the US president—is thought to require agreement from two other members of his government: the minister of defense and the chief of the general staff. “Once they all three agree, off they go,” says Woolf. Everyone else further down the chain is likely to follow their lead. “I doubt you’d get a mutiny of troops in the field.”

Finally, it’s conceivable that an enemy of Russia’s could try to sabotage its nukes, perhaps by hacking them or the systems on which they depend. But Schneider wouldn’t bet on it. “Even if you had a capability to do it today, you may not have a capability to do it tomorrow. I don’t think that’s really credible,” he says.

When it comes to nuclear weapons, it’s probably best to just assume they’re going to work. Consider the possibility that certain people who question the reliability of extant nuclear weapons may have a hawkish motive. “There does seem to be a significant appetite to resume nuclear testing under some factions of the Republican Party,” notes Emma Claire Foley at Global Zero, a nongovernmental organization that works to eliminate nuclear weapons.

These weapons’ deployment, though, is unlikely. Woolf argues that Putin has little to gain from using a nuke in Ukraine, no matter how frustrated he gets at Russia’s mounting failures and losses. Rusten agrees. “Breaking that taboo would have unbelievable consequences globally, diplomatically, economically,” she says. “There’s no way that turns out well for Russia.”

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