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.

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