The 1986 explosion at the plant was a turning point for independence in Ukraine. Now Russia is threatening to make the country relive that trauma.

The Russian military’s capture of the Chernobyl nuclear facility in northern Ukraine last week led to heightened levels of both radioactivity and confusion. Since the infamous 1986 explosion at Chernobyl, which sent nuclear materials as high as five miles into the atmosphere and likely condemned far more people than the United Nations’ projected long-term death toll of 4,000, the plant has been radioactive. It’s defunct. Why would the Russian military want it?

Maybe Russian forces overtook the facility for the sake of convenience—after all, it’s along the route from Russian ally Belarus to Kyiv, ​​the Ukrainian capital, which is now under assault. Or maybe, as Russia’s Defense Ministry claimed, the military wanted to protect the plant’s infrastructure, preventing any staging of a “nuclear provocation.” Or maybe, as a Russian security source told Reuters, it was a warning to NATO.

Whatever the Russian army’s reasoning, the implication for Ukrainians is clear: the potential for a repeat of the disaster, which they have spent three decades and considerable resources trying to prevent. I interviewed scores of cleanup workers in the ’90s for my book Life Exposed: Biological Citizens After Chernobyl, and learned just how deeply the memory of the explosion is carved into Ukraine. Russian control of the site “is one of the most appalling threats to Europe today,” Ukraine’s Ministry of Energy said in a statement last week. “Any provocation by the Chernobyl invaders … could turn into another world environmental catastrophe.”

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This violent encounter between “Chernobyl invaders” and Chernobyl survivors is its own act of aggression. The disaster at Chernobyl became a rallying cry for Ukrainian independence in the late ’80s and early ’90s, and processing its traumatic effects on the country’s people and environment became an important facet of Ukrainian national identity. By seizing the plant as part of a brutal invasion, Russia is stirring up radioactive particles, and also Chernobyl’s painful legacy: Ukrainians’ memory of the Soviet Union’s disregard for their lives.

The initial blast at Chernobyl on April 26, 1986, and the massive fire that followed sent fallout across Belarus, Ukraine, Russia, and Europe. More hidden, and more costly to Ukraine, was the process of radiological containment. The efforts lasted more than 30 years, until a structure designed to safely hold the unit’s highly radioactive remains for a century was completed in 2019. And the work was punishingly physical: Some people removed chunks of radioactive nuclear core near the No. 4 reactor unit with no more equipment than shovels and buckets.

More than 600,000 soldiers, firefighters, and other workers from across the Soviet Union were sent to the disaster site to clean up or contain the radiation. Some bulldozed contaminated topsoil while others, in the most dangerous job of all, shoveled highly radioactive debris into the mouth of the ruined reactor in one-minute stints—enough time for their bodies to absorb a lifetime’s worth of radiation exposure. They called themselves “bio-robots,” and the one-minute rule was not evenly enforced. During an interview I conducted, a man on a two-week break from work at the site lifted his pant leg and showed me a patch of skin that had puckered up to form a strange ring above his ankle. “This is from radiation,” he told me. He counted himself among the “living dead”: “Our memory is gone. You forget everything—we walk like corpses.” The Chernobyl Exclusion Zone, where people cannot live and scientists can stay for only short amounts of time, extends 1,000 square miles around the reactor site.

Read: The creatures that remember Chernobyl

The Chernobyl disaster became a turning point for Ukrainian independence. By the ’90s, the Soviet industrial framework was falling apart. Household financial savings were wiped out by hyperinflation. Meanwhile, a Chernobyl health crisis was unfolding as people who developed cancers, heart and autoimmune problems, and other disorders poured into clinics. They were looking for relief from ills that they claimed were related to Chernobyl, but such connections were dismissed by international scientific experts and their Soviet counterparts because the patients had little or no documentation of their exposure. They were faced with an impossible burden of proof, even as the devastating public-health consequences of the disaster were downplayed.

In taking over Chernobyl, Russia is implicitly threatening to cause all that pain all over again. The 15 active, aging nuclear reactors that are spread around Ukraine were not built to withstand an all-out military invasion. Some can survive airplane crashes, but probably not inadvertent strikes from missiles or artillery. Nor can they ward off a destabilizing cyberattack, or protect crucial staff members from being held hostage, as the Ukrainian Ministry of Energy said the Russian army has done at Chernobyl. Some of those staff may decide to flee due to threats of violence. An invading military, in control of those reactors, could dial up the threat of nuclear terror to engage in a wider threat of nuclear blackmail.

Tom Nichols: How Ukraine could become a nuclear crisis

Russian control of Ukraine’s functioning and decommissioned nuclear power plants would be, in the words of one analyst, like having “nuclear warfare without bombs” if these plants were to be tampered with. When the Russian military captured Chernobyl, Vladimir Putin seized the means by which to inflict nuclear damage through a new form of “dirty” power. Russia is now in a position to cause immediate disaster by reopening a toxic legacy that was meant to be sealed. It could also create uninhabitable zones all around Ukraine and force the country’s people back into inhumanely dangerous cleanup work.

Everyone I met in Ukraine in the ’90s either knew bio-robots or had one in their family. They were protecting Europe at their own peril, but they knew that it had to be done. The bio-robots’ children and grandchildren in Ukraine know exactly how hard-won nuclear containment at Chernobyl was, and just how tenuous it is. Nuclear stability, like democracy, is a delicate balance. As Ukrainians take up arms around their country, they are fighting to defend both.