It took only hours for the Russian invasion of Ukraine to hang a nuclear threat over Europe. But the defunct Chernobyl power plant may pose less of a hazard than the forest surrounding it, or the 15 nuclear reactors still operating in the country.

On Thursday morning, Ukrainian officials reported a fierce fight in the exclusion zone around the dead  Chernobyl nuclear power plant, which in 1986 blanketed parts of Europe with radioactive fallout after a meltdown that remains the worst nuclear accident in history.

“Our defenders are giving their lives so that the tragedy of 1986 will not be repeated,” Ukrainian President Volodymyr Zelensky reported on Twitter. “This is a declaration of war against the whole of Europe.”

The Russian attack “may cause another ecological disaster,” the Ukrainian Foreign Ministry reported. Chernobyl “can happen again in 2022.”

By Thursday afternoon, Ukraine reported that Russian troops had captured the facility and detained the staff overseeing cleanup and maintenance of the site, intensifying alarm in world leaders already horrified by the Russian invasion.

“We’re outraged by credible reports that Russian soldiers are currently holding the staff of the Chernobyl facility hostage,” White House Press Secretary Jen Psaki said during a news conference Thursday. “This unlawful and dangerous hostage-taking, which could upend the routine civil service efforts required to maintain and protect the nuclear waste facilities, is obviously incredibly alarming and gravely concerning. We condemn it and request their release.”

The 1986 explosion of Chernobyl’s Reactor 4 killed two first responders, and 28 more died from radiation poisoning in the following weeks. Exposure to the radioactive fallout is believed to have caused some 5,000 cases of thyroid cancer in the region, at least 15 of them fatal, and caused thousands of other premature deaths. Estimates for the total, long-term death toll from the disaster run as high as 1 million people.

The contaminated wreckage of the plant and the nuclear fuel contained there will be dangerous for centuries and requires constant maintenance. On Friday, however, Russian forces were reported to be continuing to interrupt the mainaintence, safety and cleanup work at the site. “For the second day, the occupiers have been detaining the personnel of the [Chernobyl] NPP station, not allowing them to rotate as required by technical safety rules,” Ukraine’s Ministry of Energy reported in a statement.

Automated sensors in the area surrounding the nuclear facility have detected spikes in radiation levels since the Russian occupation of the plant, although the increases have remained below dosages believed to be dangerous to human health.

Rather than a release of material from the containment facility holding the reactor that melted down—the International Atomic Energy Agency reported that there have been no destruction or casualties at the facility—most experts believed that the increase was due to radioactive dust and soils disturbed by tanks and other vehicles during the siege. 

“It’s not surprising given that hundreds of military vehicles are passing through the area stirring up dust,” said Timothy Mousseau, a biology professor at the University of South Carolina who studies the effects of radiation on organisms in Chernobyl.

But the ability of dust surrounding Chernobyl to spread radiation points to a bigger threat in the area, one that can be set off by violent conflict.

“The larger danger is the potential for forest fires in the area to put out radioactive smoke,” Mousseau said.

Once known as the “Red Forest” for the rusty color the pines took after being killed off by radiation from the meltdown, the woodlands around Chernobyl have experienced an increase in wildfire in recent years. 

Radioactivity from the disaster also killed bacteria, fungi and invertebrates that would have helped break down trees, needles and leaves, slowing the decomposition of vegetation in parts of the forest by up to 40 percent. 

The soil holds more than 90 percent of the radionuclides that fell on the forest, after needles covered in fallout fell to the ground and trees killed by it were bulldozed and buried. The trees growing there today continue to absorb the fallout from the disaster when they root into soils holding carcinogenic radionuclides like cesium-137 and pull them up into their leaves. 

With most of the population removed and commercial activities forbidden in the vast exclusion zone—an area about the size of Luxembourg secured around the site of the disaster—woody fuels for wildfires have accumulated for decades. Trees cover more than twice as much land as they did before the disaster. And, like the staff of the Chernobyl plant, the few firefighters and firefighting resources assigned to the Red Forest would be even more constrained by an invading army.

“It’s a tinderbox,” Mousseau said. “With nobody to suppress fires there, it wouldn’t be difficult for a fire to spread over the whole area.” 

A large, hot fire in the Red Forest could inject a plume of smoke and ash high enough in the atmosphere to be carried hundreds or even thousands of miles away. Research in 2011 by Ukrainian forestry professor Sergiy Zibtsev and Chad Oliver, who was then director of Yale’s Global Institute of Sustainable Forestry, estimated that a fire that fully consumed the forest would blanket Kyiv with radioactive smoke, increasing the risk of cancer for its residents. Produce grown up to 90 miles away from the fire would be so contaminated that it couldn’t be safely eaten and the stigma of radiation on one of Europe’s breadbaskets would keep other countries from importing even uncontaminated Ukrainian foods.

“It is getting drier in this area due to climate change,” Mosseau said. “We are seeing an increase in wildfires. Some of the largest fires there have occurred in the last few years.”

This winter, he said, the area has received less moisture than usual. And, as in much of the rest of the world, the fire season in the area has expanded, with blazes in the forest drawing hundreds of firefighters early in the spring. In April of 2020, a blaze burned more than 150,000 acres of the forest, the most since the nuclear disaster, smothering Kyiv with smoke. Sensors in Norway, 2,000-miles away from the fires, detected increased levels of cesium in the air.

As to why Russia would prioritize capturing Chernobyl, military analysts note that it’s along the shortest route from the territory of its ally, Belarus, to Kyiv and on a path that avoids the region’s marshes, where vehicles could be mired in mud. Chernobyl is 67 miles north of Kyiv.

The wreckage of Reactor 4 was covered with a concrete “sarcophagus” in 1986, and then with a steel and concrete structure called the New Safe Confinement, which was engineered to withstand a tornado, in 2016. Russia, which already has abundant nuclear fuel and weapons, and is close enough to Chernobyl that an incident there could threaten Moscow, has no strategic reason to breach those containment structures to access the 200 tons of radioactive fuel that remains buried in contaminated debris deep within them.

But Mousseau notes another reason that Chernobyl could be valuable to an army invading Ukraine.

“The major power grid and switching station for that whole region is there,” he said. “All these grids fed into this enormous nuclear power station. By having control of those, they have control over the electricity supply.”

The ability to turn off the power can provide Russia with a big advantage in their efforts to subdue Kyiv, but also points to a bigger nuclear threat in the Ukrainian conflict.

In addition to Chernobyl, where the last reactor stopped producing electricity in 2000, Ukraine has 15 operating nuclear reactors at four electricity plants across the country. Those reactors, many of which are old Soviet designs that are operating beyond their originally intended lifespans, are dependent on a steady supply of electricity and water to maintain their safe operation and keep them from melting down.

A wayward missile striking one of those reactors could spark a nuclear disaster. But any military operation that interrupted the power supply to one of them for longer than the facilities’ backup generators could keep the plant running safely has the potential to create another Chernobyl.