Concerns mount over conflict in Chernobyl exclusion zone

As Russian troops continue to inch their way through its invasion of Ukraine, a secondary catastrophe to the fighting between the ex-Soviet neighbors is possible: another nuclear reaction at Chernobyl.

On Thursday afternoon, Russian armed forces entered the deserted exclusion zone around the Chernobyl power plant, where the world's worst nuclear accident took place in 1986. By night, Russian forces had taken full control of the area, including the plant itself, according to Ukraine Prime Minister Denys Shmyhal.

The heavy fighting inside the "exclusion zone," a vast and empty land surrounding the Chernobyl nuclear plant that includes the ghost city of Pripyat, is causing concern that it could spark another nuclear disaster. International Atomic Energy Agency Director General Rafael Mariano is watching the situation with "grave concern, appealing for "maximum restraint" amid the conflict to avoid putting the nuclear facility at risk.

"It is of vital importance that the safe and secure operations of the nuclear facilities in that zone should not be affected or disrupted in any way," Mariano said in a statement.

On April 26, 1986, reactor No. 4 at the power plant, about 65 miles north of the capital Kyiv, exploded, spewing enormous amounts of radioactive material into the atmosphere and causing more than 100,000 people in a 1,000-square-mile radius to evacuate.

The destroyed reactor itself was sealed in 2019 under a $2 billion arch-shaped shelter, a stadium-sized metal structure that was built over it to contain, but the other three untouched reactors remain "fully exposed," said Tim Mousseau, a professor of biological sciences at the University of South Carolina who has been studying Chernobyl for more than 20 years.

"There are more than 5 million pounds of spent nuclear fuel, also uranium and plutonium and a few other nasty isotopes, sitting in the cooling ponds of the the three reactors that didn't blow up in 1986," Mousseau said.

The fighting represents an "existential threat" to the environment, Mousseau told ABC News.

Should missiles hit the structure over reactor No. 4, where "quite a bit" of radioactive material is left, or the facility where the spent nuclear fuel that accumulated over the decades of operation there is stored, it would cause large amounts of radioactive nuclear dust to spread throughout the region, Mousseau said.

"If this storage area were to be hit with any kind of any kind of missile, this could release vast quantities of highly radioactive material would spread far and wide, potentially causing an even larger disaster than the 1986 disaster," he said.

In addition, the area surrounding the power plant is "absolutely the most radioactive place on the planet," Mousseau said. That, combined with the "tinderbox" conditions left by severe forest fires in recent years, could allow a fast-sweeping wildfires to spew radio nuclides back into the atmosphere and "spread it again far and wide," Mousseau said.

Russian forces in Chernobyl prove an additional worrying sign for Ukraine, as they are now about an hour's drive from the capital. Russian special forces have also managed to keep hold of a key military airport just 20 miles from the very center of Kyiv, despite fierce fighting.

"Unfortunately, we are obliged to inform that as things stand the Chernobyl Zone, the so-called 'Exclusion Zone' and all the Chernobyl nuclear power station have been taken under the control of the Russian armed groups," Shmyhal told UNIAN, Ukraine's main news wire.

Russia's decision to enter Ukraine through such a vulnerable region as Chernobyl could be emblematic of additional escalation to come, Mousseau said, describing the situation facing the disaster site as "the worst nightmare come true."

"No one in their right mind would want to engage in warfare in that region for fear of unleashing potentially the largest nuclear disaster ever," he said.

Earlier in the day, Ukrainian President Volodymyr Zelenskyy tweeted that Ukrainian troops were fighting and "giving their lives" to avoid another 1986 Chernobyl disaster.

"This is a declaration of war against the whole of Europe," Zelenskyy wrote of the occupation.

U.S. officials had predicted before the invasion that Russia would use special forces to land into the capital before forces from Belarus, which borders the northern side of the exclusion zone, would sweep down rapidly in an effort to back Russia's lightning strike to seize the city.

ABC News' Patrick Reevell, Cindy Smith and Morgan Winsor contributed to this report.