If fighting breaches the facility’s massive sarcophagus, it could mean trouble for Europe.

Russian Ground Forces have captured the decommissioned Chernobyl nuclear power plant, Ukraine’s government confirmed Thursday. Advancing Russian army troops reportedly took over the site of the 1986 nuclear disaster following combat with Ukrainian defenders. The fighting has raised concerns that the storage facilities housing dangerous radioactive waste and debris could be damaged, re-releasing radioactivity that could spread across Europe and beyond.

“It is impossible to say the Chernobyl nuclear power plant is safe after a totally pointless attack by the Russians,” a representative of the Ukrainian government told Reuters, calling it one of the “most serious threats in Europe today.”

Chernobyl was the site of the world’s worst nuclear accident. Less than 100 people died in the accident, but it caused widespread distribution of radioactive fallout across Ukraine, Russia, Western Europe, and Scandinavia. As a result, the Ukrainian government sealed the radioactive debris in concrete containers, and put the damaged No. 4 reactor itself in a 40,000-ton, $2.5 billion reinforced steel shelter. There is also a 30-kilometer Chernobyl Exclusion Zone, spanning both Belarus and Ukraine, where people are banned from entering due to lingering radiation concerns.

Russia’s 35th Combined Arms Army, which includes three battalion tactical groups, was poised before the war to advance on the capital of Kyiv. Unlike other forces, however, the Chernobyl Exclusion Zone stood in its way. Russian army units may not have set out to actually capture the site, but simply wanted to pass through it, seeking to seal off the capital city from behind.

A short video on Twitter appears to show a Russian T-80U main battle tank, a BTR-80 wheeled armored personnel carrier, and a BMP-2 infantry fighting vehicle parked on the grounds of the site. Popular Mechanics has reviewed the video and confirmed the video was in fact taken at the exact point where the Chernobyl Nuclear Power Plant is pinned on Google Maps. The large structure in the background of the video lines up with Chernobyl Reactor No. Five, which was never completed.

An unnamed military source quoted by the Associated Press has a different explanation, stating that Russia “wants to control the Chernobyl nuclear reactor to signal NATO not to interfere militarily.” That explanation does not make a lot of sense, however, as any release of radioactivity as a weapon against NATO would first go through occupied Ukraine and then Russia itself.

There are some reports of damage at the Chernobyl facility as a result of shelling, though it’s not clear if the damage is to any of the hundreds of structures there or the most important one—the giant steel sarcophagus that entombs Reactor No. Four. It seems likely that Russian forces are under orders to prevent damage to the site, given the danger escaping radioactivity poses to Russia.

There are four fully operational nuclear power plants, with a total of 15 nuclear reactors, spread across Ukraine. According to World Nuclear News, the CEO Of Energoatom, the state nuclear power company, recently reassured the public that nuclear plants will be shut down in the event of emergencies, and that they are structurally strong enough to survive aircraft crashes.