Russia took control of the Chernobyl nuclear site in Ukraine. What does that mean?

The Russian invasion of Ukraine has sent troops through Chernobyl, the site of the world’s worst nuclear power plant accident, raising questions about the site’s status and what risks this activity may pose.

What happened this week?

Early on Thursday, Feb. 24, the first day of their invasion, Russian troops occupied Chernobyl, the site of a 1986 explosion that spewed radioactive material into the surrounding area. Although Ukraine had stationed security forces in the area to monitor the situation, they weren’t able to fight off the special forces and airborne troops sent by Russia. Chernobyl sits between the Ukranian capital of Kyiv and the country’s border with Belarus.

The Ukraine government said radiation monitors in the area showed a spike in radiation levels after the Chernobyl attack. Safecast, a non-profit environmental data monitoring group, said on Twitter that the most likely explanation was that Russian military moving through the area were stirring up dust. Safecast said the monitoring equipment had gone offline at 1 a.m. Eastern European time on Friday, but before that several sensor readings had already returned to normal. The spike may also have been caused by a sensor malfunction, experts said.

Claire Corkhill, a materials scientist and chair in Nuclear Material Degradation at the University of Sheffield, UK, said on Twitter that while the spikes were up to 20 times normal, they were still “not especially high in relative terms,” about 65 microSieverts/h, compared to about 13 µSv/h for a transatlantic flight.

What happened at Chernobyl in 1986?

At the time of the accident in 1986, Chernobyl was the site of a Soviet power plant with four active nuclear reactors and two under construction. On April 26 of that year, operators running a test lost control of the Number Four reactor, causing a fire that destroyed the building. Steam pressure built up and caused the core to explode, spewing radioactive material into the air. Uranium fuel overheated and melted through barriers. Large amounts of radioactive iodine, cesium, strontium, and plutonium, as well as other isotopes produced during nuclear fission, were scattered over an area of 150,000 km².

Two workers died in the initial explosion, and 28 emergency responders died of radiation sickness in the three months afterwards. At least 1,800 children in the area developed thyroid cancer. Fifty thousand residents were evacuated from the nearby city of Pripyat, which remains a ghost town.

What’s at the site today?

In the weeks following the accident, workers enclosed the reactor building—which still contains highly radioactive fuel material—in a concrete and steel structure called “the sarcophagus.” That eventually began to deteriorate. To address this, a “New Safe Confinement” structure, completed in 2019, was built over the original enclosure. It is designed to withstand events such as tornados and earthquakes.

Another facility at the site holds spent fuel, stored in water for radiation shielding and cooling. Once it cools sufficiently, the fuel is transferred to dry storage in the exclusion zone, a process that began last year.

The most dangerous isotopes released by the accident were iodine-131, strontium-90, and cesium-137. Iodine-131 becomes concentrated in the thyroid gland and can quickly deliver cancer-causing levels of radiation, but has decayed by half in only eight days. The chemical structure of strontium-90 allows it to replace calcium and become assimilated in teeth and bones, exposing the body to radiation over a longer period with its half-life of 29 years. Cesium-137, with a half-life of 30 years, is one of the most common byproducts of fission; is highly radioactive, chemically reactive, and soluble; and is light enough that it spread the farthest of all the contamination. A 2009 study by researchers at the Savannah River National Laboratory found that the contamination in the area did not appear to be disappearing as fast as expected.

Researchers have conducted a range of environmental and biological studies in the area, which has been opened to limited tourism. A few residents of the area have returned.

What risk does the occupation pose?

Steven Arndt, a nuclear engineering expert and president-elect of the American Nuclear Society, the professional organization of nuclear scientists and engineers, says that it’s very unlikely that the New Safe Confinement structure could be accidentally breached. “I’m not overly concerned that this is going to spread radiation a large distance outside the exclusion zone,” he says.

Troops entering the area were likely briefed about precautions to take, he says, and military equipment likely provides adequate protection. Even if troop movements are stirring up contaminated soil, that’s not likely to drive radiation very far, unlike the heat from the fires during the original accident.

Kate Brown, an Massachusetts Institute of Technology historian who has studied Chernobyl, says she spoke to a friend who used to work at the plant and lives nearby. He told her plant security tried to avoid a firefight that might cause damage and release radiation. “As a captured installation, there is no danger right now,” she says.