Engineers from all over the world worked for more than a decade on the modern equivalent of the Acropolis when they contributed to the construction of a giant protective shield for the exploded No. 4 reactor at Chernobyl, which was designed to protect the world from further fallout from the world’s worst nuclear disaster.

The steel shell, having been dragged into place over the plant’s Reactor No. 4 on rails in 2016, is the world’s largest movable structure. It is as tall as a football field and weighs some 31,000 tons. More than 45 countries and organizations channeled upwards of €1.5 billion through the European Bank of Reconstruction and Development to build it.

“We did a lot of safety analysis, considering a lot of bad things that could happen,” said Eric Schmieman, a retired civil engineer from Washington state who was a senior technical adviser on the project. “We considered earthquakes, tornadoes, heavy winds, 100-year snowfalls, all kinds of things. We didn’t consider acts of war.”

In the early hours of February 14, 2025, a cheap drone blew a very expensive hole in the giant steel shell. Ukrainian officials said the Russians deliberately targeted the structure with a Shahed 136 drone—once again striking frayed nerves that fear a repeat of the original Chernobyl disaster as ordinance continues to fly around civilian nuclear installations in Ukraine. Moscow denied responsibility.

While the initial fire was quickly extinguished, a waterproof membrane inside the insulation of the arch burned and smoldered for almost three weeks, said Tetiana Maltseva of the State Agency of Ukraine on Exclusion Zone Management, which is overseeing repairs the shell.

Emergency workers outfitted with mountaineering gear scrabbled along the shell— known as the New Safe Confinement—punching holes in its outer layer to hunt down fire, spraying water inside the structure, which is meant to stay dry to prevent corrosion, Maltseva explained.

On March 7, Ukrainian officials declared the fire officially extinguished. But by that point, roughly half of the northern section of the shield had been damaged, Maltseva said. A few days later, on March 13, the International Atomic Energy Agency reported that the fires and smoldering had caused “extensive damage, including to the northern side and to a lesser extent to the southern side of its roof,” according to an evaluation that Ukraine shared with the agency.

Radiation levels outside Chernobyl are still normal, the IAEA and Ukrainian nuclear regulators say. But the repair process, which Ukraine has begun with international assistance, is laborious and costly, and officials remain uncertain about how long it will take to fully restore the arch.

That could also delay the removal of the estimated 200 tons of fuel remaining inside it.

And there is a risk that the steel shell could start to corrode, or that the so-called “sarcophagus,” a hastily-constructed cement and steel structure that Soviet engineers built to entomb the remains of the exploded reactor, which sits within the shell, could deteriorate further.

The drone attack was only the latest military incursion into Chernobyl’s the 2,600-square-kilometer exclusion zone, an enormous swathe of irradiated forest and workers’ ghost towns that were evacuated after the 1986 disaster—and one of only many attacks jeopardizing Ukraine’s civilian nuclear infrastructure.

In the opening days of the war, Russian troops overran the zone, their transports and tanks churning up radioactive dust while soldiers looted and vandalized workshops necessary to the ongoing decommissioning of the defunct plant. Then, amid unconfirmed reports of radiation illness among the invaders, Russian troops withdrew, and, in a weirdly official flourish, handed the exclusion zone back to the Ukrainians.

The Zaporizhzhia nuclear plant, seized by Russian shortly after Moscow’s invasion began, has been struck repeatedly, but each time disaster has been nail-bitingly averted. In 2023, an explosion at the Kakhovka Dam drained the primary source of cooling water for the plant’s six reactors, forcing a pivot to a backup cooling pond. The site now relies on two electrical transmission lines—one of which is periodically severed by fighting—to keep its shutdown reactors cool.

The Kharkiv Institute of Physics and Technology, which houses a research reactor, presently shutdown, but still containing several dozen kilograms of enriched uranium, has also been hit repeatedly by Russian ordinance.

The drone strike on Chernobyl came the night before the Munich Security Conference opened in Germany, timing that some Ukrainian and Western officials interpreted as a message from Moscow. According to Ukrainian officials, drones continue to fly past the Chernobyl site almost nightly, many likely headed toward the capital of Kyiv, 130 kilometers to the south.

The Chernobyl disaster remains a defining moment in the twilight years of the Soviet Union. Moscow sought to obscure the disaster while quietly evacuating more than 116,000 people from the area surrounding the plant in the days after the reactor exploded. It would finally be Swedish officials who pierced Moscow’s official silence when they announced mysterious spikes in their own radiation monitoring systems. What they detected was a plume of radioactive material ejected into the atmosphere, causing a public health emergency across Europe and leading to a skepticism toward nuclear energy that would last decades.

The explosion’s official death toll was 31—a figure many experts say is ludicrously low. In the following years, hundreds of people got sick, and many eventually died. Cancer rates, especially for thyroid cancer, increased in areas heavily exposed to radiation. In later interviews, Mikhail Gorbachev, the last Soviet president on whose watch the Chernobyl accident occurred, would identify the catastrophe as one of the most important factors hastening the Soviet collapse.

The sarcophagus, which has become increasingly unstable, was never meant to last. By as soon as the mid-1990s, cracks had opened, leaks had formed, and the whole shell was sagging under its own weight. Figuring out how to replace it, however, took decades, and lots of money.

The confinement structure at Chernobyl that Schmieman and others developed was an engineering and construction marvel designed to protect the crippled reactor for 100 years. To minimize radiation exposure, the structure was built about a half a kilometer away from the reactor, then moved into place on rails. In places, the structure measures about 12 meters between its inner and outer shells, and the space between them is kept at below 40 percent humidity to prevent corrosion.

The outer shell is the critical to keeping out precipitation, Schmieman said. The inner shell is designed to keep the radioactive dust inside the structure, especially when the cranes already set up within it start taking apart the sarcophagus and the damaged reactor before safely disposing of the waste in smaller containers.

Specialists had aimed to finish the initial plans outlining that first dismantlement stage, but the drone attack has made that impossible, said Maltseva. “None of this work can continue until the shelter functions are fully restored,” she said.

She added that experts—including some of the initial designers, like Schmieman—had evaluated measures for fixing the shell. Workers have almost closed the initial 15-square meter hole in the outer shell left by the drone attack. But all of the smaller holes left by workers who were chasing the fire must also be sealed. They also would have to somehow repair the damaged membrane and insulation and any damaged internal structures. And they would have to reduce the humidity that resulted from hundreds of workers spraying high-powered hoses inside the structure.

For now, Maltseva says the entire New Safe Confinement structure is considered by Ukrainian regulators to be in an “emergency situation.”  In July, Kyiv and the EBRD inked an agreement to fund further assessments of the damage, all of which will contribute to a comprehensive overview of what needs to be done to bring the shell back to its full function, Maltseva said.

What that will cost, however, remains unknown, though the EBRD  has estimated that repairs could cost more than €100 million.