The first, at 1:23 a.m. on April 26, 1986, spread a cloud of deadly radiation that raised fears across Europe and shook the very foundations of the Soviet Union. Some say it led to its eventual collapse.

The second, at 1:59 a.m. on Feb. 14, 2025, was blamed by Ukrainian officials on a Russian drone with an explosive warhead. While not as catastrophic, it sparked new anxieties about Moscow’s invasion of its neighbor, striking the site that symbolized so much suffering for Ukraine.

“What once seemed unthinkable — strikes on nuclear facilities and other hazardous sites — has now become reality,” said Oleh Solonenko, head of a radiation safety shift at Chernobyl, which Ukrainians transliterate as Chornobyl.

The drone hit the outer layer of what is known as the New Safe Confinement structure, or NSC, the vast, $2.1 billion archlike shell that was completed in 2019 to enclose the original, hastily built concrete “sarcophagus” to keep the damaged Reactor No. 4 and its deadly debris from leaking radiation. Moscow denied targeting the plant, alleging Kyiv staged the attack.

It sparked a fire on the structure — which is tall enough to cover the Statue of Liberty — but did not penetrate it, damaging an area with low contamination. Monitors detected no rise in radiation levels outside the arch, and no one was injured.

Still, the International Atomic Energy Agency warned that the damage could significantly shorten the arch’s 100-year lifespan, upending its core safety function.

For Klavdiia Omelchenko, who works with over 2,200 engineers, scientists and others at the defunct plant, it rekindled memories of a horrible spring day 40 years ago.

A lifetime near Chernobyl

Omelchenko was a 19-year-old textile factory worker in 1986, asleep in her home in Pripyat, where most of Chernobyl’s workers lived. She didn’t hear the explosion at Reactor No. 4 during a routine test.

She woke to rumors of an accident, but only understood its scale weeks later — after being evacuated with a small bag holding her documents and some cosmetics. Her former home was now inside Chernobyl’s “exclusion zone,” a 2,600-square-kilometer (1,000-square-mile) area that remains uninhabited.

Soviet authorities did not immediately reveal the scope of what became known as the world’s worst nuclear disaster, which spewed a cloud of radiation over what is now Ukraine and Belarus, and caused alarm across Europe. Dozens of people died in the immediate aftermath, while the long-term death toll from radiation is unknown.

Omelchenko never found another home and came back in 1993 to work in the plant’s cafeteria. That return “wasn’t as scary as now. Back then, at least, there was no bombing,” she added.

To her, the full-scale invasion in 2022 and last year’s drone attack are more fearful than radiation.

She said she got headaches after the 1986 accident and later had surgery for a precancerous condition, but at age 59, she dismisses the risk of contamination.

“We grew up in it,” she said. “We don’t pay attention to it anymore.”

Covering the sarcophagus

Yellow daffodils bloom beside wartime fortifications at the Chernobyl plant as workers in ordinary clothes, with badges and special permits, pass through the restricted zone.

It has not produced electricity since 2000, when the last of four reactors was shut down. A global effort built the protective NSC — a landmark project designed to stabilize the site and enable the dismantling of the crumbling Soviet-era sarcophagus covering the reactor.

But Russia’s invasion has put that project on hold.

Liudmyla Kozak, an engineer who has worked at Chernobyl for over two decades, was on duty when Russian troops seized the plant in February 2022. The staff kept operations running under armed guard for nearly three weeks, exposing personnel to radiation doses well beyond the limits of their normal rotation schedules.

“We had no hope we would make it out alive — it was really that scary,” she said.

Kozak said workers slept on floors and desks, with Russian soldiers occupying key areas. Equipment was damaged and stolen, she added. The troops also drove heavy vehicles through contaminated areas and dug trenches, stirring up radioactive dust.

“With the drone strike as well, it will be much more complicated,” Kozak said.

The IAEA found the damage has left the arch unable to fully perform its core functions, which is containing radioactive material and enabling the safe dismantling of the reactor remains. Left unrepaired, the structure would gradually weaken, increasing radiation exposure risks to Ukraine and other countries.

Dismantling work on hold

Serhii Bokov, who oversees operations for the NSC, said he was on duty early on Feb. 14, 2025, when the dull blast from the drone rippled through the structure.

He and his colleagues ran outside, smelling smoke, but initially saw nothing. A nearby military checkpoint confirmed a strike, and firefighters arrived about 40 minutes later.

Climbing up into the structure, they finally found fire smoldering through the outer membrane. Hoses were stretched across the arch as crews battled flames that kept resurfacing. The fire took more than two weeks to extinguish fully.

“There was no feeling of fear, none at all. It was just a fire — something we practice in drills — only this time it was real,” he said. “I didn’t think, honestly, that we could lose the entire arch.”

The damage is patched and hidden on the inside, while a sealed breach is visible on the outside.

Every night, Bokov walks more than a kilometer (about 1,100 yards) through the structure via what workers call the “golden corridor” — a passageway lined with yellow panels shielding them from radiation. It passes abandoned control rooms, including that of Reactor No. 4.

When the NSC was completed in 2019, he was proud of being part of something extraordinary, watching it rise and take shape, and being a member of the team keeping it running.

Now, however, the structure is no longer fully sealed. While there is no immediate radiation risk, work on dismantling the sarcophagus is on hold — set back, Bokov believes, by at least a decade.

“Everything depends on how quickly we can restore this and return to normal operations — and to preparing for dismantling,” he said.

Bokov believes the arch can continue functioning in its current state for some time. But the real concern is the stability of the sarcophagus beneath it — and why it’s urgent to resume its dismantling.

Oleh Solonenko, head of a radiation safety shift at the plant, said the drone damaged the outer layer of the protective NSC but did not fully penetrate it. The damage occurred in an area with low contamination, with no rise in radiation detected beyond the arch.

Still, the incident showed how the war has upended assumptions about nuclear safety, he said.

Without urgent repairs, the risk of the sarcophagus collapsing significantly increases, Greenpeace Ukraine warned in a report by engineer Eric Schmieman, who spent years at Chernobyl and helped design the NSC.

“It is difficult to comprehend the scale of the deadly, hazardous conditions inside the sarcophagus,” he said. “There are tons of highly radioactive nuclear fuel, dust and debris. Now it is critical to find a way to restore the key functions of this facility.”

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AP reporters Vasilisa Stepanenko and Volodymyr Yurchuk in Kyiv contributed.