By Henry Fountain

New York Times News Service

CHERNOBYL, Ukraine — Against the decaying skyline here, a one-of-a-kind engineering project is rising near the remains of the world’s worst civilian nuclear disaster.

An army of workers, shielded from radiation by thick concrete slabs, is constructing a huge arch, sheathed in acres of gleaming stainless steel and vast enough to cover the Statue of Liberty. The structure is so otherworldly it looks as if it could have been dropped by aliens onto this Soviet-era industrial landscape. If all goes as planned, by 2017 the 32,000-ton arch will be delicately pushed on Teflon pads to cover the ramshackle shelter that was built to entomb the radioactive remains of the reactor that exploded and burned here in April 1986.

When its ends are closed, it will be able to contain any radioactive dust should the aging shelter collapse.

By all but eliminating the risk of additional atmospheric contamination, the arch will remove the lingering threat of even a limited reprise of those nightmarish days 28 years ago, when radioactive fallout poisoned the flatlands for miles around and turned villages into ghost towns, filled with the echoes of abandoned lives.

The arch will also allow the final stage of the Chernobyl cleanup to begin — an arduous task to remove the heavily contaminated reactor debris for permanent safe storage. That this job will fall from international hands to those of Ukraine presents new worries, especially as Russia threatens the nation’s borders.

For now, though, the rising arch is a sign of progress.

“It’s an amazing structure,” said Nicolas Caille, project director for Novarka, the consortium of French construction companies that is building it. “You can’t compare it to anything else.”

With nations debating the future of atomic power as one way to reduce greenhouse-gas emissions and fight climate change, the arch is also a stark reminder that nuclear energy, for all of its benefits, carries enormous risks. When things go wrong, huge challenges follow.

Containment and cleanup push engineering capabilities to their limits, as Japan is also finding out since the meltdowns at the Fukushima power plant three years ago.

The costs are enormous — the Chernobyl arch alone will end up costing about $1.5 billion, financed largely by the United States and about 30 other nations.

And making the site of a radioactive disaster truly secure can take generations.

Engineers have designed the Chernobyl arch to stand for 100 years; they figure that is how long it may take to fully clean the area. But there have always been questions about Ukraine’s long-term commitment, and the political turmoil and tensions with Russia have raised new concerns. So even a century might not be enough.

The arch, though, is a formidable structure, said Vince Novak, the director of nuclear safety for the European Bank for Reconstruction and Development, which administers the project’s financing. If necessary, he said, “it might be able to last 300 years or more.”

The Chernobyl accident can be likened to a huge dirty bomb, an explosion that spewed radioactive material in all directions. The blast was followed by a fire that sent even more contaminants into the atmosphere that were then carried by winds across the region and into Western Europe.

In this way the disaster differs from nuclear power’s two other major accidents, at Three Mile Island in Pennsylvania in 1979 and Fukushima in 2011. At both of those plants, reactor cores melted down, but the core material — the nuclear fuel — remained within protective containment structures.

The four reactors at the Chernobyl plant had no such containment. But that was only one aspect of their flawed design. The system for controlling the nuclear fission reaction was temperamental, and under certain conditions reactor power could quickly soar out of control.

That is what happened in the early hours of April 26, 1986, at Chernobyl’s Unit 4, during an ill-advised test of some of the reactor’s safety systems. In a matter of seconds, the reactor power rose exponentially and the core was blasted apart by steam.

A few workers died immediately, but most of the technicians in Unit 4, and the firefighters who initially responded, suffered agonizing deaths over the ensuing weeks from exposure to high levels of radiation.

Officially, several dozen people were killed, and many others became sick. The radiation also caused thousands of later cancers — though just how many is still the subject of much debate.

In the immediate aftermath, the Soviet authorities brought in the military to fight the reactor fire and evacuate nearby villages and the city of Pripyat, home to most of the plant workers and their families. Laborers were enlisted to hastily build the concrete-and-steel shelter, known as the sarcophagus. When their radiation exposure grew too high, the workers were replaced by others; in all, more than half a million people were involved in the initial cleanup.

That was nearly three decades ago. But in and around Chernobyl, it is as if the calendar froze.

An exclusion zone of about 1,000 square miles still exists around the plant, with access controlled through checkpoints. Although radiation levels have declined somewhat through the natural process of radioactive decay, the zone remains virtually empty. Many of the villages were bulldozed; forest has overtaken others. In Pripyat, where 45,000 people once lived, paint peels off the murals in the community center and a tree grows in the middle of a gym floor.

Radiation levels around the site are carefully mapped and arch workers have to stay within proscribed areas. Everyone wears dosimeters, detectors that would sound an alarm if there were a release from the sarcophagus and radiation levels increased. But otherwise the workers have normal schedules and wear regular work clothes.

Yet working there can be anything but normal. “When you arrive here for the first time, it’s quite emotional,” Caille said. “We have a lot of people who have been involved or were here when they had the accident. It’s still very, very present.”

After the arch is in place, Dodd said, the plan is for Ukraine to eventually begin removing the unstable structures and the remaining fuel. That will ensure that the radiation does not eventually reach groundwater, which would endanger the water supply for the 3 million people of Kiev. Ukraine also must build a repository for all the high-level waste it recovers.

It is unclear where the money for that work will come from — especially now with the country in turmoil. Officials with the Ukrainian Embassy in Washington did not respond when asked for comment.

But even if there is enough money, there are technical questions as to whether the work can be accomplished, and if so how long it will take.

But that is all in the future. The more immediate problem is completing the arch in an unstable political environment.

“I am very concerned how the current situation will affect this initiative,” Novak said.

Chernobyl, near Ukraine’s northern border, is far from the Crimea and other disputed territory. “Chernobyl and the exclusion zone remain calm, and our contractors continue their work,” Novak said, although Western experts were evacuated for a week in March.