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 like 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.”