The arch, called the New Safe Confinement, is being built — at a cost of at least $1.7 billion — to last 100 years. Inside, the radioactivity levels will be so high that normal maintenance, like painting, will not be possible. So inside and out, the arch is covered in stainless steel, and dehumidified air will be circulated around the structure’s steel trusses to prevent rust.

It was built several hundred yards from the destroyed No. 4 reactor, and later this year will be slid in place over the reactor building. That will eliminate one of the greatest risks that still exists at Chernobyl: a structural collapse that could raise a cloud of radioactive dust and spread more contamination across Ukraine and into Western Europe.

But it will also mark the start of a new phase in coping with what is generally considered the world’s worst nuclear power disaster. Inside the arch will be a heavy duty crane and other remote-operated equipment to be used to start removing the crumbling radioactive fuel that remains in Unit 4.

It will not be an easy task. There is a lot of fuel — 195 tons, by one estimate — along with tons of lead, sand and other materials that were dropped on the reactor by helicopter in a desperate effort to extinguish the fire. It all melted together into a lethal lava-like substance that poured through pipes and holes in the structure and solidified.