Enter the Chernobyl zone, and you might expect the worst: Security guards at a checkpoint 19 miles away from the site of the world’s worst nuclear accident scan departing vehicles for signs of radiation, as wolfish strays linger around the checkpoint.

But pass the derelict villages and collective farms evacuated after the disaster 30 years ago this month, and a new skyline emerges over the ill-fated nuclear plant.

A workforce of around 2,500 people is finishing a massive steel enclosure that will cover Chernobyl’s reactor 4, where the radioactive innards of the nuclear plant are encased in a concrete sarcophagus hastily built after the disaster. The zone is now aglow with the reflective safety vests of construction workers.

If all goes to plan, the new structure—an arch more than 350 feet high and 500 feet long—will be slid into place late next year over the damaged reactor and its nuclear fuel, creating a leak-tight barrier designed to contain radioactive substances for at least the next 100 years.