On November 29th, officials involved with the New Safe Confinement (NSC) project gathered near the site of the Chernobyl Nuclear Power Plant in Ukraine, to celebrate the installation of the massive NSC structure over top of the plant’s aging containment building. The NSC building, meant to isolate the entire power plant from the surrounding environment for the next century, required a major feat of engineering to bring into being.

On April 26, 1986, reactor #4 at the power plant exploded after pressure inside the reactor grew beyond its safety limits, the result of a horribly mismanaged safety test. A secondary explosion in the core ejected a large portion of its fuel into the air, and a fire in the reactor continued to eject radioactive material into the atmosphere until May 10th.

A containment vessel, nicknamed the sarcophagus, was erected around the destroyed reactor building, to isolate the highly-radioactive material from the surrounding environment; however, because of the necessary haste involved in constructing the structure, it was expected to only last for thirty years before it would need to be replaced.

And now, thirty years after the now-deteriorating sarcophagus was built, a massive new structure, the NSC, has been put into place to contain both the reactor and its encasement. Costing 1.5 billion euros ($1.58 billion), the structure is a massive, quonset-like arch, 108 meters (354 feet) tall and 257 meters (843 feet) across. The radiation still being emitted by the reactor made it impossible to construct the NSC directly over the sarcophagus, so instead it was constructed 180 meters (590 ft) to the west of the reactor building.

The structure, mounted on large rails, was slowly moved into position by hydraulic jacks over the course of several days. At 36,000 metric tons (39,700 tons), the NSC is the world’s largest mobile metal structure, and is designed to withstand the force of a class-3 tornado or a 6.0 magnitude earthquake.

The foundation and end walls of the structure are expected to be completed over the course of 2017, effectively sealing off the radioactive material inside. The NSC also features two remote-controlled demolition cranes to aid in the demolition of the aging reactor building inside, and a new radioactive waste storage facility is being built nearby, that will be able to contain 75,000 cubic meters (265,000 cubic feet) of spent nuclear fuel.