On Tuesday, officials from all over the world gathered about a football field away from the Chernobyl disaster site in Ukraine. They were there to celebrate the final placement of a massive, high-tech shelter over reactor 4, which exploded in April 1986.

The shelter, called the New Safe Confinement (NSC), is a feat of engineering. Because it was too dangerous to assemble the NSC over the original shelter that was built in the weeks after the explosion, the NSC was instead built at a distance and moved—slowly, over days—on a pair of tracks parallel to the original shelter. But even that was no simple task. The NSC is 354ft (108m) tall and 843ft (257m) wide, making it the largest mobile metal structure in the world.

Why it’s necessary

Thirty years ago, when the reactor exploded, plumes of radioactive debris spread across what are now the countries of Ukraine, Belarus, and Russia. Soviet authorities evacuated a 30-kilometer radius around the explosion site, but tens of thousands of kilometers outside that zone also experienced contamination. Workers were called in to encase the reactor in a sarcophagus to minimize contamination. Ultimately at least 28 people died as a result of the accident, according to the World Nuclear Association, and another 237 workers involved in the cleanup and shelter construction were later diagnosed with acute radiation poisoning, with 134 of those cases being confirmed later.

Unfortunately but understandably, the structure encasing reactor 4 was hastily built and risked corrosion and leaking that could lead to renewed contamination of water or air at any time. By the late '90s, parts of the structure risked “imminent collapse” according to the European Bank for Reconstruction and Development (EBRD), which has funded the new high-tech shelter that will encase the original one. Emergency repairs were made, but it wasn’t until 2004 that the site received the necessary equipment to do biomedical screening and conduct construction activities in a way that would minimize harm to workers. To make matters worse, according to EBRD, radioactive water was present in the shelter basement and concerns were growing that the contaminated water would leak into the nearby Dnipro Basin. In 2010, an automated monitoring system was added to keep tabs on the water situation.

100 years of confinement

The NSC was built to address these issues. Experts are confident that the radioactive water will dry out, and the double walls of the shelter are guaranteed to prevent any radiological releases for the next 100 years. Besides also keeping out any rain or snow and providing a barrier against extreme hot or cold, the external cladding will be able to withstand a class 3 tornado or a 6.0 magnitude earthquake. The structure's internal wall is smooth to “minimize the risk of dust deposition and accumulation," according to the EBRD, and it’s made of corrosion-resistant, fire-resistant, and non-magnetic steel.

Due to the radiation condition around the NSC, the space between the interior and exterior walls “has to have an active corrosion-control system,” the EBRD writes. After the two halves of the shelter were put together in 2014, the air between the outer and inner walls was dried and pressurized, and the complete shelter has a ventilation system to prevent the humidity level from rising past 40 percent.

The NSC also includes two remote-controlled cranes that will disassemble the original shelter after the NSC has been sealed off. The crane system was “specifically designed for dismantling the main structures of the destroyed reactor and original sarcophagus and for handling heavily shielded waste disposal,” says the EBRD. An auxiliary building, where humans will work, was erected to contain all the control systems for the NSC.

Throughout 2017, additional construction will make the shelter airtight.

Ultimately, 40 countries raised money for the New Safe Confinement, which cost roughly €1.5 billion ($1.58 billion) to build. Those countries hope that for the next 100 years, Chernobyl's danger will definitively be a thing of the past.

Correction: Ars originally stated that the celebration of the structure occurred on Wednesday; it actually occurred on Tuesday, Nov 29.

Listing image by European Bank for Reconstruction and Development