It was the worst nuclear plant accident in history, measured in both casualties and cost. And though the death count paled in comparison to the more than 100,000 killed by the atomic bomb in Hiroshima, the amount of radioactive material released in Chernobyl was 400 times higher.

Six months after the 1986 disaster, the Soviet Union hastily covered the damaged reactor 4 with a massive sarcophagus of metal and concrete. Only expected to last 20 years, the structure has shown many signs of aging in the past decade, including a leaky roof that led to corrosion. Now, 30 years later, its French-built replacement structure has been put into place, and will be inaugurated today in a ceremony attended by some 500 people, Paris-based daily Les Echos reports.

Since 2012, French consortium Novarka has been constructing a new arched structure that has been sliding into place over the site. Designed to seal the reactor complex, keeping it environmentally secure for an estimated 100 years, the 25,000-ton steel framework of the shelter is the largest mobile land structure in the world.

The shelter, which cost more than two billion euros and was largely funded by the European Bank for Reconstruction and Development, will allow the partial demolition of the original sarcophagus and the reactor, at some point in the future. Beyond just the heavy lifting, the work on the most contaminated areas of the site will be carried out by robots.