Sergei Supinsky / AFP | A picture taken on November 29, 2016 through a fence shows Chernobyl's New Safe Confinement covering the destroyed 4th block of Chernobyl Nuclear power plant.

In a vast exclusion zone in northern Ukraine, the world's largest land-based moving structure has been slid over the Chernobyl nuclear disaster site to stop deadly radiation spewing from the stricken reactor for the next 100 years.

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On April 26, 1986, a botched test at the Soviet nuclear plant sent clouds of smouldering nuclear material across large swathes of Europe, forced over 50,000 people to evacuate and poisoned unknown numbers of workers involved in its clean-up.

A concrete sarcophagus was hastily built over the site of the stricken reactor to contain the worst of the radiation, but a more permanent solution has been in the works since 2001.

Easily visible from kilometres away, the 36,000 tonne 'New Safe Confinement' arch has been slowly pulled over the site over the past four days to create a casement to block radiation and allow the remains of the reactor to be dismantled safely.

On Tuesday, a ceremony was held at Chernobyl to mark this major milestone in the decades of work to secure the site that has been funded by donations amounting to over 2 billion euros from over 40 countries and organisations.

"Let the whole world see today what Ukraine and the world can do when they unite, how we are able to protect the world from nuclear contamination and nuclear threats," Ukrainian President Petro Poroshenko said.

The structure, which resembles a vast aircraft hangar, has been designed to withstand extreme temperatures, corrosion and tornadoes.

The European Bank for Reconstruction and Development, which has managed the funding for the clean-up, said the programme to transform Chernobyl into an environmentally safe and secure condition by November 2017 was on track.

Even with the new arch, the surrounding zone, which at 2,600 square kilometres is roughly the size of Luxembourg, will remain largely uninhabitable and closed to unsanctioned visitors.

Over 30 years on from the disaster, nature has reclaimed much of the area's abandoned infrastructure. Trees sprout from the rusted roofs of apartment blocks in the ghost town of Pripyat, while some animal populations are booming in the absence of humans.

(FRANCE 24 with REUTERS)

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