Branded as the world's largest moveable metal structure, the so-called New Safe Confinement seals the remains of the fourth reactor at the nuclear plant that was the site of the massive Chernobyl disaster in 1986.

The 1.5 billion euro ($1.7 billion) structure was paid for via a special fund launched by the European Bank for Reconstruction and Development (EBRD) and sponsored by 45 countries.

Its gigantic 108-metre-high dome could cover the Notre Dame cathedral in Paris and weighs 36,000 tonnes.

"Today we get the keys to the construction that was created by joint efforts of dozens of countries to protect the entire planet and humanity from radioactive contamination," Ukrainian President Volodymyr Zelensky said at the ceremony.

He noted that "until this moment" Chernobyl used to be a negative part of Ukraine's brand, but "it's time to turn our problem into our advantage".

The structure is strong enough to withstand a tornado and is built to last a century, the EBRD said.

The dome was put into place in 2016 and completely covers a hastily-built confinement which was erected immediately after the disaster struck.

"This marked the next step in the transformation of Chernobyl into an environmentally safe and secure state," the EBRD said of the Wednesday launch, which formally handed over the confinement to Ukrainian authorities.

The Chernobyl nuclear power station was the scene of the world's worst civilian nuclear accident on April 26, 1986.

Thirty people were killed in an explosion and many hundreds died of related illnesses, though the exact figure remains disputed. The then Soviet authorities initially tried to play down the disaster.

Eventually 350,000 people were evacuated from within a 30-kilometre radius around the plant, an exclusion zone that remains uninhabited, apart from some 150 elderly residents who returned despite an official ban.

Authorities now say it will only be safe for humans to live there again in 24,000 years.

The new dome will have to undergo a year-long pilot operation period before the power plant can obtain a licence from Ukraine's regulatory authorities and begin dismantling the unstable structures of the original Soviet shelter.

Under the new arch an AFP correspondent saw a number of cranes, a ventilation system and facilities where radioactive materials will be "packed" in the future.