A new shelter being built over the Chernobyl nuclear plant is intended to secure the site for 100 years.
In a ceremony on the 26th anniversary of the plant's meltdown, President Viktor Yanukovych of Ukraine thanked foreign countries that contributed to the $US2 billion cost of the covering.
The disaster in 1986 spread clouds of radiation across Europe and forced hundreds of thousands of people to be evacuated.
It was caused by an explosion and fire in the main reactor at the station.
The BBC reports says the current protective covering is leaking radiation. The new covering is due to be finished in three years.
