The structure – which took nine years to build – was constructed to secure the molten reactor core and 200 tons of radioactive material at the site.
Officials have described the shelter as the largest moveable land-based structure ever built, with a span of 257 metres and a total weight of over 36,000 metric tons.
Thirty workers died either from the explosion or from acute radiation sickness within several months.
About 600,000 people had exposure to radiation at elevated levels while fighting the fire at the plant or working to clean up the contamination.
The disaster’s eventual death toll has been subject to speculation and dispute, but the World Health Organisation’s cancer research arm has estimated that 9,000 people were to die of exposure-related cancer and leukaemia if Chernobyl disaster’s health effects follow a similar pattern to the Hiroshima and Nagasaki atomic bombings.
The new confinement structure – opened by Ukraine’s president Volodymyr Zelenskiy – was designed to safeguard radioactive debris and prevent further crumbling of the reactor.
Deputy project manager Victor Zalizetskyi, who has been part of construction and repairs at the Chernobyl plant since 1987, said he was “filled with pride” that he got to work on a job “that has such a big importance for all humankind.”
However, Mr Zalizetskyi previously expressed concern that war-torn Ukraine might struggle to cover the maintenance costs for the reactor’s new enclosure.
He noted that costly and complicated work such as dismantling unstable sections of the power plant still needs to be done.
“It looks like Ukraine will be left alone to deal with this structure,” he said. “The work is not done yet, and we need to think about how to finance this project in the future.”