Ukraine was on Tuesday due to unveil the world's largest moveable metal structure over the Chernobyl nuclear power plant's doomed fourth reactor to ensure the safety of future generations across Europe.

The giant arch's height of 108 metres (355 feet) makes it taller than New York's Statue of Liberty while its weight of 36,000 tons is three times heavier that the Eiffel Tower in Paris.

The 1.5-billion-euro ($1.6-billion) structure has been edged into place over an existing crumbling dome that the Soviets built in haste when disaster struck three decades ago on April 26.

Radioactive fallout from the site of the world's worst civil nuclear accident spread across three-quarters of Europe and promoted a global rethink about the safety of atomic fuel.

Work on the previous dome began after a 10-day fire caused by the explosion was contained but as radiation still spewed.

"It was done through the super-human efforts of thousands of ordinary people," the Chernobyl museum's deputy chief Anna Korolevska told AFP.

"What kind of protective gear could they have possibly had? They worked in regular construction clothes."

About 30 of the cleanup workers known as liquidators were killed on site or died from overwhelming radiation poisoning in the following weeks.

The toll from the accident caused by errors during an experimental safety check remains under dispute because the Soviet authorities did their best to cover up the tragedy.

A United Nations estimate in 2005 said around 4,000 people had either been killed or were left dying from cancer and other related disease.

But the Greenpeace environmental protection group believes the figure may be closer to 100,000.

Authorities maintain a 30-kilometre-wide (19-mile-wide) exclusion zone around the plant in which only a few dozen elderly people live.

Concerns over the safety of the disintegrating concrete shelter -- built by 90,000 people in just 206 days -- prompted the European Bank for Reconstruction and Development (EBRD) to spearhead a 2.1-billion-euro ($2.2-billion) project to install a new safety dome.

- 30-year lifespan -

The numerous problems with the Soviet-era solution included the fact that it only had a 30-year lifespan.

Yet its deterioration began much sooner than that.

"Radioactive dust inside the structure is being blown out through the cracks," Sergiy Paskevych of Ukraine's Institute of Nuclear Power Plant Safety Problems told AFP.

Paskevych added that the existing structure could crumble under extreme weather.

The new arch should be able to withstand tremors of 6.0 magnitude -- a strength rarely seen in eastern Europe -- and tornados that strike the region only once every million years.

- Long time coming -

Chernobyl's dangers are real but Kiev complains Europe's help took a long time coming.

The EBRD found 40 state sponsors to fund a competition in 2007 to choose who should build the massive moveable dome.

A French consortium of two companies known as Novarka finished the designs in 2010 and began construction two years later.

The shelter was edged towards the fourth reactor in just under three weeks of delicate work this month that was interrupted by inclement weather and other potential dangers.

It will later be fitted with radiation control equipment as well as air vents and fire protective measures.

That equipment inside the arch is due to start working by the end of 2017.

"And only then will we begin to disassemble the old, unstable structure," the head of Ukraine's State Nuclear Regulation Inspections agency Sergiy Bozhko told AFP.

But he said no timeframe had yet been set for the truly hazardous work of removing all the remaining nuclear fuel from inside the plant or taking apart the old dome.

Novarka believes that its arch will keep the continent safe from nuclear fallout for the next 100 years.