CHERNOBYL NUCLEAR POWER STATION, Ukraine - Workers have raised the first section of a colossal arch-shaped structure that eventually will cover the exploded nuclear reactor at the Chernobyl power station.

Project officials on Tuesday hailed the raising as a significant step in a complex effort to clean up the consequences of the 1986 explosion, the world's worst nuclear accident. Upon completion, the shelter will be moved on tracks over the building containing the destroyed reactor, allowing work to begin on dismantling the reactor and disposing of radioactive waste.

Suma Chakrabati, president of the European Bank for Reconstruction and Development, which is leading the project, called Tuesday "a very significant milestone, which is a tribute to the ongoing commitment of the international donor community, and an important step toward overcoming the legacy of the accident."

The shelter, shaped like a gargantuan Quonset hut, will be 843 feet by 492 feet when completed and at its apex will be higher than the Statue of Liberty.

The April 26, 1986, accident in the then-Soviet republic of Ukraine sent a cloud of radioactive fallout over much of Europe and forced the evacuation of about 115,000 people from the plant's vicinity. A 19-mile area directly around the plant remains largely off-limits and the town of Pripyat, where the plant's workers once lived, today is a ghostly ruin of deteriorating apartment towers.

At least 28 people have died of acute radiation sickness from close exposure to the shattered reactor, and more than 6,000 cases of thyroid cancer have been detected in people who, as children or adolescents, were exposed to high levels of fallout after the blast.

Officials who showed reporters around the construction site Tuesday were clearly delighted at the colossus taking shape before them, but concerned about the challenges ahead. The shelter is to be moved over the reactor building by the end of 2015 - a deadline that no one wants to miss given that the so-called sarcophagus hastily built over the reactor building after the 1986 explosion has an estimated service life of about 30 years.

The arch now under construction is only one of two segments that will eventually form the shelter, and so far it's only been raised to a height of 72 feet. More structural elements have to be added before it reaches its full height of 354 feet, and the work so far has taken seven months.

"There's no room for error ... the schedule is very tight," said Vince Novak, director of the EBRD's nuclear safety department, who added that staying within budget is also a concern.

The overall shelter project is budgeted at $2 billion - $1.3 billion of that for the structure itself.