CHERNOBYL, Ukraine—Imagine a metal arch taller than the Statue of Liberty. Now picture it sliding a distance of roughly three football fields, making it the largest movable structure ever . Under this steel rainbow engineers are planning to entomb the site of the worst nuclear accident in history, the destroyed reactor at the Chernobyl power plant, using robotic cranes to dismantle the ruins and keep its deadly remains from poisoning the rest of the planet.

After reactor No. 4 exploded at Chernobyl in 1986 due to errors in both design and operation it sent plumes of radioactive dust as far away as Japan and the U.S. To contain the fallout, the Soviet Union constructed a metal and concrete structure commonly known as the sarcophagus over the wreckage.

"It was really quite a remarkable feat, but after 25 years, it's in danger of collapse," civil and environmental engineer Eric Schmieman of Battelle Memorial Institute explains in an interview in Kiev.

The sarcophagus, technically known as the Shelter Object, was made of more than 7,000 metric tons of metal and 400,000 cubic meters of concrete. It was erected as quickly as possible to limit worker exposure to radiation, and was never meant to last forever. In many ways it was designed "like a house of cards," Schmieman says, with pieces of metal essentially leaning against each other and hooked together. "There are no welded joints or bolted joints—it wouldn't take much of a seismic event to knock it down."

At the same time, when the sarcophagus was completed, "there were over 1,000 square meters of openings in the roof where joints didn't match up," Schmieman says. These holes allowed water in, resulting in corrosion that is hastening the structure's decline. Since then, workers have patched many of these holes, but 100 square meters of gaps remain. To help keep radioactive matter from leaking , a dust- suppression system inside relies on sprinklers that periodically spray a watery solution to prevent it from becoming airborne.

Now, to safely enclose the ailing sarcophagus, the French consortium Novarka is working on a replacement: the New Safe Confinement, a steel structure 110 meters high at its tallest point, 164 meters wide, spanning across 257 meters and weighing more than 29,000 metric tons. In comparison, the Statue of Liberty from the ground to the tip of its torch is about 93 meters high, says Schmieman, who helped lead New Safe Confinement's conceptual design .

Because the destroyed reactor is still highly radioactive, to protect workers, the arch will not be constructed over the sarcophagus. R ather, it will be assembled nearby from prefabricated segments each about 25 meters high and weighing an average of 300 metric tons. Once complete, hydraulic jacks will then slide the arch approximately 300 meters on Teflon bearings during the course of a week to enclose the sarcophagus. Walls on either side of the structure, making it resemble an aircraft hangar, will help isolate debris. "All told, it has a design life of 100 years," Schmieman says.

Inside the structure, three robotic cranes capable of lifting up to 50 metric tons each will be equipped with tools to help dismantle the sarcophagus, using drills, manipulator arms and concrete crushers, along with vacuum cleaners that can suck up to 10 metric tons of dust. The cranes will also employ radioactivity monitors as well as cameras to help remotely operate the tools . Once the sarcophagus and its contents are dismantled , it remains to be seen where the most radioactive material will be buried, but there are facilities to store the less radioactive remains.

During the first week of March, I saw deep trenches and large steel piles here meant for the foundation of the arch. Currently, the goal is to finish the New Safe Confinement by 2014, although contractors are giving themselves a year leeway. "Keep in mind, this is a one-of-a-kind structure, and nothing like this has ever been attempted," Schmieman cautions. "Further, Chernobyl is one of the most hazardous working sites in the world, and we frequently discover unexpected radiological hazards in excavation works. The combination of these factors introduces many uncertainties into any schedule."

In addition, some of the money needed to complete the project has yet to be raised. Twenty-nine countries have pledged funds to the Shelter Implementation Plan creating the New Safe Confinement, but so far another $835 million are needed; also, the storage facility designed to hold spent nuclear fuel from reactor Nos. 1 to 3 still requires funding to the tune of $195 million . Fundraising events to coincide with the 25th anniversary of the disaster in April are now underway, according to the European Bank for Reconstruction and Development, which is managing these efforts.

"I am fully aware that this is a considerable amount of money which is particularly difficult to raise at a time of universal fiscal constraints," Thomas Mirow, president of the European Bank for Reconstruction and Development, said in a statement. "Nevertheless, we must not forget that it is in the best interest of Ukraine and the international community to bring to a successful conclusion the important work we have started in Chernobyl."

The sacrifices made by the clean up workers immediately after the Chernobyl tragedy are driving those at the project to work to a much higher standard, says structural engineer Randy Jorissen , deputy manager for technical direction for the New Safe Confinement. "All they did to limit the extreme disaster, giving their lives for the task—I just hope we learned our lesson and that it never happens again," he says. "It's very satisfying to me to be part of a very significant effort to bring to a conclusion what those heroes started 25 years ago."

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