Plans are being finalized for what may be the largest moveable structure ever built - a 20,000-ton steel shell to enclose Chernobyl Reactor 4, the site more than 16 years ago of a nuclear accident whose effects are still being felt.

By mid-year an international consortium will finish the design for the hangar-shaped arch that would be slid along greased steel plates to contain the reactor's remains in a weather-tight shelter. The arch will be about 131 yards high and 295 yards long, with a 14 yard thick shell.

Inside, robotic cranes and, where possible, human workers will then begin prying apart the wreckage, removing radioactive dust from twisted girders, storing pieces of radioactive core in canisters and cutting up old steel.

The shelter design and stabilization of the derelict reactor 81 miles north of Kiev is part of a 10-year plan devised by the Group of 7 nations in 1997.

The massive structure is designed to keep water out and dust in for 100 years, or for as long as it takes the Ukrainian Government to designate a permanent storage facility and dispose of the more than 200 tons of uranium and one ton of lethally radioactive plutonium that remain in the ruins.

Most of the radioactive material is contained in a solid lava formed by the fusion of molten fuel, concrete, 30 tons of fuel dust and 2000 tons of combustibles.

"We will need a lot of shielding," said Vincent Novak, director of the nuclear safety department at the European Bank for Reconstruction and Development, which is overseeing the project.

"If it weren't for the radioactivity, I could almost call the job a piece of cake, but the radiation makes it hugely complex and extremely difficult."

In 1986 a nuclear reaction blew off the roof of the steel reactor building, spewing tons of radioactive material into the air.

The amount of radioactivity released was up to 40 times the combined amount from the Hiroshima and Nagasaki atomic bombs. It was the worst nuclear accident in history. Thirty workers died immediately, and 135,000 people were evacuated.

In the six months after the explosion, the Soviets erected an improvised shelter, known as the sarcophagus. But within 10 years there were leaks and the building threatened to collapse.

Then the Group of 7, along with Russia, the European Union and Ukraine, set up the Chernobyl fund. The reconstruction bank established a plan for the shelter, estimated cost $1.36 billion to be paid for with donations from 28 nations.

In the first phase, finished in 1999, the roof of the sarcophagus and pillars were strengthened, and the ventilation stack stabilized.

But these were emergency measures only. "Safety analyses show there are still about 1200 square yards of holes in the roof and sides," said Eric Schmieman, an engineer at Battelle Memorial Institutes Pacific Northwest National Laboratory, of Washington State, one of the consortium companies.

"A significant amount of water can go in, and dust can go out, and birds and squirrels and birds come and go all the time."

The designers, led by Bechtel International Systems, of San Francisco, have stuck with the tried and true in designing the $250 million shelter.

"We're trying to build it for 100 years, and using brand new technologies increases the risks," Dr Schmieman said. "If a human being has to intervene, there's a consequence. We need to minimize the danger."

Lethal gamma rays escaping from the core would make the centre of the arch too hot for humans to work, and building it in place was impossible. So the sliding shelter was born.

The shelter will not contain the radioactivity but will keep out the weather. However, one problem will be managing the microclimate inside the shell. "It's so big, it could even rain inside, so we have to keep the moisture down," said Bechtel's project manager, Matthew Wrona.

The Washington Post
By Guy Gugliotta - January 2 2003