Engineers are completing plans for what may be the largest movable structure ever built -- a 20,000-ton steel shell to enclose Chernobyl Reactor 4, site of an apocalyptic nuclear accident whose consequences are still being felt more than 16 years later.

By next summer an international consortium led by Bechtel International Systems Corp., of San Francisco, will finish the conceptual design for a hangar-shaped arch nearly 370 feet high -- the height of a 35-story building -- that would be slid into place along greased steel plates to cover the ruined remains in a snug, weather-tight shelter.

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

The whole job -- design, construction and "stabilization" of the derelict reactor 80 miles north of Kiev -- is part of a fully funded 10-year plan set in motion by the Group of 7 industrialized nations in 1997. The $768 million project, including the shell, is scheduled for completion in 2007, according to officials involved with the project.

And then the world will wait.

The shelter 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 more than 200 tons of uranium and nearly a ton of lethally radioactive plutonium that remain inside the ruins.

Most of the fuel-containing material lies as a solid "lava" formed by the fusion of molten fuel, concrete, 30 tons of fuel dust and 2,000 tons of combustibles.

In the basement, rainwater and fuel dust have mixed together in a dangerous radioactive "soup." Lethal chunks of the reactor core lie unseen in the rubble and in the earth alongside the building. More pieces of core were boxed and buried in a "cascade wall" built and bulldozed into place by Soviet workers in the immediate aftermath of the explosion.

"We will need a lot of shielding," said Vincent Novak, director of the Nuclear Safety Department for the European Bank for Reconstruction and Development, overseers of 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."

The Chernobyl explosion occurred April 26, 1986, when an out-of-control nuclear reaction blew off the roof of the steel building and spewed tons of radioactive material into the air, releasing 30 to 40 times as much radioactivity as the Hiroshima and Nagasaki atomic bombs combined in 1945.

It was the worst nuclear accident in history. Thirty workers died immediately at the facility, and 135,000 people were evacuated from the surrounding "Exclusion Zone." As recently as 2000, the Ukrainian government was spending 5 percent of its gross domestic product to mitigate consequences of the disaster.

In the six months immediately following the explosion, the Soviets erected an improvised shelter known as the "sarcophagus," but within 10 years scientists became alarmed because of leaks and the building's threatened collapse. The walls were weakening, Novak said, and there was tremendous uncertainty because "it was almost impossible to determine" the real dangers.

In 1997, the Group of 7, plus Russia, the European Union and Ukraine, set up the Chernobyl Shelter Fund with the European reconstruction bank in charge. The bank established a shelter implementation plan, estimated the project cost at $768 million, and funded it with donations from 28 nations, ranging from $170 million from the United States to Iceland's $10,000.

In the first phase, completed in 1999, the sarcophagus's roof and structural pillars were strengthened, and the reactor's rickety ventilation stack, jutting more than 150 feet above the sarcophagus, was stabilized. The stack was an added concern, because it was shared by the contiguous Reactor No. 3, which was still operating.

But these were emergency measures. "Safety analyses show there are still about 1,000 square meters [1,200 square yards] of holes in the roof and sides," said Eric Schmieman, chief engineer for environmental technology at Battelle Memorial Institute's Pacific Northwest National Laboratory in Richland, Wash. "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 Bechtel-led consortium designing the $250 million structure to cover the sarcophagus had to make several decisions early. None of the three design contractors, including Battelle and the French state utility Electricite de France, will be allowed to bid on the actual work.

Doubts arose as to whether a steel structure could last a century. With lethal levels of radiation inside the shell, opportunities for repair and maintenance could be limited.

"It's doable," said Bechtel's Matthew Wrona, project manager. "There are paints that last a long time and maintenance techniques for harsh environments." The Eiffel Tower is perhaps the best-known large, century-old steel structure fully exposed to the elements, but Wrona noted that several large suspension bridges are aging elegantly.

The team also avoided experimental technologies in favor of the tried and true.

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

The team settled on a steel arch 40 feet thick. The inside dimensions would be 803 feet -- almost three football fields -- across and 330 feet high. Up to that point, planning was relatively simple, because "it had all been done before," said Philippe Convert, technical manager for Electricite de France, but the next steps were a different story: Lethal gamma rays escaping from the reactor's damaged core would make the center of the arch too hot for humans to work. Building the arch in place was impossible.

Instead, the team decided to construct the arch in four 120-foot sections, then link the sections together and slide the entire structure along a track made of steel plates built on each side of Reactor No. 4. When completed, the project managers believe the new shelter will be the largest movable structure ever built.

One end will be fully enclosed, while the other will be a "cutout" that fits snugly over Reactor No. 3's building, which connects to the ruins. Current plans call for the stack to be taken down, and the junction between the arch wall and Reactor No. 3 to be sealed.

The new shelter will not "contain" the core's radioactivity but will be weatherproof.

The tracks will be made by driving piles into the ground at relatively close intervals, then filling the gaps with concrete. The planners want to avoid seating the concrete in a deep trench, for fear of unearthing radioactive material during excavation.

The concrete will then be covered with stainless steel plates and coated with a lubricant, while the bottom of the new steel shell will have Teflon pads for easier sliding. Convert said the sliding technique is used extensively to move heavy machinery.

While workers will be able to enter some parts of Reactor No. 4 and work on the wreckage in relative safety, the most routine tasks can suddenly turn deadly.

"Surprises are inevitable," Novak said. During the initial roof and structural repair, "we found a large piece of core embedded in the wall. Everything stopped until we could build a device and get the shielding to handle it. Each case is different."

To help deconstruct parts of the reactor building and the sarcophagus, the new shell will have four ceiling cranes designed to pluck heavy steel beams from the old reactor and to wrestle pieces of twisted metal from the ruins. They will also be equipped with hydraulic cutters to chop wreckage into manageable chunks.

One unusual problem is the need to manage the new shell's microclimate. "It's so big, it could even rain inside, so we have to keep the moisture down," Wrona said. Air conditioning would be prohibitively expensive, so "we'll try to use natural air currents. It's like the inside of an automobile on a cold morning."

In April 1986, a nuclear reaction at the Chernobyl power plant went out of control, causing an explosion that blew the roof off, killing 30 workers immediately and releasing tons of radioactive material into the atmosphere.