The scale and complexity of the challenge are unprecedented. No nuclear reactor has ever been fully decommissioned in Japan, let alone the four certain to be dismantled at Fukushima Daiichi after being flooded with seawater to avert meltdowns and after suffering explosions and other damage. The final fates of the two other reactors there have not been announced, but they, too, may need to be decommissioned.

The 1979 accident at Three Mile Island in Pennsylvania involved just one reactor, and though there was a partial meltdown of the nuclear fuel rods, the chamber holding them did not rupture. The cleanup there still took 14 years and cost about $1 billion. (Two reactors that continue to operate at the site are set to be decommissioned in 2014.)

Recovery from the 1986 disaster at Chernobyl in Ukraine, meanwhile, is an example engineers are not eager to follow. Following explosions and a fire that sent huge radioactive plumes into the atmosphere, workers covered the remains of the reactor with sand and lead and eventually entombed it with concrete to halt the release of radiation. The concrete coffin still remains at Chernobyl, and the area is uninhabitable.

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For now, workers in Japan are still trying to stem leaks of highly radioactive water from the plant even as they add to the flow by continuing to pump in water — now fresh, not saltwater. They are also racing to revive the contained cooling systems that circulate water and do not bleed contaminants.

But serious challenges remain, including what Japan’s nuclear regulator said Thursday were rising temperatures at one of the units, as well as a series of strong aftershocks. Later, Hidehiko Nishiyama, the deputy director general of Japan’s Nuclear and Industrial Safety Agency, said the situation at the plant remained “difficult.”

Still, Toshiba’s engineers expect the plant to stabilize “in several months,” Mr. Sasaki said, and for full-scale cooling to resume. It would be five years before engineers would be able to open the pressure vessels to remove the nuclear fuel, he said, and dismantling the reactors and cleaning up radiation at the plant would take at least another five years.

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Toshiba’s team includes engineers from Westinghouse, whose majority owner is Toshiba, and the Babcock & Wilcox Company, an energy technology and services company that handles the disposal of hazardous materials. The two companies helped shut down the damaged reactor at Three Mile Island.

A Hitachi spokesman in Tokyo, Yuichi Izumisawa, said that the 10-year projection was overly optimistic. He said that Hitachi’s engineers expected it to take that long just to remove the nuclear fuel rods from the plant and place them in casks to transport to a safe storage facility.

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Only then can the dismantling of the plant’s structures begin, he said, followed by the cleanup of the remaining radiation.

Hitachi, the country’s second biggest supplier of reactors, has a team of 50 experts working on its dismantling plan. It has a joint nuclear venture with General Electric and is also working with the American nuclear operator Exelon and Bechtel, an engineering company.

“You basically need to dismantle the plant from the inside, and the inside is still very radioactive,” Mr. Izumisawa said. “At Hitachi, we are baffled over what kind of technology would allow everything to be finished in 10 years.”

Tetsuo Matsumoto, a professor of nuclear engineering at Tokyo City University, said that how long the decommissioning process would take depended heavily on the state of the nuclear fuel.

“Will it still be shaped like rods? Or will it have melted and collapsed into a big mass?” he said. “It could be 10 years or it could be 30. You just won’t know until you open up the reactor.”