According to the Japanese cabinet, 78,200 people lived inside a 12-mile radius before the earthquake, tsunami and nuclear accident. Many of them have been staying at evacuation centers for nearly six weeks, after fleeing the tsunami without time to gather their valuables; growing activity in the evacuation area has prompted concern among evacuees that their homes may be robbed while they are away, although there have been no documented cases of looting.

Mr. Shikata said the government was sensitive to the needs of residents who had fled with nothing more than “the clothes on their backs.” One person from each evacuated household would be allowed to enter the zone as part of two-hour bus trips to recover belongings, Mr. Edano said, but not even the buses would be allowed within two miles of the reactors. Mr. Nishiyama said the government’s main concern in drafting legal measures to bar entry to the evacuation zone involved public health and safety. But he said a secondary concern was that interlopers could spread the hot spots of radiation. Decontamination of a large area after a nuclear accident consists of very carefully mapping the hot spots. Contaminated objects are then sent to a specially lined landfill; even the dirt may have to be dug up if contamination is high enough.

Michael Corradini, the chairman of engineering physics at the University of Wisconsin, said that with power crews already setting up electricity transmission lines across the evacuation zone to the plant, and with heavy repair equipment being brought in as well, the movement of private individuals and their vehicles would probably not have much additional effect in spreading out the hot spots.

But Matthew Kozak, a principal consultant at Intera, an environmental consulting firm in Denver that has worked on radioactive waste management issues in Japan since the late 1980s but is not involved in the Fukushima cleanup, said that workers could be trained to minimize disruption to hot spots and be confined to easily traceable routes. Finding and cleaning up the radiation spread by interlopers would be more costly, he said in an e-mail, although he doubted residents would spread enough contamination to pose a serious health risk.

“Bottom line for me is that it is a good idea to keep people out of the controlled area, at least for now,” he said.

About 62,400 people lived about 12 to 18 miles from the power plant before the accident. They were initially told to stay indoors but have since been asked to leave voluntarily, along with residents of the five other communities that received some radioactive fallout because of wind and rain patterns. The cabinet has not released an estimate for the population of the other communities.

After the Chernobyl accident, the Soviet Union established a more stringently enforced exclusion zone, with an initial radius of about 18 miles. .

In Japan, weather patterns appear to have pushed much of the radiation from the coastal Fukushima reactors straight east and out to sea.