TOKYO — Japan’s effort to contain the crisis at the Fukushima Daiichi nuclear plant suffered a setback, an official said on Friday, citing evidence that the reactor vessel of the No. 3 unit may have been damaged.

The development, described at a news conference by Hidehiko Nishiyama, deputy director-general of the Japan Nuclear and Industrial Safety Agency, raises the possibility that radiation from the mox fuel in the reactor — a combination of uranium and plutonium — could be released.

Photo

One sign that a breach may have occurred in the reactor vessel, Mr. Nishiyama said, took place on Thursday when three workers who were trying to connect an electrical cable to a pump in a turbine building next to the reactor were injured when they stepped into water that was found to be significantly more radioactive than normal in a reactor. The No. 3 unit, the only one of the six reactors at the site that uses the mox fuel, was damaged by a hydrogen explosion on March 14. Workers have been seeking to keep it cool by spraying it with seawater along with a more recent effort to restart the reactor’s cooling system.

In another development on Friday, the Japanese government said it would help people who wish to leave the area around the crippled plant, a sign that efforts to reassure frightened residents have failed to persuade people to stay.

Photo

There is no evacuation order for a zone from 12 to 19 miles around the nuclear plant, the chief cabinet secretary, Yukio Edano, said in a televised news conference, but officials recognize that some people may wish to leave and it will assist them. The government has ordered all people living within about 12 miles of the plant to evacuate, but has only advised those in the outer zone to stay indoors.

Advertisement Continue reading the main story

That advice has not changed, Mr. Edano said. The United States has recommended that its citizens stay at least 50 miles away from the plant.

Photo

Levels of a radioactive isotope found in Tokyo’s water supply fell by more than half on Thursday, testing below the country’s stringent maximum for infants. The lower readings were made hours after Mr. Edano said the isotope, iodine 131, had been detected in the water supply of Kawaguchi City, just north of Tokyo, as well as in those of two of Tokyo’s neighboring prefectures, Chiba and Saitama. On Wednesday, the authorities cautioned those in the affected areas not to give infants tap water.