Hidehiko Nishiyama, deputy-director general at the safety agency, said that he expected the iodine to dilute rapidly, minimizing the effect on wildlife, and pointed out that fishing had been suspended in the area after the quake and tsunami.

One sign of possible deterioration in the plant itself came at Reactor No. 3. 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. On Friday, officials and experts offered conflicting explanations of what had gone wrong — but all pointed to greater damage to the reactor’s systems and more contamination there than officials had indicated earlier.

Two workers were exposed to radiation and burned when water poured over their boots and down around their feet and ankles, officials said. A third worker was wearing higher boots and did not suffer the same exposure.

Like the injured workers, many of those risking their lives are subcontractors of Tokyo Electric Power, who are paid a small daily wage for hours of work in dangerous conditions. In some cases they are poorly equipped and trained for their task.

On Saturday, workers were focused on trying to restore the lighting to Reactor No. 2’s central control room, an important step toward restoring the unit’s cooling system. They were also preparing to pump fresh water on the No. 1, No. 2 and No. 3 units, after days of spraying with saltwater.

The National Institute of Radiological Sciences said that the radioactivity of the water that the three injured workers had stepped into was 10,000 times the level normally seen in coolant water at the plant. It said that the amount of radiation the workers were thought to have been exposed to in the water was two to six sieverts.