"After pouring seawater on them ... I believe we cannot use them anymore," Katsumata said.
Japan's government has been saying since March 20 that the entire plant must be scrapped.
On Wednesday, nuclear safety officials said seawater 300 yards (meters) outside the plant contained 3,355 times the legal limit for the amount of radioactive iodine — the highest rate yet and a sign that more contaminated water was making its way into the ocean.
The amount of iodine-131 found south of the plant does not pose an immediate threat to human health but was a "concern," said Hidehiko Nishiyama, a Nuclear and Industrial Safety Agency official. He said there was no fishing in the area.
Radioactive iodine is short-lived, with a half-life of just eight days, and in any case was expected to dissipate quickly in the ocean. It does not tend to accumulate in shellfish.
"We will nail down the cause, and will do our utmost to prevent it from rising further," he said.
Highly toxic plutonium also has been detected in the soil outside the plant, TEPCO said. Safety officials said the amounts did not pose a risk to humans, but the finding supports suspicions that dangerously radioactive water is leaking from damaged nuclear fuel rods. There have been no reports of plutonium being found in seawater.
The latest findings on radioactive iodine highlighted the urgent need to power up the power plant's cooling system. Workers succeeded last week in reconnecting some parts of the plant to the power grid.
But as they pumped in water to cool the reactors and nuclear fuel, they found pools of radioactive water in the basements of several buildings and in trenches outside.
The contaminated water has been emitting many times the amount of radiation that the government considers safe for workers, making it a priority to pump the water out before electricity can be restored.