Workers have been dousing the nuclear fuel rods with seawater and now from freshwater sources to prevent full meltdowns, but they have had to release harmful amounts of radioactive steam into the atmosphere, and their efforts have set off leaks of highly contaminated water.
Iodine 131, one of the radioactive byproducts of nuclear fission, can accumulate in the thyroid and cause cancer. But unlike cesium 137, it degrades relatively rapidly, becoming half as potent every eight days. The risk can be diminished by banning fishing.
Mr. Nishiyama said the new readings posed no immediate threat to public health, and no fishing was occurring in the area.
Regarding the international agency’s findings, he said on Thursday, “One sample is over the limit, so we have to continue monitoring the situation.” As to whether the evacuation zone should be expanded, he said, “We’ll be considering that.”
The plant’s operator, the Tokyo Electric Power Company, acknowledged that it would write off Reactors 1 through 4.

Matthew L. Wald reported from Washington, and David Jolly from Tokyo. Moshe Komata and Ayasa Aizawa contributed reporting from Tokyo, and Kevin Drew from Hong Kong.