Tokyo Electric Power Co. officials said employees at the plant, located in Okuma, pumped excess rainwater, which had accumulated Wednesday when Typhoon No. 26 hit, into small, temporary storage tanks to assess the radioactivity level.
Due to a shortage of pumps, about 7.9 inches of the radioactive water remained within the plant's 11.8-inch-high barriers when another downpour hit Sunday, officials said.
The rainwater breached the barriers at 11 locations during, causing the radioactive water to accumulate on the ground, mix with rainwater and run into the ocean, the plant's operator said.
At least six of those sites showed levels of radioactivity exceeding safety standards, the Asahi Shimbun reported.
Tepco said it plans to erect new barriers 24 to 51 inches high by the end of the year.