Japan's largest power utility acknowledged the leaks following allegations on Wednesday by a citizens group that had been tipped off by a whistle-blower.A TEPCO official said the leaks had not posed a safety threat because the density was much less than the 1/10,000th safety level set by the government.
Because of the low density level, the utility had no obligation to report the leaks to the government, the official said.
The revelation follows news in late August that TEPCO's employees had doctored safety records on inspections at nuclear reactors.
Nuclear power provides a third of resource-poor Japan's energy, but a series of accidents has eroded public faith in the industry.
As of 0330 GMT, TEPCO's shares were down 0.87 percent at 2,290 yen while the Nikkei stock average was down 1.28 percent.