Nuclear Regulation Authority Chairman Shunichi Tanaka told a news conference that the small leak and possible other leaks have added to concerns about the plant's stability.
They follow a major leak two weeks ago. TEPCO reported a loss of 300 tons of highly radioactive water from a steel tank on Aug. 19, saying most of it is believed to have seeped underground but some might have escaped into the sea. The company has yet to determine the cause or exactly where the water went.
More than 300 tanks there are of a similar type. About 1,000 tanks hold 330,000 tons of contaminated water at the plant, and the amount grows by 400 tons daily. The water is part of the makeshift system to keep the radioactive material at the plant stable.
Tanaka said he believed the discoveries of the subsequent leak and signs of possible additional leaks were the result of closer inspections after the large leak two weeks ago. That leak was the worst from a tank at the plant, which was badly damaged by a massive March 2011 earthquake and tsunami.
The investigation into the tanks has revealed sloppy record keeping and tank management by TEPCO. The company has acknowledged that it used to assign only two workers to visually inspect all 1,000 tanks in about two hours until the leak, and none of the tanks had water gauges. TEPCO has increased the patrol staff to nearly 60 and is adding other early detection measures.
TEPCO also said it overlooked several signs of leaks increases of radioactivity near the tanks and higher exposure levels for workers for more than a month.