While not revealing the water's radiation level, TEPCO said the water did not escape the complex, Kyodo News reported.
The leak came to light after a leak detector sounded an alarm about 3 a.m., local time, TEPCO said. A plant worker found the water in a building housing the desalination unit and suspended the desalination process.
The desalination unit removes salt from radioactive water after cesium and other radioactive materials are filtered out.
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