Tepco, which operates the plant, said it was confident the water, contained in a pit, is not leaking and said the area will be sealed off and checked regularly, The Asahi Shimbun reported Saturday.
Foreign nuclear experts contradicted Tepco's assessment and criticized the company for failing to disclose the leaks that outside observers said are reaching the Pacific, The New York Times said.
The water inside the No. 1 reactor has high levels of radioactive cesium -- 2.35 billion becquerels per liter.
The Fukushima Daiichi nuclear power plant was knocked off line and nearly melted down, spilling radioactive gas into the surrounding area, following a devastating earthquake and tsunami in 2011.