Some 7.71 tonnes of radioactive water has leaked from a reactor at Japan's crippled Fukushima Daiichi nuclear power plant but it has not flowed outside the reactor building, Kyodo News said, quoting the plant's operator.
Tokyo Electric Power Co said the leak occurred in the No 4 reactor after a pipe connected to the reactor dropped off, the news agency reported.
The leak was discovered on Tuesday night and was stopped shortly afterwards.
The Fukushima Daiichi plant was crippled by meltdowns and explosions caused by a massive earthquake and tsunami in March last year.
Radiation was scattered over a large area and made its way into the oceans, air and food chain in the weeks and months after the disaster.
Tens of thousands of people were evacuated from their homes in a large area around the plant and swathes of this zone remain badly polluted, with the clean-up proceeding slowly amid warnings that some towns could be uninhabitable for three decades.