Tokyo Electric Power Co (Tepco) said some of the water was seeping over or around a “liquid glass wall” it has created by injecting chemicals into the soil that solidify into a wall.
The latest problem involves underground water which has built up over the last month, since the company started creating the chemical walls in an embankment to stop leaks after it detected radiation spikes in water samples.
Tepco spokesman Yoshikazu Nagai said the company was slow to deal with the underground water problem while focusing on the melted reactors, which pose greater risks.
Measures to contain contaminated underground water leaks have lagged while “we devoted ourselves to cool the reactors”, which was the foremost task, Mr Nagai said.
The Fukushima Dai-ichi plant has suffered multiple meltdowns since the March 2011 earthquake and tsunami destroyed power and cooling systems. The plant still runs on makeshift equipment and has been plagued with blackouts and leaks from underground tanks.
Tepco has been repeatedly criticised for delayed handling and disclosures of problems and mishaps at the plant.
Japan’s nuclear watchdog set up a special panel with Tepco and met on Friday to assess the water problem and discuss measures to resolve it. Watchdog officials have urged Tepco to pump the contaminated water inland and expand underground and seawater sampling. Tepco is also building more layers of chemical walls around the embankment.
Officials acknowledged last month for the first time that the plant has been leaking radioactive water into the ocean for some time. Following a major leak a month after the meltdowns, Tepco said it had contained the problem, and denied further underground leaks into the ocean until recently.
Data provided by Tepco showed underground water at coastal monitoring points has risen over the chemical wall, leaking into the sea.