Water containing radioactive materials leaked from a treatment machine at the tsunami-hit Fukushima Daiichi Nuclear Power Plant, but radiation monitoring shows no impact to the outside environment, the utility operator said Thursday.

A plant worker found the leak Wednesday morning during valve checks at a SARRY treatment machine called, designed to remove cesium from the contaminated water, the Tokyo Electric Power Company Holdings said. The machine has been idled for maintenance work.

The estimated amount of water that leaked was approximately 5.5 tonnes – enough to fill two ordinary backyard swimming pools – leaked out through an air vent, leaving a pool of water on an iron plate outside and seeping into the soil around it, TEPCO said, but no radioactive water escaped the compound.

The leak may have been caused by valves accidentally left open while workers flushed the machine with filtered water. TEPCO said that 10 of 16 valves that should have been closed were left open during the flushing.

The filtering machine is part of TEPCO's controversial wastewater discharge project, which began in August. 

The discharges, which are expected to continue for decades, have been strongly opposed by fishing groups and neighboring countries, including China, which immediately banned imports of all Japanese seafood.

The latest leak comes only months after another accidental leak at a separate treatment facility called the Advanced Liquid Processing System, or ALPS.

(With input from AP; cover photo: aerial shot of Fukushima Daiichi Nuclear Power Plant, August 24, 2023. /CCTV Plus)