Highly radioactive water leaked from a treatment machine at the tsunami-hit Fukushima Daiichi nuclear power plant, but no one was injured and radiation monitoring shows no environmental impact, the utility operator says.

A plant worker found the leak on Wednesday morning during valve checks at a treatment machine designed to mainly remove cesium and strontium from the contaminated water, the Tokyo Electric Power Company Holdings said.

The machine has been idled for maintenance work.

An estimated 5.5 metric tons of radioactive water – 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, the power company said, but no radioactive water escaped the compound.

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Radioactivity of the leaked water was 10 times the legally releasable limit, the power company said, but no radioactive water escaped the compound.

The pool of water had been wiped off and the contaminated soil was being removed, spokesperson Kenichi Takahara said.

It’s not clear when the machine began leaking, but no problems were detected in an inspection on Tuesday.

Radiation levels around the plant and inside gutters on the compound have showed no increase.

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

The Fukushima Daiichi plant suffered triple meltdowns following the 2011 earthquake and tsunami.

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

Chinese Foreign Ministry spokesperson Wang Wenbin said the latest incident highlighted management problems and questioned the operator’s ability to safely carry out the decades-long treated water discharge.

Wang urged Japan to respond to the concerns of the international community and handle the discharge responsibly while co-operating with an independent and long-term monitoring system involving neighbouring countries and other stakeholders.

-AP