Tokyo Electric Power Co. said it had suspended a radioactive water treatment system at its Fukushima Daiichi nuclear plant early on Tuesday.

The operator halted the Advanced Liquid Processing System at 9 a.m. (0000 GMT) after it found higher calcium levels than normal in water in one of the system’s three lines.

The other two lines had already been stopped for not removing radioactive material sufficiently.

The system is a key component in the effort to deal with the massive amounts of toxic water at the Fukushima Daiichi Nuclear Power Station, as the operator keep injecting water into three of its six reactors to keep them cool.

The plant suffered meltdowns at the three units after it was hit by an earthquake and tsunami in March 2011.

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