Radioactive water may have leaked into the ground from a tank at the Fukushima Daiichi nuclear power plant, the operator said on Saturday, the latest in a series of troubles at the crippled facility.

Up to 120 tonnes of contaminated water may have escaped from one of the seven underground reservoir tanks at the tsunami-damaged plant, according to a Tokyo Electric Power Co (TEPCO) spokesman.

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The tank stores water used to cool down the reactors after radioactive caesium is removed but other radioactive substances remain.

“We are transferring the remaining water from the tank to others,” the TEPCO spokesman said, adding that the company believes the contaminated water was unlikely to flow into the sea.

The leakage came after one of the systems keeping spent atomic fuel cool at the plant temporarily failed on Friday, the second outage in a matter of weeks, underlining the precarious fix at the plant.

Nuclear fuel, even after use, has to be kept cool to prevent it from overheating and beginning a self-sustaining atomic reaction that could lead to meltdown.

The plant was hit by the giant tsunami of March 2011 as reactors went into meltdown and spewed radiation over a wide area, forcing tens of thousands of people from their homes and polluting farmland.