Systems for cooling spent fuel at a stricken Japanese nuclear plant were shut down by a power cut, operator Tokyo Electric Power Co said on Tuesday.

The Fukushima Daiichi Nuclear Power Station, hit in a 2011 earthquake and tsunami, lost power before 7 pm on Monday (10.00 GMT), affecting nine facilities, including the spent fuel storage at reactors 1, 3 and 4 of the six-reactor plant, company spokesman Akitsuka Kobayashi said.

The operator “failed to do the most important job at the moment - cooling reactors,” said Masako Sawai, a researcher at the Citizens’ Nuclear Information Center in Tokyo, an anti-nuclear advocacy group.

The cooling systems at reactor 1 and 4 were put back online on Tuesday afternoon while the operator was aiming to resume the other cooling system at reactor 3 by 8 pm, it said.

The building of reactor 4 holds a storage pool filled with 1,533 nuclear fuel rod assemblies, which experts warned could trigger a major catastrophe.

The outage did not interrupt the water injection being used to cool reactors 1, 2 and 3, which suffered severe damage and meltdowns in the wake of the March 2011 quake and tsunami.

The operator was investigating the cause of the outage while the Nuclear Regulation Authority said it might have stemmed from the power distribution board or cables attached to it.

“It has taken so long to restore them and find the cause,” Mr. Sawai said.

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