Monday's outage knocked out power to nine facilities at the plant, its largest simultaneous electricity failure since it was brought under control in December 2011.

The firm says the incident did not affect the injection of cooling water into reactors whose cores melted down soon after the start of the 2011 nuclear crisis.

The meltdown of three of Fukushima's six reactors occurred after an earthquake and huge tsunami on March 11, 2011, which shut off the power supply and cooling system.

TEPCO drew flak for playing down the scale of the disaster in the first few months. It has since admitted it had been aware of the potential dangers of a big tsunami but did nothing for fear of the reputational and financial cost.

The latest incident rekindled public concern about whether the politically connected utility is being fully transparent.

TEPCO informed the government's regulatory agency about the blackout shortly after it started, but waited three hours before issuing a public press release.

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