Parliamentary vice defence minister Masahisa Sato said the incident raised even more worrying concerns, remarking on his Twitter feed that the makeshift switchboard "is vulnerable to terrorism".

"The government has to take a leadership role in improving" the decommissioning work, Sato said.

TEPCO has stressed the glitch was fixed before any damage was caused, saying the temperatures of all the fuel pools remained well below the safety limit of 65 degrees Celsius (149 degrees Fahrenheit).

But questioned by reporters about the lack of backup power supply, TEPCO official Masayuki Ono earlier admitted shortcomings.

"We can't deny criticism that our decision-making and handling (of the decommissioning work) has not been perfect."

The firm added that it was building a backup power supply to the pools.

Company officials say there has been no major change to the level of radioactivity at nearby monitoring spots.

TEPCO says the incident did not affect the injection of cooling water into the reactors that melted down in 2011.

Total decommissioning of the nuclear plant and cleaning up of areas around it is expected to take up to four decades.

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