The crisis began Monday night when power was cut at a building that serves as the central command for work to contain the nuclear accident and dismantle the reactors.

By Tuesday afternoon some electricity had been restored, but it was not until 29 hours after the original outage that the supply was normalised.

TEPCO and the government said in December 2011 that the crippled reactors were "in a state of cold shutdown" -- a phrase carefully chosen, commentators said, to imply the normality of units that were so broken they would not easily fit classical descriptions.

Authorities insist they are getting on top of the problem and the reactors are not leaking significant amounts of radiation.

"Despite the fact it is now two years since the crisis began, TEPCO is still doing a very poor job," Muneo Morokuzu, professor of nuclear regulation at Tokyo University, told AFP.

"A short-circuit caused by a small animal is not an unforeseeable event," he said, adding that it was something many construction sites would experience.

"At the very least, the switchboard that provides power to the cooling system for the pool on reactor 4 should have been more reliable" because that pool contains more than 1,100 nuclear fuel rods, Morokuzu said.

Used nuclear fuel must be kept cool to stop it from melting and releasing radiation.

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