A rat may have caused a power cut that knocked out cooling systems at Japan's tsunami-wrecked Fukushima nuclear plant, the operator said Thursday, an episode that highlighted the jerry-rigged nature of the fix.

Equipment keeping spent nuclear fuel at a safe temperature in four different pools was out for up to 29 hours from Monday, with Tokyo Electric Power Company (TEPCO) admitting its recovery work was sometimes less than perfect.

The incident was a reminder of the precarious state of the plant two years after the tsunami, which sparked meltdowns in three reactors, spewing radiation over land and sea and forcing tens of thousands from their homes.

"We suspect a small animal may have caused a short-circuit in a switchboard" leading to the outage and disabling cooling systems for used fuel pools, a spokesman for TEPCO said.

"We cannot be sure exactly what it was, but can say what we saw at the scene was the body of a dead animal below the switchboard," he said.

"In our investigation we will concentrate on getting assurances that it was definitely this animal that caused the short-circuit."

A photograph TEPCO released showed a creature that appeared to be a rat, with a body around 15 centimetres (six inches) long.

The switchboard was a temporary one on the back of a vehicle, which was due to be replaced by a permanent one, TEPCO said, without specifying a timeframe.

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.

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.