Engineers at Fukushima were scrambling Tuesday to restore power to cooling systems after an outage, in the latest incident to underline the precarious fix at the tsunami-crippled nuclear plant.
Equipment on pools used to keep used fuel cool -- necessary to prevent any spontaneous nuclear reactions -- have not had any electricity since it was cut at 7 pm (1000 GMT) on Monday, operator TEPCO said.
The company, which has regularly been criticised in the past for not telling the public or press exactly what is happening at the damaged atomic plant, said there was no immediate danger, although they admitted they do not know why the fault occurred.
"Electricity has been cut to pools used to cool spent fuel at reactors 1, 3 and 4" as well as to the equipment used to treat contaminated discharge including radioactive caesium, TEPCO spokesman Kenichi Tanabe said.
The incident has not so far affected cooling-water injection to the number 1, 2 and 3 reactors themselves, which suffered core meltdowns soon after the start of the March 2011 nuclear crisis, he said.
The temperatures of all fuel pools remains well below the safety limit of 65 degrees Celsius (149 Fahrenheit), but was rising by 0.3-0.4 degrees every hour, he said.
Tanabe said at its present rate and if power is not restored, the used fuel pool at reactor 4 could reach 65 degrees in four days.
"We are trying to restore power by then," he said, adding the deadline would be about 14 days and 26 days for the other two.
Masayuki Ono, acting head of TEPCO's nuclear power and facilities section, told a news conference on Tuesday morning the warmest pool was 30.5 degrees.
"We are prioritising recovering power and restarting operations... but if it takes a long time we can pour cooling water whenever necessary so that the worst case scenario can be avoided," he said.
Tanabe said there had been no major changes in the level of radioactivity at nearby monitoring spots.
The meltdowns of three of Fukushima's six reactors occurred after the March 11, 2011, earthquake and ensuing huge tsunami shut off the power supply and cooling system.
TEPCO was repeatedly criticised for downplaying the scale of the disaster in the first few months and has since admitted it had been aware of the potential dangers of a big tsunami but had done nothing for fear of the reputational and financial cost.
The government in Tokyo on Tuesday gave its backing to the company's handling of the power outage incident, saying a serious crisis appeared unlikely.
"As they are planning to take all possible substitute measures for cooling, we do not need to worry at all in a sense," Chief Cabinet secretary Yoshihide Suga told a regular press conference.
That view was echoed by Akio Koyama, professor at Kyoto University's department of reactor safety management.
"At this point, I don't think anything serious will occur immediately," he told AFP.
"The important thing is to continue injecting water to the nuclear fuel in the reactors and continue cooling used fuel in the pool.
"Even if the water temperature goes up to 65 degrees Celsius, it would not cause anything critical right away, as long as the fuel bars are covered in water.
"If the water levels get lower to the point where the fuel bars are exposed to the air, then we would have to worry."