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Engineers at Fukushima Tuesday partly succeeded in getting cooling systems back online after a power outage underlined the still-precarious state of the nuclear plant two years after the tsunami.
Equipment on pools used to keep used fuel cool -- necessary to prevent any spontaneous nuclear reactions -- lost their power supply at 7 pm (1000 GMT) on Monday, operator TEPCO said.
By Tuesday afternoon, one of the cooling systems was back up and two others were likely to be online later in the day, the company said.
Even used nuclear fuel becomes dangerous if its temperature is allowed to rise uncontrollably to the point where a self-sustaining critical reaction begins, causing a meltdown.
TEPCO, which has previously been criticised for not telling the public or press exactly what is happening at the atomic plant, said there was no imminent danger, although they admitted they do not know what caused the fault.
"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 earlier.
The power supply was restored to part of the equipment, with the cooling system on the fuel pool at reactor 1 having returned to full operation at 2:20 pm (0520 GMT) Tuesday, TEPCO said in a statement.
Two other cooling systems on pools at reactors 3 and 4 will likely return to full operation at 8:00 pm (1100 GMT), and a separate cooling system that works on a common pool is expected to be back online at 8:00 am on Wednesday (2300 GMT Tuesday), ending the current power outage problem, it said.
The common pool contains more than 6,000 used fuel rods.
The incident has not so far affected cooling-water injection to reactors 1, 2 and 3 themselves, which suffered core meltdowns soon after the start of the March 2011 nuclear crisis, Tanabe said.
The temperatures of all fuel pools remains well below the safety limit of 65 degrees Celsius (149 Fahrenheit), and had been 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.
TEPCO officials 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 drew flak 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."