TEPCO has been criticised for playing down the severity of the Fukushima disaster Engineers at Japan's crippled Fukushima nuclear plant partially succeeded in restarting cooling systems on Tuesday after a power cut underlined its still-precarious state two years after a tsunami struck. Equipment in pools used to keep used fuel cool -- necessary to prevent any spontaneous nuclear reactions -- lost their power supply at 1900 hrs (1000 hrs GMT) on Monday, operator TEPCO said.
By late Tuesday two of the cooling systems were back up, reports said. One other was likely to be online later in the day, the company said earlier.
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 was previously criticised for not telling the public or press exactly what was happening at the atomic plant, said there was no imminent danger, although it admitted it does not know what caused the fault.
"Electricity has been cut to pools used to cool spent fuel at reactors one, three and four" as well as to the equipment used to treat contaminated discharge including radioactive caesium, TEPCO spokesman Kenichi Tanabe said earlier.
Power was restored to part of the equipment, with the cooling system for the fuel pool at reactor 1 having returned to full operation at 1420 hrs on Tuesday, TEPCO said in a statement.
Kyodo news reported the cooling system on the fuel pool at reactor four was also working again.
The system on the pool at reactor three will likely return to full operation Tuesday evening, the company said.
 A separate cooling system for a common pool is expected to be back online at 0800 hrs on Wednesday, ending the current problem, it said. The common pool contains more than 6,000 used fuel rods.
The incident has not so far affected the injection of cooling water to reactors one, two and three themselves, Tanabe said. These suffered core meltdowns soon after the start of the March 2011 nuclear crisis.
The temperatures of all fuel pools remains well below the safety limit of 65 degrees Celsius, and had been rising by 0.3-0.4 degrees every hour, the TEPCO spokesman said.
TEPCO officials said there had been no major changes in the level of radioactivity at nearby monitoring spots.
The meltdown of three of Fukushima's six reactors occurred after an earthquake and ensuing huge tsunami on March 11 2011, shut off the power supply and cooling system.
TEPCO drew flak for playing down the scale of the disaster in the first few months. It has since admitted it had been aware of the potential dangers of a big tsunami but did nothing for fear of the reputational and financial cost.
The government gave its backing to the company's handling of the latest 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," said Chief Cabinet Secretary Yoshihide Suga.
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,” he said.
"If the water levels get lower to the point where the fuel bars are exposed to the air, then we would have to worry."