Engineers at Japan's crippled Fukushima nuclear plant succeeded Tuesday in restarting three cooling systems 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 7:00 pm (1000 GMT) on Monday, operator TEPCO said.
By late Tuesday evening engineers had managed to restart cooling systems in three reactor pools, Kyodo news agency reported.
A separate cooling system for a common pool, which contains more than 6,000 used fuel rods, is expected to be back online at 8:00 am on Wednesday, ending the current problem, Kyodo reported.
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.
The incident has not so far affected the injection of cooling water to reactors 1, 2 and 3 themselves, Tanabe said. These suffered core meltdowns soon after the start of the March 2011 nuclear crisis.
TEPCO spokesman Masayuki Ono told a press conference that the cause of the incident, which knocked out power to nine facilities, had yet to be determined, but that a "makeshift power switchboard" was suspected, Kyodo reported.
Ono admitted it was the first time such a large number of facilities had suffered an electricity failure simultaneously since the plant was brought under control in December 2011.
TEPCO, which was previously criticised for not telling the public or press exactly what was happening at the atomic plant, said following the power cut that there was no imminent danger.
"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.
Power was restored to the cooling system for the fuel pool at reactor 1 at 2:20 pm Tuesday, TEPCO said in a statement.
Kyodo news later reported that cooling systems on the pools at reactors 3 and 4 were also up and running.
The temperatures of all fuel pools remain well below the safety limit of 65 degrees Celsius (149 degrees Fahrenheit), 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, that 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.
"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."