Power has been restored to the cooling systems for the spent fuel ponds at the damaged Fukushima nuclear plant in Japan, operator Tepco says.
A power cut shut down ponds at reactors 1, 3 and 4 on Monday evening, although cooling to the reactors themselves was not affected.
Pool temperatures were well within safe levels and the reactors were unaffected, Tepco said.
The Fukushima plant was crippled by the March 2011 earthquake and tsunami.
Cooling had initially been restored to the pool at reactor 1 first, with cooling at the reactor 3 and 4 pools resuming on Tuesday afternoon, Tepco said.
The "highest priority" was placed on restoring the cooling system to the spent fuel pool at reactor 4, Kyodo news agency quoted Tokyo Electric Power Company (Tepco) spokesperson Masayuki Ono as saying.
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Back in 2011 the world looked on aghast as emergency teams struggled to shut down Japan's three troubled nuclear reactors at Fukushima.
International experts were already warning that the real threat came from the vast quantities of used fuel stored in enormous cooling ponds on site.
Globally such radioactive waste ponds are common and therefore vulnerable to freak accidents. Most - though not all - nuclear reactors worldwide need to let their used fuel cool for at least five years before they do anything with it; many for much longer.
Even if the waste is subsequently reprocessed - which itself creates radioactive waste - the plutonium retrieved has to be guarded because of its attractiveness to nuclear bomb-makers.
The IAEA estimates that by 2020 the total amount of spent nuclear fuel worldwide will have reached 440,000 tonnes and three-quarters of it will be in storage often at the reactor sites and requiring, monitoring, safeguarding and protection.
There are more than 430 reactors worldwide but so far no country has developed a long-term underground disposal facility to house the deadly waste for the millennia to come. It appears as if much of the world's nuclear waste will remain on or near the surface and vulnerable to unforeseen calamities.
The hottest of the ponds, its temperature stood at 30.5C on Tuesday morning, Tepco said, well below the safety limit of 65C. It estimated it would take four days to reach that limit.
The two ponds in Units 1 and 3 had lower temperatures and would have taken even longer to reach the safety limit, Tepco said.
The cooling system at a fourth, common pool was restarted at 00:12am local time (4:12 GMT) on Wednesday, Tepco said.
The shared pool contained the largest number of spent fuel rods that needed to be cooled down constantly, state broadcaster NHK reported.
'Back-up system'The ponds store spent fuel from the nuclear reactors. They cool the fuel - which generates intense heat - and provide shielding from radiation. The spent fuel remains in the ponds for a year or more.
There was no immediate threat of a radiation release. But if fresh cooling water was not restored within the deadlines, it is possible that the water in the ponds could have started to boil.
That could have led to a loss of water and eventually to the exposure of the spent fuel rods to air. If that had happened, reported the BBC's Rupert Wingfield-Hayes in Tokyo, it would have been a very serious situation and could have led to a release of radiation.
On 11 March 2011, an earthquake and tsunami crippled the plant. Waves knocked out cooling systems to the reactors, leading to meltdowns at three of them.
Engineers have since stabilised the plant but years of work lie ahead to fully contain the disaster and tackle its effects.
Tens of thousands of evacuees remain unable to return home.
Source: http://www.bbc.co.uk/news/world-asia-21840080#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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