Engineers at Fukushima reactor 4 Works at the Fukushima Daiichi plant are spraying water on the damaged reactors to cool them
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Japan quake
* A test of Japan's 'stoicism'
* Disaster or distraction?
* Q&A: Fukushima alert
* Chance of a 'big one' in Tokyo?
Lighting has been restored in the control room of one of the most badly-damaged reactors at Japan's stricken Fukushima nuclear plant, officials say.
It is hoped the development will speed up work to restore cooling systems vital for stabilising the reactor.
Meanwhile, the UN's nuclear watchdog says radiation is still leaking from the quake-hit plant, but scientists are unsure exactly where it is coming from.
Japan estimates more than 21,000 people died in the 11 March quake and tsunami.
The lights came back on in the control centre of reactor 3, hours after power cables were connected to all six reactors for the first time.
The BBC's Mark Worthington in Tokyo says the hope is that as visibility within the plant improves, so will the chances of restarting cooling systems and monitoring equipment.
The Fukushima Daiichi plant's operators, Tokyo Electric Power (Tepco), said engineers will try to power up water pumps to reactor 3 some time on Wednesday.
However, they warned that safety checks had to be made to damaged equipment and any volatile gases vented, to avoid an explosion when the electricity is switched back on.
They said restoring power to all the reactor units could take weeks or even months.
Workers have been battling to cool the reactors and spent fuel ponds to avoid a major release of radiation.
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FUKUSHIMA UPDATE
* Reactor 1: Damage to the core from cooling problems. Building holed by gas explosion. Power lines attached.
* Reactor 2: Damage to the core from cooling problems. Building holed by gas blast; containment damage suspected. Power lines attached.
* Reactor 3: Damage to the core from cooling problems. Building holed by gas blast; containment damage possible. Spent fuel pond partly refilled with water after running low. Power lines attached.
* Reactor 4: Reactor shut down prior to earthquake. Fires and explosion in spent fuel pond; water level partly restored. Power lines attached.
* Reactors 5 & 6: Reactors shut down. Temperature of spent fuel pools now lowered after rising high. Power lines attached.
* Q&A: Fukushima radiation alert
* Misunderstanding the nuclear danger
Emergency teams at Fukushima have also poured seawater into a boiling storage pond housing spent nuclear fuel rods, cooling it and stopping clouds of steam - possibly radioactive - rising from it.
On Tuesday, the International Atomic Energy Agency (IAEA) said that radiation was continuing to be emitted from the plant but it was difficult to pinpoint its exact source.
Senior official James Lyons said the IAEA and Japanese officials could not confirm that the damaged reactors were "totally intact" or if they were cracked and leaking radiation.
"We continue to see radiation coming from the site... and the question is where exactly is that coming from," Mr Lyons told a news conference.
The government has evacuated tens of thousands of people within a 20km (12-mile) radius of the plant and told residents 10km beyond that to stay indoors. The US has recommended an 80km exclusion zone.
Tepco vice president Norio Tsuzumi has visited evacuation centres to meet those forced from their homes.
Bowing deeply, he said: "Since I have tried to manage this problem hand-in-hand with the government, my visit here to directly meet you was belated. For this I also apologise from the bottom of my heart."
Higher than normal levels of radiation have been detected in seawater about 16km (10 miles) off the coast near the plant, but the government said they did not pose an immediate danger to human health.
Officials did however stop food shipments in nearby prefectures after detecting higher-than-normal levels of radiation in milk and certain vegetables, although authorities again insisted there was no health hazard.
Meanwhile, strong aftershocks are continuing to rattle the north-east of Japan, adding to the misery of more than 300,000 people still huddled in evacuation centres across 16 prefectures.
Tens of thousands of homes are still without power and more than two million people have no running water, officials say.
Police say the confirmed death toll from the earthquake and tsunami is now 9,079, with 12,645 missing.
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