Tests on Friday showed iodine 131 levels in seawater 30 km (19 miles) from the coastal nuclear complex had spiked 1,250 times higher than normal, but it was not considered a threat to marine life or food safety, the Nuclear and Industrial Safety Agency said.
"Ocean currents will disperse radiation particles and so it will be very diluted by the time it gets consumed by fish and seaweed," said Hidehiko Nishiyama, a senior agency official.
Despite that reassurance, the disclosure may well heighten international concern over Japanese seafood exports. Several countries have already banned milk and produce from areas around the Fukushima Daiichi plant, while others have been monitoring Japanese seafood.
The prolonged efforts to prevent a catastrophic meltdown at the plant have also intensified concern around the world about nuclear power UN Secretary-General Ban Ki-moon said it was time to reassess the international atomic safety regime.
Engineers were trying to pump radioactive water out of the power plant 240 km (150 miles) north of Tokyo, after it was found in buildings housing three of the six reactors. On Thursday, three workers sustained burns at reactor No. 3 after being exposed to radiation levels 10,000 times higher than usually found in a reactor.
The crisis at the nuclear plant has overshadowed a big relief and recovery effort from the magnitude 9.0 quake and the huge tsunami it triggered on March 11 that left more than 27,500 people dead or missing in northeast Japan.
Plant operator Tokyo Electric Power Co said it was using fresh water instead of seawater to cool down at least some of the reactors after concern arose that salt deposits might hamper the cooling process.
Two of the plant's reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke. However, the nuclear safety agency said on Saturday that temperature and pressure in all reactors had stabilised.
Prime Minister Naoto Kan said on Friday the situation at Fukushima was "nowhere near" being resolved. His chief cabinet secretary said the following day that at least it was not deteriorating.
"We are preventing the situation from worsening - we've restored power and pumped in fresh water - and making basic steps towards improvement but there is still no room for complacency," Yukio Edano told a news briefing on Saturday.
More than 700 engineers have been working in shifts to stabilise the plant with no end in sight.
At Three Mile Island, the worst nuclear power accident in the United States, workers took just four days to stabilise the reactor, which suffered a partial meltdown. No one was injured and there was no radiation release above the legal limit.
At Chernobyl in Ukraine, the worst nuclear accident in the world, it took weeks to "stabilise" what remained of the plant and months to clean up radioactive materials and cover the site with a concrete and steel sarcophagus.
So far, no significant levels of radiation have been detected beyond the vicinity of the plant in Fukushima.
The US Department of Energy said on its website no significant quantities of radiological material had been deposited in the area around the plant since March 19, according to tests on Friday.
In Tokyo, a metropolis of 13 million people, a Reuters reading on Saturday morning showed ambient radiation of 0.22 microsieverts per hour, about six times normal for the city. That was well within the global average of naturally occurring background radiation of 0.17-0.39 microsieverts per hour, a range given by the World Nuclear Association.
The Japanese government has prodded tens of thousands of people living in a 20 km-30 km (12-18 mile) zone beyond the stricken complex to leave. Edano said the residents should move because it was difficult to get supplies to the area, and not because of elevated radiation.
Kazuo Suzuki, 56, who has moved from his house near the nuclear plant to an evacuation centre, said neighbours he had talked to by telephone said delivery trucks were not going to the exclusion zone because of radiation worries.
"So goods are running out, meaning people have to drive to the next town to buy things. But there is a fuel shortage there too, so they have to wait in long queues for gasoline to use the car."
Radiation levels at the evacuation centre were within a normal range of about 0.16 microsievert, according to a Reuters geiger counter reading.
In Japan's northeast, more than a quarter of a million people remain in shelters, and the impact on livelihoods is becoming clearer. The quake and tsunami not only wiped out homes and businesses but also a fishing industry that was the lifeblood of coastal communities.
"Fishermen lost their gear, ships and just about everything. About half will probably get out of the business," said Yuko Sasaki, a fishmonger in the tsunami-hit city of Kamaishi.
The double disaster probably destroyed aqua farms for abalone, sea urchins, oysters, scallops and seaweed that authorities say account for 80 percent of the revenue of the region's fisheries.
The tsunami obliterated centuries-old fishing ports along the northeast coast, sending ships adrift in the Pacific Ocean, to the bottom of the sea, or depositing them on land, where they now lie among the splintered remains of homes.
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