By Kazunori Takada and Yoko Nishikawa
TOKYO (Reuters) - One of Japan's six tsunami-crippled nuclear reactors appeared to stabilize on Saturday but the country suffered another blow after discovering traces of radiation in food and water from near the stricken power plant.
Fire trucks sprayed water for nearly half a day on reactor No.3, the government said, cooling overheating nuclear fuel rods considered the most dangerous in the ravaged Fukushima Daiichi complex because of their use of highly toxic plutonium.
"The situation there is stabilizing somewhat," Chief Cabinet Secretary Yukio Edano told a news conference.
But traces of radiation exceeding government limits were found in milk from a farm about 30 km (18 miles) from the plant in Fukushima prefecture and spinach grown in neighboring Ibaraki prefecture, he said.
Tiny levels of radioactive iodine were also found in tap water in Tokyo, one of the world's largest cities about 240 km (150 miles) south, where many tourists and expatriates have already left and where many residents are staying indoors.
The sample contained 1.5 becquerals per kg of iodine 131, well below the tolerable limit for food and drink of 300 becquerals per kg, the government said.
Edano said higher radiation levels still posed no risks. But the International Atomic Energy Agency said radioactive iodine in food can cause short-term health problems.
"Though radioactive iodine has a short half-life of about eight days and decays naturally within a matter of weeks, there is a short-term risk to human health if radioactive iodine in food is absorbed into the human body," it said in a statement.
The government ordered a halt to all food product sales from Fukushima prefecture, where the plant is located, said the U.N. atomic watchdog.
It was the first discovery of contaminated food since the March 11 earthquake and tsunami that left nearly 18,300 people dead or missing, turned entire towns into wastelands and triggered the world's biggest nuclear crisis in 25 years.
It is likely to heighten scrutiny of Japanese food exports, especially in Asia, their biggest market. Even before the discovery of contamination, several restaurants in Singapore said they were considering importing sushi, sashimi and other Japanese ingredients from other countries.
Despite the contaminated food and water, there has been very little radiation found in the air in Tokyo since Thursday, when the city recorded a slight rise that was well below levels considered dangerous to human health.
Prime Minister Naoto Kan, facing Japan's biggest disaster since World War Two, sounded out the opposition about forming a government of national unity to deal with the crisis. But the leader of the largest opposition party rejected the idea out of hand.
In a desperate attempt to re-start water pumps that would cool overheating nuclear fuel rods and prevent a deadly radiation leak, a 1.5 km (one mile) power cable was connected to the outside of the mangled nuclear plant Saturday.
Four of the worst-hit reactors in the complex should have electricity by Sunday, Japan's nuclear safety agency said, a potential milestone in efforts to fix the world's worst nuclear accident in 25 years.
Engineers attached the power cable to the No. 2 reactor but have yet to turn on its coolers, and they plan to test power in reactors No. 1, 2, 3 and 4 Sunday, Hidehiko Nishiyama, deputy director general at the Japan Nuclear and Industrial Safety Agency, told a news conference.
"This is an absolutely necessary step," he said. "To return the situation to normal we need power to bring the temperature down with normal methods."
Restarting the coolers would be "a significant step forward in establishing stability," added Eric Moore, a nuclear power expert at U.S.-based FocalPoint Consulting Group.
Working inside a 20-km (12-mile) evacuation zone at Fukushima, nearly 300 engineers restarted a second diesel generator for reactor No. 6, the nuclear safety agency added. They used the power to restart cooling pumps on No. 5.
Thousands living outside that danger zone but within a 30 km (18 miles) radius face dwindling supplies of heating fuel, food and water, heeding a government request to stay indoors and close all windows, doors and vents.
The U.N. nuclear watchdog says radiation levels outside the plant are safe. It said Japanese authorities have urged people in the area to ingest iodine, which can be used to help protect against thyroid cancer in the event of radioactive exposure.
The operator of the 40-year-old plant, Tokyo Electric Power Co, is facing mounting criticism in Japan, including questions over whether it hesitated too long before dousing the reactors with seawater, which permanently damages them.
Saturday, its president issued a public apology for "causing such great concern and nuisance."
Plant officials say a last resort, if all else fails, would be to bury the sprawling old plant in sand and concrete to prevent a catastrophic radiation release. The method was used at the Chernobyl reactor in 1986, scene of the world's worst nuclear reactor disaster.
Underlining their desperation, fire trucks sprayed water through much of Saturday, a day after Japan raised the severity rating of the nuclear crisis to level 5 from 4 on the seven-level INES international scale, putting it on a par with the Three Mile Island accident in Pennsylvania in 1979.
Some experts say it is more serious. Chernobyl, in Ukraine, was a 7 on that scale.
The operation to avert large-scale radiation has largely overshadowed the humanitarian crisis caused by the 9.0-magnitude quake and 10-meter (33-foot) tsunami.
Nearly 340,000 people, many elderly, have evacuated, living in shelters in northeastern coastal areas. Food, water, medicine and heating fuel are in short supply and a Worm Moon, when the full moon is closest to Earth, could bring floods to devastated areas.
In an example of the sheer power of the earthquake, the largest in Japan's recorded history, Oshika peninsula in Miyagi prefecture shifted an entire 5.3 meters (17 ft) to the east and its land sank 1.2 meters (4 ft).
"Everything is gone, including money," said Tsukasa Sato, a 74-year-old barber with a heart condition, as he warmed his hands in front of a stove at a shelter for the homeless.
Health officials and the U.N. atomic watchdog have said radiation levels in the capital Tokyo were not harmful. But the city has seen an exodus of tourists, expatriates and many Japanese, who fear a blast of radioactive material.
"I'm leaving because my parents are terrified. I personally think this will turn out to be the biggest paper tiger the world has ever seen," said Luke Ridley, 23, from London as he sat at Narita international airport using his laptop.
Though there has been alarm around the world, experts say dangerous levels of radiation are unlikely to spread to other nations. The U.S. government said "minuscule" amounts of radiation were detected in California consistent with a release from the damaged facility but there were no levels of concern.
But the immediate problems remained huge for many people. About 257,000 households in the north still have no electricity and at least one million lack running water.
Aid groups say most victims are getting help, but there are pockets of acute suffering.
"We've seen children suffering with the cold, and lacking really basic items like food and clean water," Stephen McDonald of Save the Children said in a statement.
At least 7,348 people were confirmed dead, exceeding 6,434 who died after the Kobe earthquake in 1995. But 10,947 people are still missing, National Police Agency of Japan said on Saturday.
(Writing by Jason Szep. Additional reporting by Nathan Layne, Kiyoshi Takenaka, Ealine Lies Mayumi Negishi in Tokyo, and Yoko Kubota and Chang-ran Kim in Rikuzentakata. Editing by Miral Fahmy and Sugita Katyal)