TOKYO — Japanese nuclear safety officials said Friday that they suspect that the reactor core at one unit of the troubled Fukushima Dai-ichi nuclear power plant may have breached, raising the possibility of more severe contamination to the environment.
"It is possible that somewhere at the reactor may have been damaged," said Hidehiko Nishiyama, a spokesman for the nuclear safety agency. But he added that "our data suggest the reactor retains certain containment functions," implying that the damage may have occurred in Unit 3's reactor core but that it was limited.
Officials say the damage could instead have happened in other equipment, including piping or the spent fuel pool.
Operators have been struggling to keep cool water around radioactive fuel rods in the reactor's core after the March 11 earthquake and tsunami cut off power supply to the plant and its cooling system.
Damage could have been done to the core when a March 14 hydrogen explosion blew apart Unit 3's outer containment building.
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This reactor, perhaps the most troubled at the six-unit site, holds 170 tons of radioactive fuel in its core. Previous radioactive emissions have come from intentional efforts to vent small amounts of steam through valves to prevent the core from bursting. However, releases from a breach could allow uncontrolled quantities of radioactive contaminants to escape into the surrounding ground or air.
Operators stopped work Friday at units 1 through 3 to check on radiation levels.
Meanwhile, radiation injuries to workers complicated the battle to control the plant on Friday, two weeks after a quake and tsunami that also left more than 27,000 people dead or missing.
More than 700 engineers have been working in shifts around the clock to stabilize the six-reactor Fukushima complex since the multiple disaster.
But they had to pull out of some parts of the complex, 150 miles north of Tokyo, when three workers replacing a cable at one reactor were exposed to high contamination by standing in radioactive water on Thursday, officials said.
Two were taken to a hospital with possible radiation burns after the water seeped over their boots.
"We should try to avoid delays as much as possible, but we also need to ensure that the people working there are safe," said Japanese nuclear agency official Hidehiko Nishiyama.
Safety fears at the plant and beyond — radiation particles have been found as far away as Iceland — are compounding Japan's worst crisis since World War Two.
More than two dozen people have been injured trying to bring the plant under control.
Two of the reactors are now regarded as safe in what is called a cold shutdown. Four remain volatile, emitting steam and smoke periodically, but work is advancing to restart water pumps needed to cool fuel rods inside those reactors.
"It's much more hopeful," said Tony Roulstone, a nuclear energy expert at Cambridge University.
The United States has been offering aid to its ally Japan, and two of its barges will together provide 525,000 gallons (2.0 million liters) of water for cooling the reactors.
Tokyo Electric Power Co (TEPCO) said the three injured workers were carrying radiation meters but ignored an alarm when it rang. Engineers would be briefed again on safety.
Water rush continues
Heightened by widespread public ignorance of the technicalities of radiation, alarm has been spreading.
Vegetable and milk shipments from the areas near the plant have been stopped, and Tokyo's 13 million residents were told this week not to give tap water to babies after contamination from rain put radiation at twice the safety level.
But it dropped back to safe levels the next day, and the city governor cheerily drank water in front of cameras at a water purifying plant.
Despite government reassurances and appeals for people not to panic, there has been a rush on bottled water and shelves in many Tokyo shops remained empty of the product on Friday.
"Customers ask us for water. But there's nothing we can do," said Tokyo supermarket worker Masayoshi Kasahara.
The government is dipping into stockpiles, and there have been donations of bottled water from abroad.
The National Police Agency said Friday the death toll has topped 10,000, and more than 17,440 people are listed as missing. Hundreds of thousands have been left homeless.Video: Tokyo still tepid about water safety (on this page)
The U.S. and Australia halted imports of Japanese dairy and produce from the region, Hong Kong said it would require that Japan perform safety checks on meat, eggs and seafood, and Canada said it would upgrade controls on imports of Japanese food products. Singapore, too, has banned the sale of milk, produce, meat and seafood from areas near the plant.
Concerns also spread to Europe. In Iceland, officials said they measured trace amounts of radioactive iodine in the air but assured residents it was "less than a millionth" of levels found in Europe in the wake of the 1986 Chernobyl disaster — the world's worst nuclear accident.
Radioactive iodine is short-lived, with a half-life of eight days — the length of time it takes for half of it to break down harmlessly. However, experts say infants are particularly vulnerable to radioactive iodine, which can cause thyroid cancer.
New readings Thursday showed the city's tap water was back to levels acceptable for infants, but the relief was tempered by elevated levels of the isotope in two neighboring prefectures: Chiba and Saitama. A city in a third prefecture, just south of the plant, also showed high levels of radioactive iodine in tap water, officials said.
Tap water in Kawaguchi City in Saitama, north of Tokyo, contained 210 becquerels of radioactive iodine — well above the 100 becquerels considered safe for babies but below the 300-becquerel level for adults, Health Ministry official Shogo Misawa said.
In Chiba prefecture, the water tested high for radiation in two separate areas, said water safety official Kyoji Narita. The government there warned families in 11 cities in Chiba not to give infants tap water.
"The high level of iodine was due to the nuclear disaster," Narita said. "There is no question about it."
Radiation levels also tested dangerously high in Hitachi in Ibaraki prefecture, about 70 miles south of the Fukushima plant, city water official Toshifumi Suzuki said, adding that officials were distributing bottled water.
The limits refer to sustained consumption rates, and officials said parents should stop using tap water for baby formula, although it was OK for infants to consume small amounts.
Video: U.S. sees progress in Japan relief operation (on this page)
Despite the appeals, shelves were bare in many stores across Tokyo.
Maruetsu supermarket in the city center sought to impose buying limits on specific items to prevent hoarding: only one carton of milk per family, one 5-kilogram (11-pound) bag of rice, one package of toilet paper, one pack of diapers. Similar notices at some drugs stores told women they could only purchase two feminine hygiene items at a time.
Maruetsu spokeswoman Kayoko Kano acknowledged that the earthquake and tsunami resulted in delays of some products.
Some frustrated shoppers have turned to the city's many vending machines as an alternative. The machines are found everywhere in the city and one can feature about three dozen different beverages — ranging from hot coffee and green tea to power drinks and juice. A 500-milliliter bottle of imported water costs about 100 yen (about $1.25).
A spokesman for Procter & Gamble Japan said its plant was fully operational but that rolling blackouts in Tokyo may be affecting distribution. "Consumers are nervous, and they may be buying up supplies," Noriyuki Endo added.
Worse hardships continued in the frigid, tsunami-struck northeast. Some 660,000 households still do not have water, the government said. Electricity has not been restored to some 209,000 homes, Tohoku Electric Power Co. said. Damage is estimated at $309 billion, making it the most costly natural disaster on record.
In one bright spot of economic news, Toyota Motor Corp. — which had suspended production due to damage to suppliers' factories and power shortages in the quake zone — said it will soon resume production of the Prius and two other hybrid models.
But rival Honda Motor Co. said the suspension of car production at its Saitama and Suzuka factories will be extended to April 3.
The economic woes spawned by the disasters were especially painful for farmers in the region near the nuclear plant.
Sumiko Matsuno, a 65-year-old farmer in Fukushima, spent Thursday frantically harvesting vegetables from her fields.
"We are digging up all our carrots and onions as fast as we can. We can't sell them but we need them ourselves for food," she said. "We are really worried about our future. If this goes on, it is going to really hurt us."
Exhausted rescuers are still sifting through the wreckage of towns and villages, retrieving bodies.
Amid the suffering, though, there was a sense that Japan was turning the corner in its humanitarian crisis. Aid flowed to refugees, and phone, electricity, postal and bank services began returning to the north, sometimes by makeshift means.
"Things are getting much better," said 57-year-old Tsutomu Hirayama, staying with his family at an evacuation center in Ofunato town.
"For the first two or three days, we had only one rice ball and water for each meal. I thought, how long is this going to go on? Now we get lots of food, it's almost like luxury."
The Associated Press, Reuters and msnbc.com staff contributed to this report.