Tokyo Electric Power Co. workers began pumping freshwater instead of seawater to cool reactors at the quake-damaged Fukushima Daiichi nuclear power plant on Friday, amid concerns over leakage of highly radioactive water that injured two workers the previous day.

Seawater that has been used to cool the fuel rods during the emergency could be leaving a salt crust, increasing the difficulty of cooling the rods and adding to the risk of a meltdown.

A Japan Self Defence Forces auxiliary ship tows a U.S. navy barge, filled with freshwater to cool reactors at the Fukushima Daiichi nuclear plant, in Tokyo harbour on Friday. Kyodo/Reuters

Tokyo Electric said it began injecting freshwater into the No. 1 and No. 3 reactor cores Friday afternoon and evening, and was also preparing to pump freshwater into the No. 2 reactor core.

On Thursday, three workers were exposed to water containing radioactivity 10,000 times the normal level while they were working in the turbine building of the No. 3 reactor. Two of the workers suffered radiation burns on their legs and required hospitalization.

On Friday, highly radioactive water was also found in the turbine buildings of the No. 1 and No. 2 reactors, increasing the risk for workers attempting to deal with the nuclear crisis at the plant.

Earlier Friday, concerns were aired that the high radioactivity levels in water from the No. 3 reactor might have resulted from a breach of the reactor's containment vessel, raising the threat of wider contamination from a reactor that uses a more hazardous plutonium-uranium fuel mixture.

But a spokesman for the government's nuclear agency, Hidehiko Nishiyama, said there was no change in the pressure level to suggest a crack in the reactor's containment vessel. He said further investigation would be needed to determine how the radioactive water reached the underground area where the workers were contaminated.

Prime Minister Naoto Kan said at a press conference Friday evening that current situation at the plant ''still does not warrant optimism.''

'We are not in a position where we can be optimistic.'—Naoto Kan, Japanese PM

Japanese Prime Minister Naoto Kan sounded pessimistric in a televised address to the country Friday.

"The situation today at the Fukushima Daiichi power plant is still very grave and serious," Kan said. "We must remain vigilant. We are not in a position where we can be optimistic. We must treat every development with the utmost care."

Japanese Prime Minister Naoto Kan delivers a message to the Japanese people at his official residence in Tokyo on Friday. Yoshikazu Tsuno/AFP/Getty Images

He apologized to farmers and business owners around the plant for any damage He also thanked utility workers, firefighters and military personnel for "risking their lives" to cool the overheated facility.

The prime minister was speaking two weeks to the day after a magnitude-9.0 earthquake and subsequent tsunami set in motion unprecedented damage and explosions at the Daiichi nuclear site.

The nuclear safety agency has ordered the plant operator, Tokyo Electric Power Co., to improve radiation management, the Kyodo News reported on its website.

The government, which had originally told residents within a 20-kilometre radius of the stricken plant to leave, on Friday encouraged those within a 30-kilometre radius to leave voluntarily because the release of radiation from Daiichi is expected to carry on for sometime, Kyodo reported.

Engineers have been working round the clock trying to gain control of the plant 220 kilometres northeast of Tokyo two weeks after a magnitude-9.0 quake triggered a tsunami that engulfed the facility and knocked out itscrucial cooling system.

The plant has been releasing radiation, with elevated levels of radiation turning up in raw milk, seawater and 11 kinds of vegetables, including broccoli, cauliflower and turnips.

Tokyo water concerns ease

After several days of concern about elevated radiation levels in Tokyo tap water, lower levels were reported late Friday at a Tokyo water purification plant, the Tokyo metropolitan government said.

The government's Bureau of Waterworks detected 51 becquerels of radioactive iodine per kilogram of water sampled Friday morning at the Kanamachi water purification plant in the capital's Katsushika Ward.

That's within the normal range for consumption and below the central government's limits of 100 becquerels for safe consumption by infants and 300 becquerels for adults, the local government said.

Tap water in several areas of Japan — including Tokyo — had earlier been found to have radiation levels considered unsafe for infants, who are particularly vulnerable to cancer-causing radioactive iodine, officials said.

The scare caused a run on bottled water in the capital, and prompted city officials to distribute the products to families with babies, Steve Futterman reported for CBC News.

Police who have finished checking Minamisoma City for radiation are screened for radiation contamination in Fukushima prefecture in northeastern Japan. Kyodo/Reuters

"Once they gave this warning a couple of days ago … it set the alarm bells going, and people don't want to use tap water now."

Distribution centres in Tokyo continued to hand out bottles of water to families with children under a year old, he said.

Officials are also grappling with a humanitarian crisis in the northeast, where hundreds of thousands of survivors remain camped out in schools and civic buildings two weeks after the tsunami swallowed up swaths of the coast. Much of the frigid northeast remains in despair and devastation, with the country struggling to feed and house homeless survivors, clear away debris and bury the dead.

Some 660,000 households are without water, and more than 209,000 lack electricity. Damage could rise as high as $310 billion US, the government said, making it the most costly natural disaster on record.

Police said the official death toll jumped past the 10,000 mark Friday. With the cleanup and recovery operation continuing, and more than 17,400 listed as missing, the final number of dead was expected to surpass 18,000, taking into account overlapping figures.

Japan's nuclear safety agency official Hidehiko Nishiyama, right, consults a staff member during a news conference on Tokyo Electric Power Co.'s Fukushima Daiichi nuclear power plant, in Tokyo on Friday. Kyodo/Reuters

At the Fukushima nuclear plant, fires, explosions and spikes in radiation have hampered efforts to contain the nuclear crisis. High radiation levels have forced repeated evacuations, and more than two dozen workers have been injured, according to NISA.

Operators have been struggling to keep cool water around radioactive fuel rods in the reactor's core after the earthquake and tsunami cut off power supply to the plant and its cooling system.

Damage may have been done to the Unit 3 core when a March 14 hydrogen explosion blew apart its outer containment building. This reactor, the most troubled at the six-unit site, holds 170 tonnes 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.

Meanwhile, damage to factories was taking its toll on the world's third-largest economy and creating a ripple effect felt worldwide. Nissan Motor Co. said it may move part of its engine production line to the United States because of damage to one of its plants.

"There is no doubt that we have immense economic and financial damage," Finance Minister Yoshihiko Noda said. "It will be our task how to recover from the damage."

At the port of Sendai, new Toyota cars lie crushed in piles. At the airport, flooded by the tsunami on March 11, U.S. marines used bulldozers and shovels to shift wrecked cars that lay scattered like discarded toys.

With files from Kyodo News and The Associated Press