Tokyo:
Three days after a massive earthquake and tsunami, the Japanese authorities were still struggling on Sunday to avoid full meltdowns at a plant with two stricken reactors on Japan's coast.
(See Pics: Japan quake triggers Tsunami)As of Sunday night, technicians' increasingly desperate measures to keep the reactors from overheating, and possibly melting down, had not yet brought them under control. In one case Sunday, a mechanical failure left the plant far more vulnerable to additional fuel melting and the release of more radiation.
Twenty two people who live near the plant are already showing signs of radiation exposure from earlier releases. The challenge of bringing the plant under control comes as the country is also struggling to rescue and feed people buried in rubble or stranded after the country's largest earthquake in recorded history.
(Read: Radioactive releases in Japan could last months, say experts)Japanese officials said they had ordered the largest mobilization of their Self-Defense Forces since World War II to assist in the quake relief effort, including helping with the evacuation of people around the plant, where an explosion blew the roof off a reactor containment building, allowing radiation to escape.
(Read: Japan quake - Toll may exceed 10,000)Both that plant, Fukushima Daiichi Nuclear Power Station, and Fukushima Daini, about 10 miles away, have been under a state of emergency. On Monday morning, the company that operates both plants said it had restored the cooling systems at two of three reactors experiencing problems at Daini. That would leave a total of four reactors at the two plants having pumping difficulties.
"I'm not aware that we've ever had more than one reactor troubled at a time," said Frank N. Von Hippel, a physicist and professor at Princeton, explaining the difficulties faced by the Japanese.
"The whole country was focused on Three Mile Island," he said, referring to the Pennsylvania nuclear plant accident in 1979. "Here you have Tokyo Electric Power and the Japanese regulators focusing on multiple plants at the same time. ''
In what was perhaps the clearest sign of the rising anxiety over the nuclear crisis, both the United States Nuclear Regulatory Commission and the Russian authorities issued statements Sunday trying to allay fears, saying they did not expect harmful levels of radiation reaching their territory.
(Watch: I saw my child being swept away: Japan tsunami survivor)There were two new scares on Sunday.
Late at night, the International Atomic Energy Agency announced that Japan had added a third plant to the list because radiation had been detected outside its walls. But on Monday morning, it quoted Japanese authorities as saying that the radioactivity levels at the Onagawa plant had returned to normal levels and that there appeared to be no leak there.
"The increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant," the agency said. The two plants are 75 miles apart.
Soon after that announcement, Kyodo News reported that a plant about 75 miles north of Tokyo was having at least some cooling system problems. But a plant spokesman later said a backup pump was working.
The government was scrambling Sunday to test people who lived near the Daiichi plant, with local officials saying that about 170 people had probably been exposed, but it was unclear if they or the 22 people who showed signs of exposure had received dangerous doses. The government had earlier said that three workers were suffering radiation illness, but Tokyo Electric Power, which runs Fukushima Daiichi and Daini, said Monday that only one worker was ill.
The events Sunday raised new questions about whether the government was telling its people everything it knew about the extent of the nuclear troubles.
Although the government has held a string of news conferences, officials have not provided information on whether it was the earthquake or the tsunami that began the cascading series of nuclear problems.
The crisis is sure to raise questions among Japanese about their earthquake-prone country's reliance on nuclear energy. One third of the country's electricity comes from nuclear plants.
The worst problems continued to be at Fukushima Daiichi, which appeared to be the most serious involving a nuclear plant since the Chernobyl disaster. Officials have said they presume that two of the stricken reactors at the plant suffered a partial meltdown of the reactor core -- a dangerous situation that, if unchecked, could lead to a full meltdown and the likelihood of a catastrophic release of radiation.
A partial meltdown can occur when radioactive fuel rods, which normally are covered in water, remain partially uncovered for too long. The more the fuel is exposed, the closer the reactor comes to a full meltdown.
Technicians are essentially fighting for time now at the reactors, trying to keep the rods covered despite a lack of electricity to quickly pump the water; the main electrical power was knocked out in the quake or the tsunami. The government said emergency diesel generators, intended to cope with a grid failure, initially functioned after the quake by then failed.
On Sunday, Japanese nuclear officials said operators at the plant had suffered a setback trying to bring one of the reactors under control when a valve malfunction stopped the flow of water and left fuel rods partially uncovered. The delay raised pressure at the reactor.
At a late-night news conference, officials at Tokyo Electric Power said that the valve had been fixed, but that water levels had not yet begun rising. Chief Cabinet Secretary Yukio Edano said the situation was unchanged Monday morning.
On Sunday, Masataka Shimizu, president of Tokyo Electric, apologized for the problems at the Daiichi and Daini plants.
"I offer a heartfelt apology for the widespread concerns trouble we have brought to society due to the leak of radioactive substances," Mr. Shimizu said.
He denied long-standing allegations by some nuclear experts that the Daiichi plant, which began running in 1971 was too old to be operating and suffered from structural flaws that made it more vulnerable to earthquakes. He said that the tsunami, rather than the quake was the "No. 1 problem" for the plant.
He did not explain why the backup diesel generators had not been better protected, even though the plant is along the coast, and therefore vulnerable to tsunamis.