Japanese officials are fighting a desperate multifront battle to avert a nuclear disaster in an area of the country already ravaged by a massive earthquake and devastating tsunami.
Nuclear plant operators were flooding shut-down reactors with cold water - including sea water in some cases - in a bid to keep temperatures down and prevent explosions that could spread radioactive contamination across the country and into East Asia.
The second hydrogen explosion in three days rocked the nuclear plant Monday, sending a massive cloud of smoke into the air and injuring 11 workers. The blast was felt 40 kilometers away, but the plant's operator said the radiation levels at the affected unit were still within legal limits.
Shortly after Monday's explosion, Tokyo Electric warned it had lost the ability to cool Unit 2. Hours later, the company said fuel rods in that unit were fully exposed, at least temporarily. The company was trying to channel sea water into the reactor to cover the rods, cool them down and prevent another explosion at the stricken plant.
As of early Monday, despite the latest incidents at the Fukushima Daiichi plant, Japanese officials continued to express confidence that high-level radiation that results from the partial meltdown of reactor fuel could be confined to the steel containment vessels that cover each unit. Operators have been venting steam that contains only low-level radiation.
However, the situation was far from stable and the from the magnitude 9 earthquake that ravaged the northeast coast. About 180,000 people have been evacuated from the area and scores of local residents show signs of exposure to low-level radiation.
Canadian nuclear engineer John Luxat of McMaster University said the reported levels appeared to be well below maximum exposures set by the International Atomic Energy Agency.
Should the steel containment walls be breached in an explosion, the radioactive gases could spread for hundreds of kilometres - although the worst contamination would occur within a 50-kilometre radius. Officials insist the steel containment chambers were built to withstand an explosion in the reactor core.
The Fukushima Daiichi plant was the worst hit among Japan's large fleet of nuclear stations, with two units at the six-reactor facility experiencing ongoing problems with cooling and the likelihood of partial meltdowns of the reactor cores.
But three other nuclear stations were also reporting either cooling problems or higher radiation levels outside the plants.
The failure at the Daiichi plant, some 270 kilometres north of Tokyo, is building to be the worst nuclear disaster since the Chernobyl 25 years ago, but experts say they don't expect the massive release of high-level radiation that occurred at the plant in Ukraine. Unlike the Japanese units, Chernobyl exploded while it was still in full operation and its substandard containment system was destroyed by the explosion, resulting in the release of high-level radiation.
There was an explosion on Saturday at the No. 1 reactor at Fukushima Daiichi, but officials say that was caused by the buildup of hydrogen in an outer chamber of the unit, and that the containment vessel remained intact. The loss of the outer structure, however, did cause the release of some radioactive gases.
On Monday, there was an explosion at the No. 3 reactor. Workers had been pumping sea water into the unit to maintain temperature control. Chief Cabinet Secretary Yukio Edano had warned earlier that a buildup of hydrogen at the second plant could cause an explosion similar to the one on Saturday.
"If there is an explosion, there would be no significant impact on human health," Mr. Edano told reporters, expressing confidence the containment structure could withstand any shock.
On Monday, Tokyo Electric Power Co. said radiation levels at Unit 3 were 10.65 microsieverts, significantly under the 500 microsieverts at which a nuclear operator is legally bound to file a report to the government.
Operators were forced to use sea water when they could not maintain enough cold fresh water around the reactor's fuel to keep it from melting. The corrosive salt water can damage the steel containment vessel over time, and its use ensures the 40-year-old reactors will never be restarted.
Later on Monday, a third reactor at the site, Unit 2, lost its cooling capacity.
And, some analysts noted, the length of time since the nuclear crisis began indicates that the chemical reactions inside the reactor were not moving quickly toward a complete meltdown.
"We're now into the fourth day. Whatever is happening in that core is taking a long time to unfold," said Mark Hibbs, a senior associate at the nuclear policy program for the Carnegie Endowment for International Peace. "They've succeeded in prolonging the timeline of the accident sequence."
The Japanese are hoping their nuclear nightmare resembles the one that occurred at Three Mile Island in Pennsylvania in 1979 rather than the more disastrous one at Chernobyl. The Three Mile Island unit experienced a partial meltdown of the reactor, but contamination was largely confined to the steel containment vessel, with the exception of the escape of some low-level radiation.
Japan's nuclear fleet
The loss of power at the Fukushimi Daiichi plant and the shutdown of others, including the Fukushimi Daini, is a serious blow to Japan's power supply, and government officials are warning of rolling blackouts until earthquake and tsunami damage can be assessed and reactors restarted.
Despite its location in a earthquake zone - and despite being the only country to have suffered from a nuclear-bomb attack - the resource-poor island has come to rely heavily on nuclear power for its electricity generation.
The country's 54 reactors provide 30 per cent of the country's electricity and the government proposes to increase that to 40 per cent by 2017. Construction on two new plants - both similar in design to the Fukushima reactors - has started, while some 12 more are planned for completion within 10 years.
Japan is a major player in the global reactor market. U.S.-based Westinghouse is owned by Toshiba; Hitachi is in a joint venture with General Electric, and Mitsubishi Heavy Industries has its own reactor design.
The fate of that industry depends heavily on whether operators can contain the fallout from the crippled reactors.
But a lot of questions remain about the state of the reactor cores, the release of radioactive gases and level of exposure for workers and residents, said Shawn Patrick Stensil, a nuclear campaigner with Greenpace.
He accused Japanese authorities - and the global industry - of minimizing the impacts.
Dr. Luxat of McMaster University said it would be several days at least before operators know whether they can cool the reactors enough to avoid further problems.
It the high-level contamination can be contained after such a devastating earthquake and tsunami, it would be a remarkable testament to the design safety, he said.
Three Mile Island v. Chernobyl
It has taken the nuclear industry a generation to recoverin North America from the accidents at Three Mile Island and Chernobyl, as it fought to reassure politicians and consumers that their product is a clean and safe source of power.
With the Japanese crisis, the industry faces a public-relations disaster in addition to the very real health threats to citizens and long-term environmental damage.
While Three Mile Island has become synonymous with nuclear disaster, relatively little radioactive material escaped into the atmosphere.
Studies after the accident showed the radiation exposure was well below levels that would indicate possible health effects. The Pennsylvania Department of Health maintained a registry of 30,000 people who lived within eight kilometres of the plant for 18 years, before closing it down, citing the lack of proven health impacts.
As appears to be happening at Fukushima, the Three Mile Island plant did experience a partial meltdown of one of its reactor cores after the loss of coolant, but the high-level contamination was confined to the containment chamber.
That didn't happen seven years later at Chernobyl - which stands as the worst nuclear-plant accident in history. The explosion at Chernobyl destroyed the modest containment system at the plant, and it immediately spewed not just radioactive gases, but molten plutonium and uranium into the surrounding countryside.
In 2005, a United Nations report - in collaboration with Russia, Ukraine and Belarus - concluded that 4,000 people would eventually died from cancer as a result of the radiation poisoning, although only 50 deaths had been directly attributable to it.
The report also said that, apart from the 30-kilometre-wide dead zone, the surrounding country had largely recovered from the accident of 20 years earlier.
However, environmental groups scorned the UN report, saying it underestimated the cancer rates by focusing on deaths directly attributable to the disaster - and played down the decades-long damage to the environment.
With files from AP