TOKYO – Inside the troubled nuclear power plant, officials knew the risks were high when they decided to vent radioactive steam from a severely overheated reactor vessel. They knew a hydrogen explosion could occur, and it did. The decision still trumped the worst-case alternative — total nuclear meltdown.
At least for the time being.
The chain of events started Friday when a magnitude-8.9 earthquake and tsunami severed electricity to the Fukushima Dai-ichi nuclear complex 170 miles (270 kilometers) northeast of Tokyo, crippling its cooling system. Then, backup power did not kick in properly at one of its units.
From there, conditions steadily worsened, although government and nuclear officials initially said things were improving. Hours after the explosion, they contended that radiation leaks were reduced and that circumstances had gotten better at the 460-megawatt Unit 1. But crisis after crisis continued to develop or be revealed.
Without power, and without plant pipes and pumps that were destroyed in the explosion of the most-troubled reactor's containment building, authorities resorted to drawing seawater in an attempt to cool off the overheated uranium fuel rods.
Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, said in a briefing for reporters that the seawater was a desperate measure.
"It's a Hail Mary pass," he said.
He said that the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution. He said the dousing would need to continue nonstop for days.
Another key, he said, was the restoration of electrical power, so that normal cooling systems can be restored.
Officials placed Dai-ichi Unit 1, and four other reactors, under states of emergency Friday because operators had lost the ability to cool the reactors using usual procedures.
An additional reactor was added to the list early Sunday, for a total of six — three at the Dai-ichi complex and three at another nearby complex. Local evacuations have been ordered at each location. Japan has a total of 55 reactors spread across 17 complexes nationwide.
Officials began venting radioactive steam at Fukushima Dai-ichi's Unit 1 to relieve pressure inside the reactor vessel, which houses the overheated uranium fuel.
Concerns escalated dramatically Saturday when that unit's containment building exploded.
It turned out that officials were aware that the steam contained hydrogen, acknowledged Shinji Kinjo, spokesman for the government Nuclear and Industrial Safety Agency. More importantly, they also were aware they were risking an explosion by deciding to vent the steam.
The significance of the hydrogen began to come clear late Saturday:
_Officials decided to reduce rising pressure inside the reactor vessel, so they vented some of the steam buildup. They needed to do that to prevent the entire structure from exploding, and thus starting down the road to a meltdown.
_At the same time, in order to keep the reactor fuel cool, and also prevent a meltdown, operators needed to keep circulating more and more cool water on the fuel rods.
_Temperature in the reactor vessel apparently kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit (1,200 Celsius), it reacted with the water, becoming zirconium oxide and hydrogen.
_When the hydrogen-filled steam was vented from the reactor vessel, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.
A similar "hydrogen bubble" had concerned officials at the 1979 Three Mile Island nuclear disaster in Pennsylvania until it dissipated.
If the temperature inside the Fukushima reactor vessel continued to rise even more — to roughly 4,000 degrees Fahrenheit (2,200 Celsius) — then the uranium fuel pellets would start to melt.
According to experts interviewed by The Associated Press, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.
At some point in the process, the walls of the reactor vessel — 6 inches (15 centimeters) of stainless steel — would melt into a lava-like pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one caused by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.
If the reactor core became exposed to the external environment, officials would likely began pouring cement and sand over the entire facility, as was done at the 1986 Chernobyl nuclear accident in the Ukraine, Peter Bradford, a former commissioner of the U.S. Nuclear Regulatory Commission, said in a briefing for reporters.
At that point, Bradford added, "many first responders would die."
Thousands of Japanese fled the vicinity of an earthquake-crippled nuclear plant after a radiation leak and authorities faced a fresh threat Sunday with the failure of the cooling system in a second reactor.
Operator Tokyo Electric Power (TEPCO) said it was preparing to vent steam to relieve pressure in the No.3 reactor at the plant 240 km (150 miles) north of Tokyo — which would release a small amount of radiation — following an explosion and leak Saturday from the facility's No. 1 reactor.
As strong aftershocks continued to shake Japan's main island, the desperate search pressed on for survivors from Friday massive earthquake and tsunami and the death toll was expected to rise.
Thousands spent another freezing night huddled over heaters in emergency shelters along the northeastern coast, a scene of devastation after the 8.9 magnitude quake sent a 10-meter (33-foot) wave surging through towns and cities in the Miyagi region.
"It's hard to even imagine the scale of it," a resident said. "I came to Miyagi during the last earthquake as well to help, but once there is water involved, it becomes a whole different story. It's hard to think about."
Kyodo news agency said the number of dead or unaccounted was expected to exceed 1,800. It also reported there had been no contact with around 10,000 people in one town, more than half its population.
The government insisted radiation levels were low following Saturday's explosion, saying the blast had not affected the reactor core container, and the International Atomic Energy Agency (IAEA) said it had been told by Japan that levels "have been observed to lessen in recent hours."
But Japan's nuclear safety agency said the number of people exposed to radiation from the Fukushima Daiichi plant could reach 160. Workers in protective clothing were scanning people arriving at evacuation centers for radioactive exposure.
"They are working on relieving pressure and pumping in water into the No. 3 reactor," Chief Cabinet Secretary Yukio Edano told a news briefing.
"This will result in some radiation leakage, although at a level that won't affect peoples' health. It will help stabilize the situation." He also said radiation from the No. 1 reactor was "low enough not to affect people's health."
Officials ordered the evacuation of a 20-km (12-mile) radius zone around the plant and 10 km (6 miles) around another nuclear facility close by. Around 140,000 people had left the area, the IAEA said, while authorities prepared to distribute iodine to protect people from radioactive exposure.
"There is radiation leaking out, and since the possibility (of being exposed) is high, it's quite scary," said Masanori Ono, 17, standing in line Saturday to be scanned for radiation at an evacuation center in Fukushima prefecture.
The government, which took power led by the Democratic Party of Japan for the first time less than two years ago, was already facing criticism.
"Crisis management is incoherent," blared a headline in the Asahi newspaper, charging that information disclosure and instructions to expand the evacuation area around the troubled plant were too slow.
"Every time they repeated 'stay calm' without giving concrete data, anxiety increased," it quoted an unidentified veteran party lawmaker as saying.
Edano said the government expected the 200 billion yen ($2.4 billion) budget reserve for the current fiscal year to cover its response to the crisis. "We cannot comment on the size of the necessary extra budget for the next fiscal year (starting in April) for now, it will need deliberations in parliament."
In Europe, environmentalists seized upon the accident to press demands for an end to nuclear power. Up to 60,000 protesters formed a 45-km (27-mile) human chain in Germany to denounce the government's policy of extending the life of nuclear plants.
Helicopters from Japan's Self-Defense Forces lifted 195 people from retirement homes and hospitals inside the exclusion zone.
Before news of the problem with reactor No. 3, the nuclear safety agency said the plant accident was less serious than both the Three Mile Island accident in 1979 and the 1986 Chernobyl nuclear disaster.
An official at the agency said it had rated the incident a 4 according to the International Nuclear and Radiological Event Scale (INES). Three Mile Island was rated 5 while Chernobyl was rated 7 on the 1 to 7 scale, the official said.
However, Kyodo news agency later reported that radiation levels at the Daiichi plant had risen above their limit.
Along the northeast coast, rescue workers searched through the rubble of destroyed buildings, cars and boats, looking for survivors in hardest-hit areas such as Sendai, 300 km (180 miles) northeast of Tokyo.
Aerial footage showed buildings, trains and even light aircraft strewn like children's toys after powerful walls of seawater swamped areas around Sendai.
In Iwanuma, not far from Sendai, nurses and doctors were rescued Saturday after spelling S.O.S. on the rooftop of a partially submerged hospital, one of many desperate scenes. In cities and towns, worried relatives checked information boards on survivors at evacuation centers.
Dazed residents hoarded water and huddled in makeshift shelters in near-freezing temperatures.
"All the shops are closed, this is one of the few still open. I came to buy and stock up on diapers, drinking water and food," Kunio Iwatsuki, 68, told Reuters in Mito city, where residents queued outside a damaged supermarket for supplies.
Kyodo said about 300,000 people were evacuated nationwide, many seeking refuge in shelters, wrapped in blankets, some clutching each other sobbing.
It said 5.5 million people were without power, while 3,400 buildings had been destroyed or damaged. Four trains were unaccounted for after the tsunami.
In Tokyo, the usually bustling central districts were deserted Saturday night, and the few in bars and restaurants were glued to television coverage of the disaster.
"Even in the bar we kept staring at the news," said Kasumi, a 26-year-old woman meeting a friend for a drink in the central district of Akasaka. "I looked at the tsunami swallowing houses and it seemed like a film."
Plant operator TEPCO has had a rocky past in an industry plagued by scandal. In 2002, the president of the country's largest power utility was forced to resign along with four other senior executives, taking responsibility for suspected falsification of nuclear plant safety records.
Many Japanese flooded social networking sites with worries about the plant.
"I can't trust TEPCO," said a person with the handlename Tanuki Atsushi on mixi, the Japanese social networking site.
The earthquake and tsunami, and now the radiation leak, present Japan's government with its biggest challenge in a generation.
The disaster struck as the world's third-largest economy had been showing signs of reviving from an economic contraction in the final quarter of last year. It raised the prospect of major disruptions for many key businesses and a massive repair bill running into tens of billions of dollars.
Foreign countries have started to send disaster relief teams to help Japan, with the United Nations sending a group to help coordinate work.
The earthquake was the fifth most powerful to hit the world in the past century. It surpassed the Great Kant quake of September 1, 1923, which had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.