text: 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. 
   
   In this photo taken on Aug. 10, 2006, the Fukushima Daini Nuclear Power Plant in Tmiokamachi, Fukushima Prefecture, is seen. The cooling ability had been lost at three of four reactors, in white and light... (Associated Press) 
   
   In this photo taken on Oct. 3, 2008, the Fukushima Daiichi Nuclear Power Plant in Okumamachi, Fukushima Prefecture, is seen. A strong earthquake on March 11, 2011 knocked out power at the plant, and because... (Associated Press) 
   
   Police officers wearing gas masks patrol in the area of the Fukushima power plant's Unit 1 in Okumamachi, Fukushima Prefecture (state), northern Japan, Saturday, March 12, 2011, amid fears that a part... (Associated Press) 
   
   Fukushima Daiichi power plant's Unit 1 is seen in Okumamachi, Fukushima prefecture, Japan, Friday, March 11, 2011. The nuclear power plant affected by a massive earthquake is facing a possible meltdown,... (Associated Press) 
   
   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. 
   
   Another expert on the call, Ken Bergeron, a physicist and former Sandia scientist, added that as a result of such a meltdown the surrounding land would be off-limits for a considerable period of time, and "a lot of first responders would die." 
   
   ___ 
   
   AP National Writer Jeff Donn reported from Boston. ||||| The estimated death toll from Japan's disasters climbed past 10,000 Sunday as authorities raced to combat the threat of multiple nuclear reactor meltdowns and hundreds of thousands of people struggled to find food and water. The prime minister said it was the nation's worst crisis since World War II. 
   
   EDS NOTE: GRAPHIC CONTENT - A dead man lies on the stairs of a destroyed house in Sendai, northeastern Japan, Sunday, March 13, 2011, two days after the powerful earthquake and tsunami hit the area. (AP... (Associated Press) 
   
   In this photo released by the Japanese Defense Agency, Japanese Hiromitsu Shinkawa, wrapped in a blanket, is lifted to a Maritime Self-Defense Force destroyer following his rescue on Sunday March 13,... (Associated Press) 
   
   Residents queue to collect fresh water in Koriyama, Fukushima Prefecture, Japan, Sunday, March 13, 2011, two days after a giant quake and tsunami struck the country's northeastern coast. (AP Photo/Mark... (Associated Press) 
   
   A man holds his baby as they are prepared to be scanned for levels of radiation in Koriyama, Fukushima Prefecture, Japan, Sunday, March 13, 2011. Friday's quake and tsunami damaged two nuclear reactors... (Associated Press) 
   
   A crate of beer is balanced on downed power lines on Sunday, March 13, 2011 over a canal littered with cans of alcohol that were washed into the area from a nearby brewery when a tsunami struck Sendai,... (Associated Press) 
   
   TV sets are scattered on a street in Hachinohe, northeastern Japan, on Sunday, March 13, 2011 after Friday's catastrophic earthquake and tsunami. (AP Photo/Kyodo News) MANDATORY CREDIT, NO LICENSING... (Associated Press) 
   
   A dead woman lies under a blanket near the stairs of her destroyed home in Sendai, Miyagi Prefecture, northeastern Japan, on Sunday, March 13, 2011 after Japan's biggest recorded earthquake hit its eastern... (Associated Press) 
   
   ALTERNATE CROP - Residents evacuated from areas surrounding the Fukushima nuclear facilities damaged in Friday's massive earthquake react during a check for radiation contamination, Sunday, March 13,... (Associated Press) 
   
   A woman pushes a bicycle on a street following a massive tsunami that's triggered by a catastrophic earthquake in Tagajo near Sendai, northeastern Japan, on Sunday, March 13, 2011. (AP Photo/Koji Sasahara) (Associated Press) 
   
   Buildings are covered with mud in Rikuzentakata, Iwate Prefecture, northern Japan, Saturday, March 12, 2011, after a powerful tsunami created by one of the strongest earthquakes ever recorded swept through... (Associated Press) 
   
   A stranded elderly woman is carried on the back of a Japanese soldier after being rescued from a residence at Kesennuma, northeastern Japan, on Saturday March 12, 2011, one day after a giant earthquake... (Associated Press) 
   
   A ship washed away by tsunami sits amid debris in Kesennuma, Miyagi prefecture, Sunday, March 13, 2011 after Japan's biggest recorded earthquake hit its eastern coast Friday. (AP Photo/Itsuo Inouye) (Associated Press) 
   
   A Japanese couple walk through the ruins of their neighborhood in Sendai, northeastern Japan, on Sunday, March 13, 2011. (AP Photo/David Guttenfelder) (Associated Press) 
   
   A body, covered in a blanket, lies in the rubble of a destroyed neighborhood as police officers search the area in Sendai, Miyagi Prefecture, northeastern Japan, on Sunday, March 13, 2011. Japan's biggest... (Associated Press) 
   
   People make their way along on a damaged river bank following a massive tsunami that's triggered by a catastrophic earthquake in Tagajo near Sendai, northeastern Japan, on Sunday, March 13, 2011. (AP... (Associated Press) 
   
   Destroyed cars are left out on a street following a massive tsunami triggered by a huge earthquake in Tagajo near Sendai, northern Japan, Sunday, March 13, 2011. (AP Photo/Koji Sasahara) (Associated Press) 
   
   Oil tar lie in the other side of the river bank from burning oil refinery following a massive tsunami triggered by March 11 earthquake in Tagajo, near Sendai, northern Japan, Sunday, March 13, 2011.... (Associated Press) 
   
   Smoke rise from an oil refinery on fire following a tsunami triggered by a strong earthquake in Tagajo, Miyagi Prefecture, northern Japan, Sunday, March 13, 2011. (AP Photo/Koji Sasahara) (Associated Press) 
   
   A large ship sits among rubble after it was hit by a tsunami in Kesennuma, northeastern Japan, on Saturday March 12, 2011, one day after a giant quake and tsunami struck the country's northeastern coast.... (Associated Press) 
   
   An SOS sign is written on the ground of Shizugawa High School in Minamisanrikucho in Miyagi Prefecture (state), northern Japan, Sunday, March 13, 2011, two days after the powerful earthquake and tsunami... (Associated Press) 
   
   A car sits on top of a small building in a destroyed neighborhood in Sendai, Japan, on Sunday, March 13, 2011 after it was washed into the area by the tsunami that hit northeastern Japan. AP Photo/David... (Associated Press) 
   
   Smoke rises from burning debris washed away by tsunami in Kesennuma, Miyagi Prefecture, Sunday, March 13, 2011 after Japan's biggest recorded earthquake hit its eastern coast Friday. (AP Photo/Itsuo Inouye) (Associated Press) 
   
   Black thick smokes cover the sky over Tagajo, Miyagi, northern Japan Saturday, March 12, 2011 after Japan's biggest recorded earthquake slammed into its eastern coast Friday. (AP Photo/Kyodo News) MANDATORY... (Associated Press) 
   
   Minamisanriku town is submerged after Friday's strong earthquake-triggered tsunami in Miyagi prefecture, northern Japan, Saturday, March 12, 2011. (AP Photo/Kyodo News) MANDATORY CREDIT, NO LICENSING... (Associated Press) 
   
   In this photo released by the Japanese Defense Agency, Hiromitsu Shinkawa, right, waves to rescuers before being rescued to a Maritime Self-Defense Force destroyer on Sunday March 13, 2011. When spotted,... (Associated Press) 
   
   Nuclear plant operators worked frantically to try to keep temperatures down in several reactors crippled by the earthquake and tsunami, wrecking at least two by dumping sea water into them in last-ditch efforts to avoid meltdowns. Officials warned of a second explosion but said it would not pose a health threat. 
   
   Near-freezing temperatures compounded the misery of survivors along hundreds of miles (kilometers) of the northeastern coast battered by the tsunami that smashed inland with breathtaking fury. Rescuers pulled bodies from mud-covered jumbles of wrecked houses, shattered tree trunks, twisted cars and tangled power lines while survivors examined the ruined remains. 
   
   One rare bit of good news was the rescue of a 60-year-old man swept away by the tsunami who clung to the roof of his house for two days until a military vessel spotted him waving a red cloth about 10 miles (15 kilometers) offshore. 
   
   The death toll surged because of a report from Miyagi, one of the three hardest hit states. The police chief told disaster relief officials more than 10,000 people were killed, police spokesman Go Sugawara told The Associated Press. That was an estimate _ only 400 people have been confirmed dead in Miyagi, which has a population of 2.3 million. 
   
   According to officials, more than 1,800 people were confirmed dead _ including 200 people whose bodies were found Sunday along the coast _ and more than 1,400 were missing in Friday's disasters. Another 1,900 were injured. 
   
   For Japan, one of the world's leading economies with ultramodern infrastructure, the disasters plunged ordinary life into nearly unimaginable deprivation. 
   
   Hundreds of thousands of hungry survivors huddled in darkened emergency centers that were cut off from rescuers, aid and electricity. At least 1.4 million households had gone without water since the quake struck and some 1.9 million households were without electricity. 
   
   While the government doubled the number of soldiers deployed in the aid effort to 100,000 and sent 120,000 blankets, 120,000 bottles of water and 29,000 gallons (110,000 liters) of gasoline plus food to the affected areas, Prime Minister Naoto Kan said electricity would take days to restore. In the meantime, he said, electricity would be rationed with rolling blackouts to several cities, including Tokyo. 
   
   "This is Japan's most severe crisis since the war ended 65 years ago," Kan told reporters, adding that Japan's future would be decided by its response. 
   
   In Rikuzentakata, a port city of over 20,000 virtually wiped out by the tsunami, Etsuko Koyama escaped the water rushing through the third floor of her home but lost her grip on her daughter's hand and has not found her. 
   
   "I haven't given up hope yet," Koyama told public broadcaster NHK, wiping tears from her eyes. "I saved myself, but I couldn't save my daughter." 
   
   A young man described what ran through his mind before he escaped in a separate rescue. "I thought to myself, ah, this is how I will die," Tatsuro Ishikawa, his face bruised and cut, told NHK as he sat in striped hospital pajamas. 
   
   Japanese officials raised their estimate Sunday of the quake's magnitude to 9.0, a notch above the U.S. Geological Survey's reading of 8.9. Either way, it was the strongest quake ever recorded in Japan, which lies on a seismically active arc. A volcano on the southern island of Kyushu _ hundreds of miles (kilometers) from the quake' epicenter _ also resumed spewing ash and rock Sunday after a couple of quiet weeks, Japan's weather agency said. 
   
   Dozens of countries have offered assistance. Two U.S. aircraft carrier groups were off Japan's coast and ready to help. Helicopters were flying from one of the carriers, the USS Ronald Reagan, delivering food and water in Miyagi. 
   
   Two other U.S. rescue teams of 72 personnel each and rescue dogs arrived Sunday, as did a five-dog team from Singapore. 
   
   Still, large areas of the countryside remained surrounded by water and unreachable. Fuel stations were closed, though at some, cars waited in lines hundreds of vehicles long. 
   
   The United States and a several countries in Europe urged their citizens to avoid travel to Japan. France took the added step of suggesting people leave Tokyo in case radiation reached the city. 
   
   Community after community traced the vast extent of the devastation. 
   
   In the town of Minamisanrikucho, 10,000 people _ nearly two-thirds of the population _ have not been heard from since the tsunami wiped it out, a government spokesman said. NHK showed only a couple concrete structures still standing, and the bottom three floors of those buildings gutted. One of the few standing was a hospital, and a worker told NHK that hospital staff rescued about a third of the patients. 
   
   In the hard-hit port city of Sendai, firefighters with wooden picks dug through a devastated neighborhood. One of them yelled: "A corpse." Inside a house, he had found the body of a gray-haired woman under a blanket. 
   
   A few minutes later, the firefighters spotted another _ that of a man in black fleece jacket and pants, crumpled in a partial fetal position at the bottom of a wooden stairwell. From outside, while the top of the house seemed almost untouched, the first floor where the body was had been inundated. A minivan lay embedded in one outer wall, which had been ripped away, pulverized beside a mangled bicycle. 
   
   The man's neighbor, 24-year-old Ayumi Osuga, dug through the remains of her own house, her white mittens covered by dark mud. 
   
   Osuga said she had been practicing origami, the Japanese art of folding paper into figures, with her three children when the quake stuck. She recalled her husband's shouted warning from outside: "'GET OUT OF THERE NOW!'" 
   
   She gathered her children _ aged 2 to 6 _ and fled in her car to higher ground with her husband. They spent the night in a hilltop home belonging to her husband's family about 12 miles (20 kilometers) away. 
   
   "My family, my children. We are lucky to be alive," she said. 
   
   "I have come to realize what is important in life," Osuga said, nervously flicking ashes from a cigarette onto the rubble at her feet as a giant column of black smoke billowed in the distance. 
   
   As night fell and temperatures dropped to freezing in Sendai, people who had slept in underpasses or offices the past two nights gathered for warmth in community centers, schools and City Hall. 
   
   At a large refinery on the outskirts of the city, 100-foot (30-meter) -high bright orange flames rose in the air, spitting out dark plumes of smoke. The facility has been burning since Friday. The fire's roar could be heard from afar. Smoke burned the eyes and throat, and a gaseous stench hung in the air. 
   
   In the small town of Tagajo, also near Sendai, dazed residents roamed streets cluttered with smashed cars, broken homes and twisted metal. 
   
   Residents said the water surged in and quickly rose higher than the first floor of buildings. At Sengen General Hospital, the staff worked feverishly to haul bedridden patients up the stairs one at a time. With the halls now dark, those who can leave have gone to the local community center. 
   
   "There is still no water or power, and we've got some very sick people in here," said hospital official Ikuro Matsumoto. 
   
   Police cars drove slowly through the town and warned residents through loudspeakers to seek higher ground, but most simply stood by and watched them pass. 
   
   In the town of Iwaki, there was no electricity, stores were closed and residents left as food and fuel supplies dwindled. Local police took in about 90 people and gave them blankets and rice balls, but there was no sign of government or military aid trucks. 
   
   ___ 
   
   Todd Pitman reported from Sendai. Associated Press writers Eric Talmadge and Kelly Olsen in Koriyama and Malcolm J. Foster, Mari Yamaguchi, Tomoko A. Hosaka and Shino Yuasa in Tokyo contributed to this report.
target: – When officials decided to vent radioactive steam from one of the damaged nuclear power plants in Japan, they knew it could result in a hydrogen explosion. And, even though an explosion is exactly what happened, the decision was still the best option for avoiding a total meltdown, the AP reports in an interesting piece that traces the chain of events from Friday, when the earthquake damaged the Fukushima Dai-ichi nuclear complex, to this morning, when a total of six reactors were placed under a state of emergency. More from Japan: The estimated death toll is more than 10,000 as of today, with the prime minister calling this disaster Japan’s worst crisis since World War II, the AP reports. The confirmed dead number more than 1,800. Nuclear plant operators dumped sea water into two reactors in a last-ditch attempt to keep temperatures down, as officials warned a second explosion could occur, but would not pose a health threat. Experts confirm to Reuters that “this is not a serious public health issue at the moment,” and that the winds will likely carry contamination to the Pacific, where it will not threaten any other nations. The next crisis facing Japan could be an economic one, Reuters reports. Automakers, electronics firms, and oil refineries have been forced to suspend operations at some of their factories, and many companies are not sure how soon they will be back up and running. Even so, experts say the economy could bounce back in months. Click for more on Japan.