Image: Official in protective gear scans for signs of radiation on a man from the evacuation area near the Fukushima nuclear plant in Koriyama, Japan.
Kim Kyung-Hoon  /  Reuters
An official in protective gear Saturday scans for signs of radiation on a man from the evacuation area near the Fukushima nuclear plant in Koriyama, Japan.
NBC, msnbc.com and news services
updated 3/14/2011 1:09:05 AM ET 2011-03-14T05:09:05

The second hydrogen explosion in three days rocked Japan's stricken Fukushima Daiichi nuclear plant Monday, sending a massive column of smoke into the air and wounding six workers. It was not immediately clear how much — if any — radiation had been released.

The explosion at the plant's Unit 3, which authorities have been frantically trying to cool following a system failure in the wake of a massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.

Disaster at a glance

The blast follows a similar explosion Saturday that took place at the plant's Unit 1, which injured four workers and caused mass-evacuations. More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.

Japan's nuclear safety agency said six workers were injured in Monday's explosion but it was not immediately clear how, or whether they were exposed to radiation. They were all conscious, said the agency's Ryohei Shomi.

The reactor's inner containment vessel holding nuclear rods was intact, Edano said, allaying some fears of the risk to the environment and public. TV footage of the building housing the reactor appeared to show similar damage to Monday's blast, with outer walls shorn off, leaving only a skeletal frame.

Tokyo Electric Power Co. said radiation levels at plant were 10.65 microsieverts Monday afternoon, significantly under the 500 microsieverts at which a nuclear operator is legally bound to file a report to the government.

But Pentagon officials reported Sunday that helicopters flying 60 miles from the plant picked up small amounts of radioactive particulates — still being analyzed, but presumed to include cesium-137 and iodine-121 — suggesting widening environmental contamination, according to the New York Times.

A massive column of smoke was seen belching from the Fukushima Daiichi plant's No. 3 unit Monday. Four nuclear plants in northeastern Japan have reported damage.

Operators have been dumping seawater into units 1 and 3 in a last-ditch measure to cool the reactors. They were getting water into the other four reactors with cooling problems without resorting to corrosive sea water, which likely makes the reactors unusable.

A meltdown at the No. 3 reactor could be more serious than at the other reactors because it is fueled by both plutonium and uranium, BBC News reported. The others have only uranium fuel.

Earlier, Edano said radioactivity released into the environment so far was so small it didn't pose any health threats.

Such statements, though, did little to ease public worries. In a country where memories of a nuclear horror of a different sort during the last days of World War II weigh on the national psyche and national politics, the impact of continued venting of long-lasting radioactivity from the plants is hard to overstate, the New York Times reported.

Image: The damaged roof of reactor No. 1 at the Fukushima Daiichi nuclear plant after an explosion
Tepco  /  Reuters
The damaged roof of reactor No. 1 at the Fukushima Daiichi nuclear plant after an explosion that blew off the upper part of the structure is seen in this Saturday photo released by Tokyo Electric Power Company (TEPCO) in Fukushima Prefecture, northeastern Japan.

"First I was worried about the quake," said Kenji Koshiba, a construction worker who lives near the plant. "Now I'm worried about radiation." He spoke at an emergency center in Koriyama, about 40 miles (60 kilometers) from the most troubled reactors and 125 miles (190 kilometers) north of Tokyo.

At the makeshift center set up in a gym, a steady flow of people — mostly the elderly, schoolchildren and families with babies — were met by officials wearing helmets, surgical masks and goggles.

About 1,500 people had been scanned for radiation exposure, officials said.

GE-designed reactors in Fukushima have 23 sisters in U.S.

Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. It was unclear whether any cases of exposure had reached dangerous levels.

A foreign ministry official briefing reporters said radiation levels outside the Daiichi plant briefly rose above legal limits, but had since declined significantly.

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Edano said none of the Fukushima Daiichi reactors was near the point of complete meltdown, and he was confident of escaping the worst scenarios.

Officials, though, have declared states of emergency at the six reactors where cooling systems were down — three at Daiichi and three at the nearby Fukushima Daini complex.

The U.N. nuclear agency said a state of emergency was also declared Sunday at another complex, the Onagawa power plant, after higher-than-permitted levels of radiation were measured there. But radiation levels at that plant returned to normal later Sunday.

A pump for the cooling system at yet another nuclear complex, the Tokai Daini plant, also failed after Friday's quake but a second pump operated normally as did the reactor, said the utility, the Japan Atomic Power Co. It did not explain why it did not announce the incident until Sunday.

Edano denied there had been a meltdown in the Fukushima Daiichi complex, but other officials said the situation was not so clear.

Hidehiko Nishiyama, a senior official of the Economy, Trade and Industry Ministry, indicated the reactor core in Unit 3 had melted partially, telling a news conference, "I don't think the fuel rods themselves have been spared damage," according to the Kyodo News agency.

A complete meltdown — the melting of the radioactive core — could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.

The steel reactor vessel could melt or break from the heat and pressure. A concrete platform underneath the reactor is supposed to catch the molten metal and nuclear fuel, but the intensely hot material could set off a massive explosion if water has collected on the platform. Radioactive material also could be released into the ground if the platform fails.

The explosion that destroyed the walls and ceiling of Daiichi Unit 1's containment building was much less serious that a meltdown would be — in fact, it was operators' efforts to avoid a meltdown that caused it.

Officials vented steam from the reactor to reduce pressure, and were aware that there was an explosion risk because the steam contained hydrogen, said Shinji Kinjo, spokesman for the government's Nuclear and Industrial Safety Agency. The explosion occurred when hydrogen reacted with oxygen outside the reactor.

It is unclear how far the impact of a meltdown might reach. In the United States, local communities plan for evacuation typically within 10 miles of a nuclear plant. However, states must be ready to cope with contamination of food and water as far as 50 miles away. Radioactivity can also be carried to faraway places by the winds, as it was in the 1986 Chernobyl disaster, though it will become increasingly diffuse. Acute radiation deaths would normally be expected only much closer to the plant.

Japanese authorities have classified the event at Daiichi's No. 1 as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale (INES).

The scale is used to consistently communicate the safety significance of events associated with sources of radiation. The scale runs from 0 (deviation — no safety significance) to 7 (major accident).

The 1979 Three Mile Island accident in Pennsylvania was a level 5 ("accident with wider consequences"). The 1986 Chernobyl disaster was a level 7 ("major accident").

The U.S. is not expected to experience "any harmful levels" of radiation from Japan's stricken reactors, the U.S. Nuclear Regulatory Commission said Sunday.

"All the available information indicates weather conditions have taken the small releases from the Fukushima reactors out to sea away from the population," the NRC said in a statement.

"Given the thousands of miles between the two countries, Hawaii, Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity."

Interactive: How a nuclear plant works

The reactor that exploded at Chernobyl, sending a cloud of radiation over much of Europe, was not housed in a sealed container as those atDaiichi are. The Japanese reactors also do not use graphite, which burned for several days at Chernobyl.

Prime Minister Naoto Kan on Sunday sought to allay radiation fears: "Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency quoted him as saying. "This is fundamentally different from the Chernobyl accident."

Japan's nuclear crisis was triggered by twin disasters on Friday, when an 8.9-magnitude earthquake, the most powerful in the country's recorded history, was followed by a tsunami that savaged its northeastern coast with breathtaking speed and power.

More than 1,800 people were killed and hundreds more were missing, according to officials, but police in one of the worst-hit areas estimated the toll there alone was more than 10,000.

All of the reactors in the region shut down automatically when the earthquake hit. But with backup power supplies also failing, shutting down the reactors is just the beginning of the problem, scientists said.

"You need to get rid of the heat," said Friedrich Steinhaeusler, a professor of physics and biophysics at Salzburg University and an adviser to the Austrian government on nuclear issues. "You are basically putting the lid down on a pot that is boiling."

"They have a window of opportunity where they can do a lot," he said, such as using sea water as an emergency coolant. But if the heat is not brought down, the cascading problems can eventually be impossible to control. "This isn't something that will happen in a few hours. It's days."

Japan's nuclear safety agency said 1,450 workers were at the Daiichi plant on Sunday, its usual staffing. The workers were in protective gear and were taking shorter turns than usual in units 1 and 3 to limit their exposure, agency spokesman Yoshihiro Sugiyama said.

The emergencies at the nuclear plants have led to an electricity shortage in Japan, where nearly 2 million households were without power Sunday. Starting Monday, power will be rationed with rolling blackouts in several cities, including Tokyo.

Japan has a total of 55 reactors spread across 17 complexes nationwide.

NBC News, Reuters and The Associated Press contributed to this report.

Video: Disaster in the heart of Sendai

Interactive: Japan before and after the disaster

These aerial photos show locations in Japan before and after the 9.0-magnitude earthquake and tsunami that struck March 11. Use the slider below the images to reveal the changes in the landscape.

Explainer: The 10 deadliest earthquakes in recorded history

  • A look at the worst earthquakes in recorded history, in loss of human life. (The March 11, 2011, earthquake and tsumani that affected eastern Japan is not included because the fatalities caused, about 15,000, are fewer than those resulting from the temblors listed below.) Sources: United States Geological Survey, Encyclopedia Britannica

  • 1: Shensi, China, Jan. 23, 1556

    Magnitude about 8, about 830,000 deaths.

    This earthquake occurred in the Shaanxi province (formerly Shensi), China, about 50 miles east-northeast of Xi'an, the capital of Shaanxi. More than 830,000 people are estimated to have been killed. Damage extended as far away as about 270 miles northeast of the epicenter, with reports as far as Liuyang in Hunan, more than 500 miles away. Geological effects reported with this earthquake included ground fissures, uplift, subsidence, liquefaction and landslides. Most towns in the damage area reported city walls collapsed, most to all houses collapsed and many of the towns reported ground fissures with water gushing out.

  • 2: Tangshan, China, July 27, 1976

    Chinese Earthquake
    Keystone  /  Getty Images
    1976: Workers start rebuilding work following earthquake damage in the Chinese city of Tangshan, 100 miles east of Pekin, with a wrecked train carriage behind them. (Photo by Keystone/Getty Images)
    Magnitude 7.5. Official casualty figure is 255,000 deaths. Estimated death toll as high as 655,000.

    Damage extended as far as Beijing. This is probably the greatest death toll from an earthquake in the last four centuries, and the second greatest in recorded history.

  • 3: Aleppo, Syria, Aug. 9, 1138

    Magnitude not known, about 230,000 deaths.

    Contemporary accounts said the walls of Syria’s second-largest city crumbled and rocks cascaded into the streets. Aleppo’s citadel collapsed, killing hundreds of residents. Although Aleppo was the largest community affected by the earthquake, it likely did not suffer the worst of the damage. European Crusaders had constructed a citadel at nearby Harim, which was leveled by the quake. A Muslim fort at Al-Atarib was destroyed as well, and several smaller towns and manned forts were reduced to rubble. The quake was said to have been felt as far away as Damascus, about 220 miles to the south. The Aleppo earthquake was the first of several occurring between 1138 and 1139 that devastated areas in northern Syria and western Turkey.

  • 4: Sumatra, Indonesia, Dec. 26, 2004

    Aerial images show the extent of the devastation in Meulaboh
    Getty Images  /  Getty Images
    MEULABOH, INDONESIA - DECEMBER 29: In this handout photo taken from a print via the Indonesian Air Force, the scene of devastation in Meulaboh, the town closest to the Sunday's earthquake epicentre, is pictured from the air on December 29, 2004, Meulaboh, Aceh Province, Sumatra, Indonesia. The western coastal town in Aceh Province, only 60 kilometres north-east of the epicentre, has been the hardest hit by sunday's underwater earthquake in the Indian Ocean. Officials expected to find at least 10,000 killed which would amount to a quarter of Meulaboh's population. Three-quarters of Sumatra's western coast was destroyed and some towns were totally wiped out after the tsunamis that followed the earthquake. (Photo by Indonesian Air Force via Getty Images)

    Magnitude 9.1, 227,898 deaths.

    This was the third largest earthquake in the world since 1900 and the largest since the 1964 Prince William Sound, Alaska temblor. In total, 227,898 people were killed or were missing and presumed dead and about 1.7 million people were displaced by the earthquake and subsequent tsunami in 14 countries in South Asia and East Africa. (In January 2005, the death toll was 286,000. In April 2005, Indonesia reduced its estimate for the number missing by over 50,000.)

  • 5: Haiti, Jan 12, 2010

    Haitians walk through collapsed building
    Jean-philippe Ksiazek  /  AFP/Getty Images
    Haitians walk through collapsed buildings near the iron market in Port-au-Prince on January 31, 2010. Quake-hit Haiti will need at least a decade of painstaking reconstruction, aid chiefs and donor nations warned, as homeless, scarred survivors struggled today to rebuild their lives. AFP PHOTO / JEAN-PHILIPPE KSIAZEK (Photo credit should read JEAN-PHILIPPE KSIAZEK/AFP/Getty Images)

    Magnitude 7.0. According to official estimates, 222,570 people killed.

    According to official estimates, 300,000 were also injured, 1.3 million displaced, 97,294 houses destroyed and 188,383 damaged in the Port-au-Prince area and in much of southern Haiti. This includes at least 4 people killed by a local tsunami in the Petit Paradis area near Leogane. Tsunami waves were also reported at Jacmel, Les Cayes, Petit Goave, Leogane, Luly and Anse a Galets.

  • 6: Damghan, Iran, Dec. 22, 856

    Magnitude not known, about 200,000 deaths.

    This earthquake struck a 200-mile stretch of northeast Iran, with the epicenter directly below the city of Demghan, which was at that point the capital city. Most of the city was destroyed as well as the neighboring areas. Approximately 200,000 people were killed.

  • 7: Haiyuan, Ningxia , China, Dec. 16, 1920

    7.8 magnitude, about 200,000 deaths.

    This earthquake brought total destruction to the Lijunbu-Haiyuan-Ganyanchi area. Over 73,000 people were killed in Haiyuan County. A landslide buried the village of Sujiahe in Xiji County. More than 30,000 people were killed in Guyuan County. Nearly all the houses collapsed in the cities of Longde and Huining. About 125 miles of surface faulting was seen from Lijunbu through Ganyanchi to Jingtai. There were large numbers of landslides and ground cracks throughout the epicentral area. Some rivers were dammed, others changed course.

  • 8: Ardabil, Iran, March. 23, 893

    Magnitude not known, about 150,000 deaths

    The memories of the massive Damghan earthquake (see above) had barely faded when only 37 years later, Iran was again hit by a huge earthquake. This time it cost 150,000 lives and destroyed the largest city in the northwestern section of the country. The area was again hit by a fatal earthquake in 1997.

  • 9: Kanto, Japan, Sept. 1, 1923

    Kanto Damage
    Hulton Archive  /  Getty Images
    1923: High-angle view of earthquake and fire damage on Hongokucho Street and the Kanda District, taken from the Yamaguchi Bank building after the Kanto earthquake, Tokyo, Japan. (Photo by Hulton Archive/Getty Images)
    7.9 magnitude, 142,800 deaths.

    This earthquake brought extreme destruction in the Tokyo-Yokohama area, both from the temblor and subsequent firestorms, which burned about 381,000 of the more than 694,000 houses that were partially or completely destroyed. Although often known as the Great Tokyo Earthquake (or the Great Tokyo Fire), the damage was most severe in Yokohama. Nearly 6 feet of permanent uplift was observed on the north shore of Sagami Bay and horizontal displacements of as much as 15 feet were measured on the Boso Peninsula.

  • 10: Ashgabat, Turkmenistan, Oct. 5, 1948

    7.3 magnitude, 110,000 deaths.

    This quake brought extreme damage in Ashgabat (Ashkhabad) and nearby villages, where almost all the brick buildings collapsed, concrete structures were heavily damaged and freight trains were derailed. Damage and casualties also occurred in the Darreh Gaz area in neighboring Iran. Surface rupture was observed both northwest and southeast of Ashgabat. Many sources list the casualty total at 10,000, but a news release from the newly independent government on Dec. 9, 1988, advised that the correct death toll was 110,000. (Turkmenistan had been part of the Soviet Union, which tended to downplay the death tolls from man-made and natural disasters.)

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