WASHINGTON — The chairman of the United States Nuclear Regulatory Commission gave a significantly bleaker appraisal of the threat posed by Japan’s nuclear crisis than the Japanese government, saying on Wednesday that the damage at one crippled reactor was much more serious than Japanese officials had acknowledged and advising Americans to evacuate a wider area around the plant than the perimeter established by Japan. 

The announcement marked a new and ominous chapter in the five-day long effort by Japanese engineers to bring four side-by-side reactors under control after their cooling systems were knocked out by an earthquake and tsunami last Friday. It also suggested a serious  split between Washington and Tokyo, after American officials concluded that the Japanese warnings were insufficient, and that, deliberately or not, they had understated the potential threat of what is taking place inside the nuclear facility.

Gregory Jaczko, the chairman of the commission, said in Congressional testimony that the commission believed that all the water in the spent fuel pool at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station had boiled dry, leaving fuel rods stored there exposed and bleeding radiation. As a result, he said, “We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.”

On Thursday morning a spokesman for Tokyo Electric Power, the Daiichi plant operator, and a spokesman for Japan’s nuclear regulatory agency, denied Mr. Jaczko’s account, saying the situation at reactor No. 4 had not changed and that water remained in the spent fuel storage pool. But both officials said the situation was changing and that the reactor had not been inspected in recent hours.

"We can’t get inside to check, but we’ve been carefully watching the building’s environs, and there has not been any particular problem," said Hajime Motojuku, the spokesman for Tokyo Electric.
Takumi Koyamada, the spokesman at Japan’s nuclear regulator, said that as of 12 hours ago water remained and the temperature reading was 84 degrees Celsius and that no change had been reported since then. "We cannot confirm that there has been a loss in water," he said. "But we face a very unpredictable situation." If the American analysis is accurate and Japanese workers have been unable to keep the spent fuel at that inoperative reactor properly cooled — it needs to remain covered with water at all times — radiation levels could make it difficult not only to fix the problem at reactor No. 4, but to keep workers at the Daiichi complex from servicing any of the other problem reactors at the plant.

Mr. Jaczko (the name is pronounced YAZZ-koe) said radiation levels may make it impossible to continue what he called the “backup backup” cooling functions that have so far helped check the fuel melting at the other reactors. Those efforts consist of using fire hoses to dump water on overheated fuel and then letting the radioactive steam vent into the atmosphere.
Those emergency measures, implemented by a small squad of workers and firemen, are the main steps Japan is taking at Daiichi to forestall a full blown fuel meltdown that would lead to much higher releases of radioactive material.

Mr. Jaczko’s testimony came as the American Embassy in Tokyo, on advice from the Nuclear Regulatory Commission, told Americans to evacuate a radius of “approximately 50 miles” from the Fukushima plant.

The advice represents a graver assessment of the risk in the immediate vicinity of Daiichi than the warnings made by the Japanese themselves, who have told everyone within 20 kilometers, about 12 miles, to evacuate, and those between 20 and 30 kilometers to take shelter.
Mr. Jaczko’s testimony, the most extended comments by a senior American official on Japan’s nuclear disaster, described what amounts to an agonizing choice for Japanese authorities: Send a small number of workers into an increasingly radioactive area in a last-ditch effort to cover the spent  fuel,  and the fuel in other reactors, —  with water, or do more to protect the workers but risk letting the pools of water protecting  the fuel boil away — and thus risk a broader meltdown.

 Mr. Jaczko was asked by Michael Burgess, a Republican of Texas and a member of the House committee where he was testifying, if the radiation levels at the plant boundary could be related to a chest x-ray or a cat scan. Mr. Jaczko said the Nuclear Regulatory Commission believed they were “Levels that would be lethal with a fairly short period of time.’’ 

Recently, he said, there were “very very high readings.”
Experts say workers at the plant probably could not approach a fuel pool that was dry, because radiation levels would be so high. In a normally operating pool, the water provides not only cooling but also shields workers from gamma radiation. A plan to dump water into the pool, and others like it, from helicopters was suspended because the crews would be flying right into a radioactive plume.

Mr. Jaczko’s analysis suggests that a potentially dangerous chain of events could unfold, as workers trying to cool the adjacent reactors at the facility could also be exposed to intolerable levels of radiation. If they, too, had to withdraw, the problem could worsen, as reactor cores  go uncooled and spent fuel pools run dry.

Earlier in the day, Japanese authorities announced a different escalation of the crisis at Daiichi when they said that a second reactor unit at the plant may have suffered damage to its primary containment structure and appeared to be releasing radioactive steam.
The break, at the No. 3 reactor unit, worsened the already perilous conditions at the plant, a day after officials said the containment vessel in the No. 2 reactor had also cracked.

The possibility of high radiation levels above the plant prompted the Japanese military to put off a highly unusual plan to dump water from helicopters — a tactic normally used to combat forest fires — to lower temperatures in a pool containing spent fuel rods that was dangerously overheating at the No. 4 reactor. The operation would have meant flying a helicopter into the steam rising from the plant.

But in one of a series of rapid and at times confusing pronouncements on the crisis, the authorities insisted that damage to the containment vessel at the No. 3 reactor — the main focus of concern earlier on Wednesday — was unlikely to be severe.

Yukio Edano, the chief cabinet secretary, said the possibility that the No. 3 reactor had “suffered severe damage to its containment vessel is low.” Earlier he said only that the vessel might have been damaged; columns of steam were seen rising from it in live television coverage.
The reactor’s operator, Tokyo Electric Power Company, said it had been able to double the number of people battling the crisis at the plant to 100 from 50, but that was before the clouds of radioactive steam began billowing from the plant. On Tuesday, 750 workers were evacuated, leaving a skeleton crew of 50 struggling to reduce temperatures in the damaged facility. An increasing proportion of the people at the plant are soldiers, but the exact number is not known.

The Pentagon said Wednesday that American military forces in Japan were not allowed within 50 miles of the plant and that some flight crews who might take part in relief missions were being given potassium iodide to protect against the effects of radiation. Tokyo Electric said Wednesday that some of those at the plant had taken cover for 45 minutes on site, and left water pumps running at reactors Nos. 1, 2 and 3. There was no suspension of cooling operations, said Kazuo Yamanaka, an official at Tokyo Electric. 

The vessel that possibly ruptured on Wednesday had been seen as the last fully intact line of defense against large-scale releases of radioactive material from the stricken reactor, but it was not clear how serious the possible breach might be.

The possible rupture, five days after a devastating earthquake and tsunami crippled the plant, followed a series of explosions and other problems there that have resulted in the world’s worst nuclear crisis since the Chernobyl accident in 1986.

The head of the Vienna-based International Atomic Energy Agency, Yukiya Amano, who is Japanese, said he would leave for Japan as soon as possible to assess the situation. 

The revised official assessment of the severity of the damage at the No. 3 reactor may have been intended to reduce some concerns about the containment vessel, which encloses the core, but the implications of overheating in the fuel rod pool at No. 4 seemed potentially dire.  
There are six reactors at the plant, all of which have pools holding spent fuel rods at the top level of the reactor building. Reactors 4, 5 and 6 were out of service when the earthquake and tsunami struck, and there were concerns about the pools at 5 and 6 as well, and possibly those at the other reactors.

At a hearing in Washington on Wednesday held by two subcommittee of the House Energy and Commerce Committee, Energy Secretary Steven Chu said, “We think there is a partial meltdown” at the plant.
“We are trying to monitor it very closely,’’ he said. “We hear conflicting reports about exactly what is happening in the several reactors now at risk. I would not want to speculate about what is happening.”

He said that his agency had sent 39 people to the American Embassy and to United States consulates in Japan “with the skills, expertise and equipment to help assess, survey and monitor areas.” The department has also shipped survey equipment that can measure radiation levels from the air, he said.

The developments were the latest in Japan’s swirling tragedy since the quake and tsunami struck the country with unbridled ferocity last Friday. Emperor Akihito made his first ever televised appearance on Wednesday to tell the nation he was “deeply worried” about the nuclear crisis.
International alarm about the nuclear crisis appeared to be growing, as several nations urged their citizens in Japan to head to safer areas in the south or leave the country. Prior advisories had largely been limited to simply avoiding nonessential travel. Germany urged its citizens to move to areas farther away from the stricken nuclear plant.

Earlier Wednesday morning, Tokyo Electric reported that a fire was burning at the No. 4 reactor building, just hours after officials said flames that erupted Tuesday had been doused.
A government official at Japan’s nuclear regulatory agency soon after said that flames and smoke were no longer visible, but he cautioned that it was unclear if the fire had died out. He also was not clear if it was a new fire or if the fire Tuesday had never gone out.

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Fire Erupts at Troubled Reactor; Helicopters May Be Used at Plant

TOKYO — The Japanese government reported early Wednesday that a fire was burning at the No. 4 reactor of the troubled Fukushima Daiichi plant in northeastern Japan.

Utility officials said the fire erupted because a blaze at the plant earlier had not been extinguished.
The announcement came just hours after officials with the Tokyo Electric Power Company said that they would consider using helicopters in an attempt to put cold water into a boiling rooftop storage pool for spent uranium fuel rods. The rods are still radioactive and potentially as hot and dangerous as the fuel rods inside the reactors if not kept submerged in water.

“The only ideas we have right now are using a helicopter to spray water from above, or inject water from below,” a power company official said at a news conference. “We believe action must be taken by tomorrow or the day after.”

Hydrogen gas bubbling up from chemical reactions set off by the hot spent fuel rods produced a powerful explosion on Tuesday morning that blew a 26-foot-wide hole in the outer building of Reactor No. 4 at the Fukushima Daiichi nuclear power plant. A fire there may have been caused by machine oil in a nearby facility, inspectors from the United States Nuclear Regulatory Commission said, according to an American official.
Concern remained high about the storage pools at that reactor and at two other reactors, Nos. 5 and 6. None of those three reactors at the plant, 140 miles northeast of Tokyo, were operating on Friday afternoon when an offshore earthquake with a magnitude now estimated at 9.0 suddenly shook the site. A tsunami with waves up to 30 feet high rolled into the northeast Japanese coastline minutes later, swamping the plant.

At least 750 workers were evacuated on Tuesday morning after a separate explosion ruptured the inner containment building at Reactor No. 2 at the Daiichi plant, which was crippled by Friday’s earthquake and tsunami. The explosion released a surge of radiation 800 times more intense than the recommended hourly exposure limit in Japan.

But 50 workers stayed behind, a crew no larger than would be stationed at the plant on a quiet spring day. Taking shelter when possible in the reactor’s control room, which is heavily shielded from radiation, they struggled through the morning and afternoon to keep hundreds of gallons of seawater a minute flowing through temporary fire pumps into the three stricken reactors, Nos. 1, 2 and 3, where overheated fuel rods continued to boil away the water at a brisk pace.

By early afternoon radiation levels had plunged, according to the International Atomic Energy Agency.

Workers have released surges of radiation each time they bleed radioactive steam from the troubled reactors in an attempt to manage the pressure inside them, but the reactors are not yet releasing high levels of radiation on a sustained basis, Japanese officials said.

  The United States military revised its plans as radiation from the plant worsened. Some American warships that had been expected to arrive at the tsunami-shattered northeast coast of Honshu Island were diverted to the west coast instead because of concerns about radiation, the Navy said.
The Navy also promised to continue relief missions even though several more helicopter crews were testing positive for low-level exposure to radiation, and even as American military personnel and their families at the Yokosuka and Atsugi bases were encouraged to take precautions against radiation exposure.

   Late Tuesday morning, Prime Minister Naoto Kan warned in a nationally televised address of rising radiation.

The chief cabinet secretary, Yukio Edano, urged people who live within about 18 miles of the plant to take
precautions. “Please do not go outside, please stay indoors, please close windows and make your homes airtight,” Mr. Edano said. More than 100,000 people are believed to be in the area.

Yukiya Amano, the director general of the International Atomic Energy Agency, said at a news conference at the organization’s Vienna headquarters that there was a “possibility of core damage” at reactor No. 2, but that the damage “is estimated to be less than 5 percent of the fuel.”

The sudden turn of events, after an explosion on Monday at one reactor and then an early-morning explosion on Tuesday at yet another — the third in four days at the plant — had already made the crisis at the plant the worst nuclear accident since the Chernobyl reactor disaster a quarter-century ago. It had become impossible for workers to remain at many areas within the plant for extended periods, the agency said. Japan has requested assistance from the Nuclear Regulatory Commission and the International Atomic Energy Agency.

In Tokyo, the metropolitan government said Tuesday that it had detected radiation levels 20 times above normal over the city, though it stressed that such levels posed no immediate health threat and that readings had dropped since then. The explosion in Reactor No. 2, a little after 6 a.m. on Tuesday, particularly alarmed Japanese officials and nuclear power experts around the world because it was the first detonation at the plant that appeared to occur inside one of the primary containment buildings. 


Those buildings are fortresslike structures of steel and reinforced concrete, designed to absorb the impact of a plane crash and minimize radiation leaks. After a series of conflicting reports about how much damage the reactor had sustained after that blast, Mr. Edano said, “There is a very high probability that a portion of the containment vessel was damaged.”

Japanese officials subsequently said that the explosion had damaged a doughnut-shaped steel container of water, known as a torus, that surrounds the base of the reactor vessel inside the primary containment building.
Ruptures in the torus are serious, said Michael Friedlander, a senior nuclear power plant operator for 13 years at three plants in the United States, including three years at a General Electric boiling water reactor very similar to the ones in trouble in Japan. But the torus is not as important as the reactor vessel itself, which has 6.7-inch-thick steel walls and 8.4-inch-thick steel for its roof and floor. The vessel is designed to hold very high-pressure steam as well as the uranium fuel rods.

The reactor vessel has 20 safety valves that during a shutdown of the reactor inject steam into a million-gallon “suppression pool” of water in a steel torus immediately underneath it.
“Imagine if you had a big pressure cooker and you had a tube off the pressure cooker into a big tub of water — the suppression pool is the tub of water,” said Mr. Friedlander, a defender of nuclear power who is now a money manager in Hong Kong.

Steam vented into the suppression pool from the reactor vessel is not supposed to be radioactive. But the steam becomes radioactive, and potentially very radioactive, if the fuel rods in the reactor vessel above have begun to melt.

The atmosphere in the primary containment building, around the reactor vessel and above the suppression pool, is supposed to consist of inert nitrogen, with no oxygen at all. An inert atmosphere is used in the primary containment building to avoid the risk of oxygen explosions with hydrogen if the reactor starts producing much larger quantities of hydrogen gas than usual. Hydrogen gas is highly combustible with oxygen.

The blast on Tuesday morning that broke the suppression pool at reactor No. 2 shows that the reactor vessel is producing hydrogen and that oxygen may have somehow entered the atmosphere above the suppression pool, Mr. Friedlander said.

The primary containment building, with its massive steel and concrete walls, is housed with various ducts, electrical equipment and other gear inside a bigger building, the secondary containment building. Explosions at Reactors Nos. 1 and 3 blew the roofs off those reactors’ secondary containment buildings, which are not designed to contain hydrogen explosions, unlike primary containment buildings.

The storage pool blast at reactor No. 4 also appears to have ripped a hole in a secondary containment building, based on initial descriptions from Japanese officials.

A senior nuclear industry executive who insisted on anonymity said that a compromised suppression pool made it much harder to bleed high-pressure steam from an overheating nuclear re

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U.S. Nuclear Industry Faces New Uncertainty

  WASHINGTON — The fragile bipartisan consensus that nuclear power offers a big piece of the answer to America’s energy and global warming challenges may have evaporated as quickly as confidence in Japan’s crippled nuclear reactors. 

Until this weekend, President Obama, mainstream environmental groups and large numbers of Republicans and Democrats in Congress agreed that nuclear power offered a steady energy source and part of the solution to climate change, even as they disagreed on virtually every other aspect of energy policy. Mr. Obama is seeking tens of billions of dollars in government insurance for new nuclear construction, and the nuclear industry in the United States, all but paralyzed for decades after the Three Mile Island accident in 1979, was poised for a comeback.
Now, that is all in question as the world watches the unfolding crisis in Japan’s nuclear reactors and the widespread terror it has spawned.

“I think it calls on us here in the U.S., naturally, not to stop building nuclear power plants but to put the brakes on right now until we understand the ramifications of what’s happened in Japan,” Senator Joseph I. Lieberman, independent of Connecticut and one of the Senate’s leading voices on energy, said on CBS’s “Face the Nation.”

Nuclear power, which still suffers from huge economic uncertainties and local concerns about safety, had been growing in acceptance as what appeared to many to be a relatively benign, proven and (if safe and permanent storage for wastes could be arranged) nonpolluting source of energy for the United States’ future growth.

But even staunch supporters of nuclear power are now advocating a pause in licensing and building new reactors in the United States to make sure that proper safety and evacuation measures are in place. Environmental groups are reassessing their willingness to see nuclear power as a linchpin of any future climate change legislation. Mr. Obama still sees nuclear power as a major element of future American energy policy, but he is injecting a new tone of caution into his endorsement.
“The president believes that meeting our energy needs means relying on a diverse set of energy sources that includes renewables like wind and solar, natural gas, clean coal and nuclear power,” said Clark Stevens, a White House spokesman. “Information is still coming in about the events unfolding in Japan, but the administration is committed to learning from them and ensuring that nuclear energy is produced safely and responsibly here in the U.S.”

Three of the world’s chief sources of large-scale energy production — coal, oil and nuclear power — have all experienced eye-popping accidents in just the past year. The Upper Big Branch coal mine explosion in West Virginia, the Deepwater Horizon blowout and oil spill in the Gulf of Mexico and the unfolding nuclear crisis in Japan have dramatized the dangers of conventional power generation at a time when the world has no workable alternatives able to operate at sufficient scale.
The policy implications for the United States are vexing. “It’s not possible to achieve a climate solution based on existing technology without a significant reliance on nuclear power,” said Jason Grumet, president of the Bipartisan Policy Center in Washington and an energy and climate change adviser to the 2008 Obama campaign. “It’s early to reach many conclusions about what happened in Japan and the relevance of what happened to the United States. But the safety of nuclear power will certainly be high on the list of questions for the next several months.”

“The world is fundamentally a set of relative risks,” Mr. Grumet added, noting the confluence of disasters in coal mining, oil drilling and nuclear plant operations. “The accident certainly has diminished what had been a growing impetus in the environmental community to support nuclear 
power as part of a broad bargain on energy and climate policy.”
 
Mitch McConnell of Kentucky, the Senate Republican leader, said that the United States should not overreact to the Japanese nuclear crisis by clamping down on the domestic industry indefinitely. Republicans have loudly complained that the Obama administration did just that after the BP oil spill last spring when it imposed a moratorium on deepwater oil drilling until new safety and environmental rules were written.
“I don’t think right after a major environmental catastrophe is a very good time to be making American domestic policy,” Mr. McConnell said on “Fox News Sunday.”
He said that the American public and politicians had recoiled after Three Mile Island, rejecting permits for the construction of dozens of nuclear plants on the “not in my backyard” impulse.
“My thought about it is, we ought not to make American and domestic policy based upon an event that happened in Japan,” Mr. McConnell said.

Mr. Obama has been as supportive of nuclear power as any recent president as he has tried to devise a political and technical strategy for ensuring energy supplies and reducing greenhouse gas emissions. Nuclear power, along with expanded offshore oil drilling, “clean coal” development and extensive support for renewable energy, are part of his “all-of-the-above energy strategy,” an approach and terminology borrowed from Republicans. But his support for coal and oil as part of a grand compromise on energy were set back by last year’s mining and drilling disasters, and today’s problems with nuclear in Japan cannot help.

Concerns about earthquakes and nuclear power have been around for a long time; new questions might also be raised now about tsunamis and coastal reactors.
In Mr. Obama’s State of the Union address and in his budget, he proposed an expansion of nuclear energy technology and $36 billion in Department of Energy loan guarantees for the construction of as many as 20 new nuclear plants.

That policy will be on the table at a hearing of the Energy and Commerce Committee on Wednesday, when Steven Chu, the energy secretary, and Gregory B. Jaczko, chairman of the Nuclear Regulatory Commission, are scheduled to testify.
“We will use that opportunity to explore what is known in the early aftermath of the damage to Japanese nuclear facilities,” said Representative Fred Upton, Republican of Michigan, the committee chairman, “as well as to reiterate our unwavering commitment to the safety of U.S. nuclear sites.”

Representative Edward J. Markey, Democrat of Massachusetts and a skeptic of nuclear power who nonetheless supported expansion of nuclear power as part of the House energy and climate legislation he co-sponsored, said the United States needed tougher standards for siting and operating nuclear plants.

He said regulators should consider a moratorium on locating nuclear plants in seismically active areas, require stronger containment vessels in earthquake-prone regions and thoroughly review the 31 plants in the United States that use similar technology to the crippled Japanese reactors. “The unfolding disaster in Japan must produce a seismic shift in how we address nuclear safety here in America,” Mr. Markey said.

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Emergency Cooling Effort at Reactor Is Failing, Deepening Japanese Crisis

 TOKYO — Japan’s struggle to contain the crisis at a stricken nuclear power plant worsened early Tuesday morning, as emergency operations to pump seawater into one crippled reactor temporarily failed, increasing the risk of a wider release of radioactive material, officials said.
With the cooling systems malfunctioning simultaneously at three separate reactors at the Fukushima Daiichi Nuclear Power Station after the powerful earthquake and tsunami, a more acute crisis developed late Monday at reactor No. 2 of the plant. There, a series of problems thwarted efforts to keep the core of the reactor covered with water — a step considered crucial to preventing the reactor’s containment vessel from exploding and preventing the fuel inside it from melting down.
The plant’s operator, Tokyo Electric Power, said late Monday that efforts to inject seawater into the reactor had failed. That caused water levels inside the reactor’s containment vessel to fall and exposed its fuel rods. The company said its workers later succeeded in infusing seawater in the pre-dawn hours of Tuesday morning, but fuel rods were exposed for at least several hours.
Workers had been having difficulty injecting seawater into the reactor because its vents — necessary to release pressure in the containment vessel by allowing radioactive steam to escape — had stopped working properly, they said.
But Tuesday morning Tokyo Electric announced that workers had succeeded in opening a malfunctioning valve controlling the vents, reducing pressure in the container vessel. It then resumed flooding the reactor with water.
The company said water levels were not immediately rising to the desired level, possibly because of a leak in the containment vessel. Still, a Tokyo Electric official said the situation was improving.
"We do not feel that a critical event is imminent," he told a press conference.
The release of pressure appears to avert the immediate risk that the containment vessel would explode, creating a potentially catastrophic release of radioactive material into the atmosphere. But if the vessel is cracked and is not holding water properly, the risks of a large scale release of radioactive material would remain high.
In reactor No. 2, which is now the most damaged of the three at the Daiichi plant, at least parts of the fuel rods have been exposed for several hours, which also suggests that some of the fuel has begun to melt. Government and company officials said fuel melting has almost certainly occurred in that reactor, which can increase releases of radioactive material through the water and steam that escapes from the container vessel.
In a worst case, the fuel pellets could also burn through the bottom of the containment vessel and radioactive material could pour out that way — often referred to as a full meltdown.
"There is a possibility that the fuel rods are heating up and starting to melt,” said a Tokyo Electric spokesman told a late-night conference on Monday, televised on public broadcaster NHK. “It is our understanding that we have possible damage to the fuel rods,” he said.
By Monday night, officials said that radiation readings around the plant reached 3,130 micro Sievert, the highest yet detected at the Daiichi facility since the quake and six times the legal limit. Radiation levels of that magnitude are considered elevated, but they are much lower than would be the case if one of the container vessels had been compromised.
Industry executvies in touch with their counterparts in Japan Monday night grew increasingly alarmed about the risks posed by the No. 2 reactor.
“They’re basically in a full-scale panic” among Japanese power industry managers, said a senior nuclear industry executive. The executive is not involved in managing the response to the reactors’ difficulties but has many contacts in Japan. “They’re in total disarray, they don’t know what to do.”
The venting problems made it impossible for a time to administer the emergency remedy the plant operator had been using to control heat at the three crippled Daiichi reactors, all of which experienced failures in their electronic cooling systems. That remedy involves pumping in seawater to cool the fuel rods, then opening vents to release the resulting steam pressure that builds in the container vessel. When the vessel is depressurized, workers can inject more seawater, a process known as “feed and bleed.” 

The extreme challenge of managing reactor No. 2 came as officials were still struggling to keep the cores of two other reactors, No. 1 and No. 3, covered with seawater. There was no immediate indication that either of those two reactors had experienced a crisis as serious as that at No. 2.
The United States Nuclear Regulatory Commission said Monday that the Japanese government had formally asked for assistance as it responds to the crisis in Fukushima. As part of a wider response, the United States has already dispatched two experts in boiling-water reactors, the type used at Daiichi. They are in Tokyo offering technical assistance to the Japanese, the commission said in a statement. The commission is considering further assistance, including providing technical advice, it said.
The situation at Daiichi was also complicated on Monday by another problem when the outer structure housing reactor No. 3 exploded earlier on Monday. A similar explosion destroyed the structure surrounding reactor No. 1 on Saturday. Live footage on public broadcaster NHK showed the skeletal remains of the reactor building and thick smoke rising from the building. Eleven people had been injured in the blast, one seriously, officials said.
Chief Cabinet Secretary Yukio Edano said earlier Monday evening that the release of large amounts of radiation as a result of the explosion at No. 3 was unlikely because the blast did not compromise the steel containment vessel inside the No. 3 reactor. But traces of radiation could be released into the atmosphere, and about 500 people who remained within a 12-mile radius were ordered temporarily to take cover indoors, he said.
“I have received reports that the containment vessel is sound,” Mr. Edano said. “I understand that there is little possibility that radioactive materials are being released in large amounts.”
Mr. Edano and other senior officials did not address the escalating crisis at reactor No. 2 later Monday or early Tuesday.
But the situation a reactor No. 3 was being closely watched for another reason. That reactor uses a special mix of nuclear fuel known as MOX fuel. MOX is considered contentious because it is made with reprocessed plutonium and uranium oxides. Any radioactive plume from that fuel would be more dangerous than ordinary nuclear fuel, experts say, because inhaling plutonium even in very small quantities is considered lethal.
In screenings, higher-than-normal levels of radiation have been detected from at least 22 people evacuated from near the plant, the nuclear safety watchdog said, but it is not clear if the doses they received were dangerous.
Technicians had been scrambling most of Sunday to fix a mechanical failure that left the reactor far more vulnerable to explosions.
The two reactors where the explosions occurred are both presumed to have already suffered partial meltdowns — a dangerous situation that, if unchecked, could lead to a full meltdown.

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Nuclear Plants in Europe Are Delayed

 BERLIN — With the crisis in Japan raising fears about nuclear power, Germany and Switzerland said on Monday that they would reassess the safety of their own reactors and possibly reduce their reliance on them.
Doris Leuthard, the Swiss energy minister, said Switzerland would suspend plans to build and replace nuclear plants. She said no new ones would be permitted until experts had reviewed safety standards and reported back. Their conclusions will apply to existing plants as well as planned sites, she added. Swiss authorities recently approved three sites for new nuclear power stations.
Germany will suspend “the recently decided extension of the running times of German nuclear power plants,” Chancellor Angela Merkel told reporters in Berlin. “This is a moratorium and this moratorium will run for three months.” She said the suspension would allow for a thorough examination of the safety standards of the county’s 17 nuclear power plants.
“There will be no taboos,” Mrs. Merkel said.
Even when the three months is over, Mrs. Merkel warned, there would be no going back to the situation before the moratorium.
Across Europe, officials worried about the Continent’s use of nuclear power as cooling systems failed at a third nuclear reactor in Japan and officials in that country struggled to regain control.
The European Union called for a meeting on Tuesday of nuclear safety authorities and operators to assess Europe’s preparedness. Austria’s environment minister, Nikolaus Berlakovich, called for a European Union-wide stress test “to see if our nuclear power stations are earthquake-proof.”
In Germany, with Mrs. Merkel’s center-right coalition facing important regional elections this month, the move was apparently in part an effort to placate the influential antinuclear lobby and give her coalition some breathing space before making a final decision about nuclear energy.
The foreign minister, Guido Westerwelle, called for a new risk analysis of the country’s nuclear plants, particularly regarding their cooling systems. He is the leader of the pro-business Free Democratic Party, which strongly supports nuclear power.
A previous government, led by the Social Democrats and Greens, pushed through legislation in 2001 to close all of the country’s nuclear plants by 2021. But Mrs. Merkel’s center-right government reversed that decision last year and voted to extend the lives of the plants by an average of 12 years.
Nuclear energy provides about 11 percent of Germany’s energy supply but its contribution to electricity output is about 26 percent.
In Switzerland, the suspension of plans to build and replace plants will affect all “blanket authorization for nuclear replacement until safety standards have been carefully reviewed and if necessary adapted,” Ms. Leuthard, the energy minister, said in a statement.
Switzerland has five nuclear reactors, which produce about 40 percent of the country’s energy needs.
Ms. Leuthard said she had already asked the Federal Nuclear Safety Inspectorate to analyze the exact cause of the problems in Japan and draw up new or tougher safety standards “particularly in terms of seismic safety and cooling.”
In Russia, Prime Minister Vladimir V. Putin said his government would not revise its ambitious program of building nuclear reactors but would “draw conclusions from what’s going on in Japan,” Russian news agencies reported. Nuclear power currently accounts for 16 percent of Russia’s electricity generation.

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E.U.'s Latest Rescue Package Seen Falling Short-Again

LONDON — Another missed opportunity for Europe?
Over the past year, the European Union and the International Monetary fund have pledged €640 billion in bailout funds to distressed economies on the Continent’s periphery. Yet the interest rates on benchmark bonds in Greece, Ireland and Portugal remain at or near their historic highs.
Pressed yet again by a skeptical market to take action, Europe’s leaders cobbled together a new structure over the weekend that will allow its rescue fund, the European Financial Stability Facility, to disburse its entire €440 billion, or $615 billion, allotment if needed, and to buy bonds at government auctions. They also eased the conditions on Greece’s rescue loans by reducing interest rates and extending repayment terms.
Analysts and investors greeted the effort Monday with cautious approval, as euro zone finance ministers regrouped to work out more details.
But as has often been the case with grand European rescue plans, there appears to be a catch: While the European rescue facility can now buy bonds at government auctions, it will not be able to purchase paper in the secondary market. That will heap more pressure on an ever-reluctant European Central Bank to remain in its job as peripheral Europe’s buyer of last resort.
What is more, the plan states that the fund can buy bonds only from countries that have taken bailout funds, an odd stipulation that means that the country most in need of support at the moment — Portugal — does not get any.
Consequently, pressure on Portuguese 10-year bonds eased only slightly Monday, with interest rates sticking close to the 7.4 percent level at which they have traded for the past few weeks. Greek bonds rallied on the interest rate deal, but Irish bonds made only the smallest move as they did not receive a break on their interest rate. Spanish yields fell to 5.2 percent, still close to their high of 5.4 percent.
“What we have now falls short of what is required, which is a way to bring interest rates down,” said Dirk Hoffman-Becking, a banking analyst at Sanford C. Bernstein in London.
Mr. Hoffman-Becking argued that these persistently high rates only recycled fear and uncertainty via higher funding costs for banks and, ultimately, lower economic growth. He added that unless the E.F.S.F. stepped into the secondary market to buy bonds in large quantities, rates would remain high. That is because private investors remain unwilling to buy given the risk of default and the risk that austerity programs will undermine economic recovery.
Again, the example of Portugal is relevant. Last Friday, the government in Lisbon promised more spending cuts and revenue-raising measures to make good on its promise to bring the budget deficit down to 4.6 percent of gross domestic product, from 7.4 percent this year. But with one of the lowest growth rates in Europe, an uncompetitive export industry and public debt at 80 percent of G.D.P., investors see little upside in buying Portuguese bonds now, even at such wide spreads to comparable German bonds.
For the most part, the E.C.B. has been unwilling to step in aggressively — it has purchased only €77 billion in peripheral-country bonds so far. On Monday, the bank reported that it had not bought any bonds last week, for the second week in a row.
Thus, the lack of a big buyer is expected to keep rates high and continue to feed market unease. And that in many ways remains the unsolved root of Europe’s sovereign debt crisis.
Euro zone governments and the I.M.F. are asking that investors take a leap of faith and accept that the bone-crunching austerity programs in Greece, Ireland and Portugal, underpinned by tax increases and sharp wage reductions, will reduce deficits and ultimately high levels of debt.
But, as a comprehensive new report on the origins of Europe’s debt crisis by the investment bank Nomura points out, what is more likely to happen is that the debt of countries like Greece and Ireland will merely increase in what economists call a snowball effect, as the interest rate on the government’s debt grows faster than their stagnating economies.
That problem will only be compounded when the European Central Bank increases interest rates, which it has warned it will do.
The result is a Catch-22 of sorts: the faster and deeper these countries cut, the slower they grow, with the perverse effect that their debt ends up increasing as opposed to decreasing.
“In the short term, this problem is inevitable — your debt will continue to increase as long as your growth rate is below the interest rate you are paying,” said Peter Westaway, European economist at Nomura in London and one of the authors of the report.
In 2013, for example, Greece’s debt will have increased to almost 160 percent of G.D.P. from 127 percent of G.D.P. And while the country was quick to hail its improved terms, the bottom line still remains that by 2014 it must begin paying interest equivalent to about 8 percent of its G.D.P. — a huge amount by any measure.
For now, Greece has time. But with growth expected to shrink again this year, by 3.4 percent, and with unemployment now at about 15 percent, how long the Greek government, or any government for that matter, can continue to expect so much public sacrifice to pay off its bankers remains to be seen.

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Radioactive Releases in Japan Could Last Months, Experts Say

WASHINGTON — As the scale of Japan’s nuclear crisis begins to come to light, experts in Japan and the United States say the country is now facing a cascade of accumulating problems that suggest that radioactive releases of steam from the crippled plants could go on for weeks or even months.
The emergency flooding of two stricken reactors with seawater and the resulting steam releases are a desperate step intended to avoid a much bigger problem: a full meltdown of the nuclear cores in two reactors at the Fukushima Daiichi Nuclear Power Station. So far, Japanese officials have said the melting of the nuclear cores in the two plants is assumed to be “partial,” and the amount of radioactivity measured outside the plants, though twice the level Japan considers safe, has been relatively modest.
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. In a country where memories of a nuclear horror of a different sort in the last days of World War II weigh heavily on the national psyche and national politics, the impact of continued venting of long-lasting radioactivity from the plants is hard to overstate.
Japanese reactor operators now have little choice but to periodically release radioactive steam until the radioactive elements in the fuel of the stricken reactors stop generating intense heat, a process that can continue for a year or more even after the fission process has stopped. To control that heat, the plant’s operator must constantly try to flood the reactors with seawater, then release the resulting radioactive steam into the atmosphere, several experts familiar with the design of the Daiichi facility said. That suggests that the 200,000 people who have been evacuated may not be able to return to their homes for a considerable period and that shifts in the wind could blow radioactive materials toward Japanese cities rather than out to sea.
Re-establishing normal cooling of the reactors would require restoring electric power — which was cut in the earthquake and tsunami — and now may require plant technicians working in areas that have become highly contaminated with radioactivity.
More steam releases also mean that the plume headed across the Pacific could continue to grow. On Sunday evening, the White House sought to tamp down concerns, saying that modeling done by the Nuclear Regulatory Commission had concluded that “Hawaii, Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity.”
But all weekend, after a series of intense interchanges between Tokyo and Washington and the arrival of the first American nuclear experts in Japan, officials said they were beginning to get a clearer picture of what went wrong over the past three days, and as one senior official put it, “under the best scenarios, this isn’t going to end anytime soon.”
The essential problem is the definition of “off” in a nuclear reactor. When the nuclear chain reaction is shut and the reactor shuts down, the fuel is still producing about 6 percent as much heat as it did when it was running, because of continuing heat generation by the radioactivity, the release of subatomic particles and of gamma rays.
Usually, when a reactor is first shut down, an electrically driven pump pulls heated water from the vessel to a heat exchanger, and cool water from a river or ocean is brought in to draw off that heat.
But at the Japanese reactors, after losing electric power, that system could not be used. Instead the operators are dumping seawater into the vessel, and letting it cool the fuel by boiling. But as it boils, pressure rises too high to pump in more water, so they have to vent the vessel to the atmosphere, and feed in more water, a procedure known as “feed and bleed.”
When the fuel was intact, the steam they were releasing had only modest amounts of radioactive material, in a nontroublesome form. With damaged fuel, that steam is getting dirtier.
Re-establishing normal cooling will require electric power and may require workers to function in areas that are now contaminated, American nuclear experts say.
Christopher D. Wilson, a reactor operator and later a manager at Exelon’s Oyster Creek plant, near Toms River, N.J., said, “normally you would just re-establish electricity supply, from the on-site diesel generator or a portable one.” Portable generators have been brought into Fukushima, he said. 
Fukushima was designed by General Electric, just as Oyster Creek was, at about the same time, and the two plants are very similar, he said. The problem, he said, was that the hook-up is done through electric switching equipment that is in a basement room flooded by the tsunami, he said. “Even though you have generators on site, you have to get the water out of the basement,” he said.
Another nuclear engineer with long experience in reactors of this type, who now works for a government agency, was emphatic. “To completely stop venting, they’re going to have to put some sort of equipment back in service,” he said. He asked not to be named because his agency had not authorized him to speak.
The central problem arises from a series of failures that began after the tsunami. It easily overcame the sea walls surrounding the Fukushima plant. It swamped the diesel generators, which were placed in a low-lying area, apparently because of misplaced confidence that the sea walls would protect them. At 3:41 p.m. Friday, roughly an hour after the quake and just around the time the region would have been struck by the giant waves, the generators shut down. According to Tokyo Electric Power Company, the plant switched to an emergency cooling system that operates on batteries, but these were soon depleted.
Inside the plant, according to industry executives and American experts who received briefings over the weekend, there was deep concern that spent nuclear fuel that was kept in a “cooling pond” inside one of the plants had been exposed and begun letting off potentially deadly gamma radiation. Then water levels inside the reactor cores began to fall. While estimates vary, several officials and industry experts said on Sunday that the top four to nine feet of the nuclear fuel in the core and control rods appear to have been exposed to the air — a condition that that can quickly lead to melting, and ultimately to a complete meltdown.
At 8 p.m., just as Americans were waking up to news of the earthquake, the government declared an emergency, contradicting its earlier reassurances that there were no major problems. But the chief cabinet secretary, Yukio Edano, stressed that there had been no radiation leak.
But one was coming: Workers inside the reactors saw that levels of coolant water were dropping. They did not know how severely. “The gauges that measure the water level don’t appear to be giving accurate readings,” one American official said.
What the workers knew by Saturday morning was that cooling systems at a nearby power plant, Fukushima Daini, were also starting to fail, for many of the same reasons. And the pressure in the No. 1 reactor at Fukushima Daiichi was rising so fast that engineers knew they would have to relieve it by letting steam escape, sending the first traces of radiation into the atmosphere.
Shortly before 4 p.m., camera crews near the Daiichi plant captured what appears to have been an explosion at the No. 1 reactor — apparently caused by a buildup of hydrogen. It was dramatic television but not especially dangerous — except to the workers injured by the force of the blast inside.
The explosion was in the outer container, leaving the main reactor vessel unharmed, according to Tokyo Electric’s reports to the International Atomic Energy Agency. (The walls of the outer building blew apart, as they are designed to do, rather than allow a buildup of pressure that could damage the reactor vessel.)
But the dramatic blast was also a warning sign of what could happen inside the reactor vessel if the core was not cooled. The International Atomic Energy Agency said that “as a countermeasure to limit damage to the reactor core,” Tokyo Electric proposed injecting seawater mixed with boron — which can absorb some of the reactive elements — and it began to do that at 10:20 p.m. Saturday.
It was a desperation move: The corrosive seawater will essentially disable the 40-year-old plant; the decision to flood the core amounted to a decision to abandon the facility. But even that operation has not been easy.
To pump in the water, the Japanese have apparently tried used fire-fighting equipment — hardly the usual procedure. But forcing the seawater inside the containment vessel has been extraordinarily difficult, because the pressure in the vessel has become so great.
One American official likened the process to “trying to pour water into an inflated balloon,” and said that on Sunday it was “not clear how much water they are getting in, or whether they are covering the cores.”
The problem was compounded because gauges inside the reactor seemed to have been damaged in the earthquake or the tsunami, making it impossible to know just how much water is in the core.
And workers attempting the pumping operation are presumed to be exposed to radiation; several workers, according to Japanese reports, have been treated for radiation poisoning. It is not clear how severe their exposure was.

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Danger Posed by Radioactivity in Japan Hard to Assess

The different radioactive materials being reported at the nuclear accidents in Japan range from relatively benign to extremely worrisome. 

The central problem in assessing the degree of danger is that the amounts of various radioactive releases into the environment are now unknown, as are the winds and other atmospheric factors that determine how radiation-emitting materials will disperse around the stricken plants.
Still, the properties of the materials and their typical interactions with the human body give some indication of the threat.

“The situation is pretty bad,” said Frank N. von Hippel, a nuclear physicist who advised the Clinton White House and now teaches international affairs at Princeton. “But it could get a lot worse.”

In Vienna on Saturday, the International Atomic Energy Agency said Japanese authorities had informed it that iodine pills would be distributed to residents around the Fukushima Daiichi and Daini plants in northeast Japan. Both have experienced multiple failures in the wake of the huge earthquake and tsunami that struck Friday.
In the types of reactors involved, water is used to cool the reactor core and produce steam to turn the turbines that make electricity. The water contains two of the least dangerous radioactive materials now in the news — radioactive nitrogen and tritium. Normal plant operations produce both of them in the cooling water, and they are even released routinely in small amounts into the environment, usually through tall chimneys.

Nitrogen is the most common gas in the earth’s atmosphere, and at a nuclear plant the main radioactive form is known as nitrogen-16. It is made when speeding neutrons from the reactor’s core hit oxygen in the surrounding cooling water. This radioactive form of nitrogen does not occur in nature.

The danger of nitrogen-16 is an issue only for plant workers and operators because its half-life is only seven seconds, after which it decays back into natural nitrogen. A half-life is the time it takes half the atoms of a radioactive substance to disintegrate.
The other radioactive material often in the cooling water of a nuclear reactor is tritium. It is a naturally occurring radioactive form of hydrogen, sometimes known as heavy hydrogen. It is found in trace amounts in groundwater throughout the world. Tritium emits a weak form of radiation that does not travel very far in the air and cannot penetrate the skin.
It accumulates in the cooling water of nuclear reactors and is often vented in small amounts to the environment. Its half-life is 12 years.

The big worries on the reported releases of radioactive material in Japan center on radioactive iodine and cesium.

“They imply some kind of core problem,” said Thomas B. Cochran, a senior scientist in the nuclear program of the Natural Resources Defense Council, a private group in Washington.
The active core of a nuclear reactor splits atoms in two to produce bursts of energy and, as a byproduct, large masses of highly radioactive particles. The many safety mechanisms of a nuclear plant focus mainly on keeping these so-called fission products out of the environment.
Iodine-131 has a half-life of eight days and is quite dangerous to human health. If absorbed through contaminated food, especially milk and milk products, it will accumulate in the thyroid and cause cancer. Located near the base of the neck, the thyroid is a large endocrine gland that produces hormones that help control growth and metabolism.

Dr. von Hippel of Princeton said the thyroid danger was gravest in children. “The thyroid is more sensitive to damage when the cells are dividing and the gland is growing,” he said.
Fortunately, an easy form of protection is potassium iodide, a simple compound typically added to table salt to prevent goiter and a form of mental retardation caused by a dietary lack of iodine.
If ingested promptly after a nuclear accident, potassium iodide, in concentrated form, can help reduce the dose of radiation to the thyroid and thus the risk of cancer. In the United States, the Nuclear Regulatory Commission recommends that people living within a 10-mile emergency planning zone around a nuclear plant have access to potassium iodide tablets.
Over the long term, the big threat to human health is cesium-137, which has a half-life of 30 years.
At that rate of disintegration, John Emsley wrote in “Nature’s Building Blocks” (Oxford, 2001), “it takes over 200 years to reduce it to 1 percent of its former level.”

It is cesium-137 that still contaminates much of the land in Ukraine around the Chernobyl reactor. In 1986, the plant suffered what is considered the worst nuclear power plant accident in history.
Cesium-137 mixes easily with water and is chemically similar to potassium. It thus mimics how potassium gets metabolized in the body and can enter through many foods, including milk. After entering, cesium gets widely distributed, its concentrations said to be higher in muscle tissues and lower in bones.
The radiation from cesium-137 can throw cellular machinery out of order, including the chromosomes, leading to an increased risk of cancer.

The Environmental Protection Agency says that everyone in the United States is exposed to very small amounts of cesium-137 in soil and water because of atmospheric fallout from the nuclear detonations of the cold war.
The agency says that very high exposures can result in serious burns and even death, but that such cases are extremely rare. Once dispersed in the environment, it says, cesium-137 “is impossible to avoid.”
 
This article has been revised to reflect the following correction:

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Cuba Gives 15-Year Prison Term to American

A Cuban court sentenced an American government contractor, Alan Gross, to 15 years in prison for crimes against the state, Cuban state television reported Saturday.
Mr. Gross, 61, was detained in December 2009 while on a U.S.A.I.D. mission in Cuba designed to weaken the government. Cuban authorities said that Mr. Gross was distributing satellite telephone equipment, which could be sued to access the Internet, to Jewish groups in Cuba. Those groups have denied having anything to do with him.

The prosecution was seeking a 20-year sentence.
A panel of judges found that the evidence “demonstrated the participation of the North American contractor in a subversive project of the U.S. government that aimed to destroy the Revolution through the use of communications systems out of the control of authorities,” the Associated Press reported Saturday.

The United State National Security Council spokesman, Tommy Vietor, criticized the ruling and called again for Mr. Gross to be released. “Today’s sentencing adds another injustice to Alan Gross’s ordeal,” he said. “He has already spent too many days in detention and should not spend one more.”

Mr. Gross’ detention has been a point of contention between the United States and the Cuban government, even as President Obama has loosened restrictions on travel for groups like scholars and artists and pledged renewed engagement with the Cuban people.
Most political observers have said they expect Mr. Gross to be released on humanitarian grounds. 

He has lost some 90 pounds while in detention, and his daughter had a double mastectomy after a cancer diagnosis last year.

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Japan Floods Nuclear Reactor Crippled by Quake in Effort to Avert Meltdown

TOKYO — Japanese officials took the extraordinary step on Saturday of flooding a crippled nuclear reactor with seawater in a last-ditch effort to avoid a nuclear meltdown, as the nation grappled simultaneously with its worst nuclear mishap and the aftermath of its largest recorded earthquake. 

A radiation leak and explosion at the Fukushima Daiichi Nuclear Power Station on Saturday prompted the government to expand an evacuation order to affect 170,000 people in the plant’s vicinity. And the plant’s operator issued an emergency notice early Sunday morning that a second reactor at the same aging plant was also experiencing critical failures of its cooling system and that rising pressure there risked a new explosion.
The government said radiation emanating from the first reactor appeared to be decreasing after the blast Saturday afternoon destroyed part of the facility, and they said that they had filled it with sea water to prevent full meltdown of the nuclear fuel. That step would only be taken in extreme circumstances because ocean water is likely to permanently disable the reactor.

The Japanese Nuclear and Industrial safety agency said as many as 160 people may have been exposed to radiation around the plant, and Japanese news media said three workers at the facility were suffering from full-on radiation sickness.
Even if Japan manages to avoid large, uncontrolled releases of radiation that would result from a meltdown, the problems at the Fukushima facility already amounted to the worst nuclear accident in Japan’s history and perhaps the biggest malfunction at a nuclear plant since the Chernobyl disaster 25 years ago, the worst ever.
The handling of the crisis and vulnerability of Japan’s extensive nuclear facilities to earthquakes and tsunamis will also add to long-simmering grass-roots resistance against nuclear power within Japan, where people have learned to doubt the industry’s reliability as well as anodyne official statements about safety.
Even before the explosion on Saturday, officials said they had detected radioactive cesium, which is created when uranium fuel is split, an indication that some of the nuclear fuel in the reactor was already damaged — a situation sometimes referred to as a partial meltdown. How much damage the fuel suffered remained uncertain, though safety officials insisted repeatedly through the day that radiation leaks outside the plant remained small.

Although officials said the leaks did not pose a major health risk, they also told the International Atomic Energy Agency that they were making preparations to distribute iodine, which helps protect the thyroid gland from radiation exposure, to people living near Daiichi and a second nuclear plant that suffered damage in the quake, called Daini, about 10 miles away.
Worries about the safety of the two plants were worsened on Saturday because government officials and executives of Tokyo Electric Power, which runs the plant, gave confusing accounts of the causes of the dramatic midday explosion and the damage it caused. Late Saturday night, officials said that the explosion occurred in a structure housing turbines near the No. 1 reactor at the plant rather than inside the reactor itself.

The blast, apparently caused by a sharp buildup of pressure or of hydrogen when the reactor’s cooling system failed after the quake, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside, they said. They said that raised the chances they could continue cooling the core, and prevent the release of large amounts of radioactive material and avoid a full core meltdown at the plant.

“We’ve confirmed that the reactor container was not damaged,” Japan’s chief cabinet secretary, Yukio Edano, said in a news conference on Saturday night. “The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside. At this point, there has been no major change to the level of radiation leakage outside, so we’d like everyone to respond calmly.” 

 Mr. Edano said that, in addition to filling the reactor with seawater, Tokyo Electric Power workers also added boric acid to the containment vessel on Saturday night to poison the nuclear chain reaction. Mr. Edano said that the operation could “prevent criticality.”

He said radioactive materials had leaked outside the plant before the explosion, but that the explosion did not worsen the leak and that, in fact, measured levels of radioactive emission had been decreasing. He did not specify the levels of radiation involved.
Naoto Sekimura, a professor at Tokyo University, told NHK, Japan’s public broadcaster, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.”

Japanese nuclear safety officials and international experts said that because of crucial design differences, the release of radiation at the Fukushima plant would most likely be much smaller than at Chernobyl even if the Fukushima plant had suffered a complete core meltdown, which they said it had not.

But the vulnerability of nuclear plants to earthquakes was also underscored by continuing problems at the cooling system of reactors at the second nearby plant, Daini, which prompted a evacuation of 30,000 from surrounding communities. Together, the authorities sought to move about 200,000 people around the two plants, a massive logistical task at a time when rescue workers also sought to help people trapped on injured in the earthquake.
After a full day of worries about leaking radiation at the Daiichi plant after its cooling systems malfunctioned, Tokyo Electric Power said an explosion occurred “near” the No. 1 reactor at Daiichi around 3:40 p.m. Japan time on Saturday. It said four of its workers were injured in the blast.



The decision to flood the reactor core with seawater, experts said, was an indication that Tokyo Electric and Japanese authorities had probably decided to scrap the plant, because the salt water would corrode its delicate metal innards. “This plant is almost 40 years old, and now it’s over for that place,” Olli Heinonen, the former chief inspector for the I.A.E.A., and now a visiting scholar at Harvard, said on Saturday.
Mr. Heinonen lived in Japan in the 1980s, monitoring its nuclear industry, and visited the stricken plant many times. Based on the reports he was seeing, he said he believed that the explosion was caused by a hydrogen formation, which could have begun inside the reactor core. “Now, every hour they gain in keeping the reactor cooling down is crucial,” he said.

But he was also concerned about the presence of spent nuclear fuel in a pool inside the same reactor building. The pool, too, needs to remain full of water, to suppress gamma radiation and prevent the old fuel from melting. If the spent fuel is also exposed — and so far there are only sketchy reports about the condition of that building — it could also pose a significant risk to the workers trying to prevent a meltdown in the core.
Both the Daiichi and Daini plants were shut down by Friday’s earthquake. But the loss of power in the area and damage to the plant’s generators from the subsequent tsunami crippled the cooling systems, which need to function after a shutdown to cool down nuclear fuel rods.
Malfunctioning cooling systems allowed pressure to build up beyond the design capacity of the reactors. Early Saturday, officials had said that small amounts of radioactive vapor were expected to be released into the atmosphere to prevent damage to the containment systems and that they were evacuating tens of thousands of people living around the plants as a precaution.

Those releases apparently did not prevent the buildup of hydrogen inside the plant, which ignited and exploded Saturday afternoon, government officials said. They said the explosion itself did not increase the amount of radioactive material being released into the atmosphere. But safety officials also urged people who were not evacuating but still lived relatively near the plants to cover 

David Lochbaum, who worked at three reactors in the United States similar to the Fukushima design, and who was later hired by the Nuclear Regulatory Commission to teach its personnel about that technology, said that from pictures he had seen of the stricken plant, the explosion appeared to have occurred in the turbine hall, and not the reactor vessel or the containment that surrounds the vessel.  

The technology used at Fukushima is called a boiling-water reactor, in which the reactor, inside a containment, sends its steam out of containment to a turbine. The turbine converts the steam’s energy into rotary motion, which turns a generator and makes electricity.
But as the water goes through the reactor, some water molecules break up into hydrogen and oxygen. A system in the turbine hall usually scrubs out those gases. Hydrogen is also used in the turbine hall to cool the electric generator. Hydrogen from both sources has sometimes escaped and exploded, he said, but in this case, there is an additional source of hydrogen: interaction of steam with the metal of the fuel rods. Operators may have vented that hydrogen into the turbine hall.

Earlier Saturday, before the explosion, a Japanese nuclear safety panel said the radiation levels were 1,000 times above normal in a reactor control room at the Daiichi plant. Some radioactive material had also seeped outside, with radiation levels near the main gate measured at eight times normal, NHK quoted nuclear safety officials as saying.
The emergency at the Daiichi plant began shortly after the earthquake struck on Friday afternoon. Emergency diesel generators, which had kicked in to run the reactor’s cooling system after the electrical power grid failed, shut down about an hour after the earthquake. There was speculation that the tsunami had flooded the generators and knocked them out of service.
For some time after the quake, the plant was operating in a battery-controlled cooling mode. Tokyo 
Electric said that by Saturday morning it had also installed a mobile generator at Daiichi to ensure that the cooling system would continue operating even after reserve battery power was depleted. Even so, the company said it needed to conduct “controlled containment venting” in order to avoid an “uncontrolled rupture and damage” to the containment unit.

Why the controlled release of pressure did not succeed in addressing the problem at the reactor was not immediately explained. Tokyo Electric and government nuclear safety officials also did not explain the precise sequence of failures at the plant.
Daiichi and other nuclear facilities are designed with extensive backup systems that are supposed to function in emergencies to ensure the plants can be shut down safely.
At Daiichi, a pump run by steam, designed to function in the absence of electricity, was adding water to the reactor vessel, and as that water boiled off, the steam was being released. Such water is usually only slightly radioactive, according to nuclear experts. As long as the fuel stays covered by water, it will remain intact, and the bulk of the radioactive material will stay inside. But if fresh water cannot be pumped into the containment vessel and the cooling water evaporates, the nuclear fuel is exposed, which can result in a meltdown.

Japan, with no substantial coal or oil, relies heavily on nuclear power, which generates just over one-third of the country’s electricity. Its plants are designed to withstand earthquakes, which are common, but experts have long expressed concerns about safety standards, particularly if a major quake hit close to a reactor.

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