TOKYO—Japanese officials struggled on Sunday to contain a widening nuclear crisis in the aftermath of a devastating earthquake and tsunami, saying they presumed that partial meltdowns had occurred at two crippled reactors and that they were facing serious cooling problems at four more.
The emergency appeared to be the worst involving a nuclear plant since the Chernobyl disaster 25 years ago. The developments at two separate nuclear plants prompted the evacuation of more than 200,000 people. Japanese officials said they had also ordered up the largest mobilization of their Self-Defense Forces since World War II to assist in the relief effort.
On Saturday, Japanese officials took the extraordinary step of flooding the crippled No. 1 reactor at the Fukushima Daiichi Nuclear Power Station, 270 kilometers north of Tokyo, with seawater in a last-ditch effort to avoid a nuclear meltdown.
Then on Sunday, cooling failed at a second reactor—No. 3—and core melting was presumed at both, said the top government spokesperson, Chief Cabinet Secretary Yukio Edano.
On Sunday, the Associated Press said another reactor at Daiichi was added to a list of reactors under states of emergency for a total of six.
Cooling had failed at three reactors at a nuclear complex nearby, Fukushima Daini, although conditions there were considered less dire for now.
With high pressure inside the reactors at Daiichi hampering efforts to pump in cooling water, plant operators had to release radioactive vapor into the atmosphere. Radiation levels outside the plant, which had retreated overnight, shot up to 1,204 microsieverts per hour, or over twice Japan’s legal limit, Edano said.
NHK, Japan’s public broadcaster, flashed instructions to evacuees: Close doors and windows; place a wet towel over the nose and mouth; cover up as much as possible. At a news conference, Edano called for calm. “If measures can be taken, we will be able to ensure the safety of the reactor,” he said.
One result of the venting may have been setting off an explosion, caused by either steam or hydrogen, that tore the outer wall and roof off the building housing reactor No. 1, although the steel containment of the reactor remained in place, officials said.
Fuel damage
Even before Edano’s statement on Sunday, it was clear from radioactive materials turning up in trace amounts outside the reactors that fuel damage had occurred. The existence or extent of melting might not be clear until workers can open up the reactors and examine the fuel, which could be months.
A meltdown occurs when there is insufficient cooling of the reactor core, and it is the most dangerous kind of a nuclear power accident because of the risk of radiation releases.
The radiation levels reported so far by the Japanese authorities are far above normal but still too small to pose a hazard to human health if the exposure continued for a brief period. The fear was that more core damage would bring bigger releases.
The Japanese Nuclear and Industrial Safety Agency said that as many as 160 people may have been exposed to radiation around the plant, and Japanese news media said that three workers at the facility were suffering from full-on radiation sickness.
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.
Radiation exposure
How much damage the fuel suffered remained uncertain, though safety officials insisted repeatedly through the day that radiation leaks outside the plant remained small and did not pose a major health risk.
However, 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 Daini.
Worries about the safety of the two plants worsened on Saturday because executives of the company that runs them, Tokyo Electric Power (Tepco), and government officials gave confusing accounts of the location and causes of the dramatic midday explosion and the damage it caused.
Late Saturday night, officials said that the explosion at Daiichi occurred in a structure housing turbines near its No. 1 reactor at the plant, rather than inside the reactor itself. But photographs of the damage did not make clear that this was the case.
Officials said that the blast, which may have been caused by a sharp buildup of hydrogen when the reactor’s cooling system failed, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside. This pattern of damage cast doubt on the idea that the explosion was in the turbine building.
Less serious than Chernobyl
“We’ve confirmed that the reactor container was not damaged,” 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.”
Japanese nuclear safety officials and international experts said that because of crucial design differences, the release of radiation at Daiichi would most likely be much smaller than at Chernobyl even if the plant had a complete core meltdown, which they said it had not.
Before news of the problem with reactor No. 3, the United Nations nuclear safety agency said the plant accident was less serious than both the Three Mile Island accident in 1979 and Chernobyl in 1986.
An official at the agency said it rated the incident a “4,” according to the International Nuclear and Radiological Event Scale (Ines). Three Mile Island was rated “5” while Chernobyl was rated “7” on the 1 to 7 scale.”
After a full day of worries about the radiation leaking at Daiichi, Tepco 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.
Cause of explosion
The decision to flood the reactor core with corrosive seawater, experts said, was an indication that Tepco and Japanese authorities had probably decided to scrap the plant.
“This plant is almost 40 years old, and now it’s over for that place,” said Olli Heinonen, the former IAEA chief inspector and now a visiting scholar at Harvard.
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.
Both Daiichi and Daini were shut down by Friday’s earthquake, but the loss of power in the area and damage to the plants’ generators from the ensuing tsunami crippled the cooling systems. Those are crucial after a shutdown to cool down the nuclear fuel rods.
The malfunctions 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 people in the area as a precaution.
Hydrogen buildup
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. However, safety officials urged people who were not evacuating but still lived relatively nearby to cover their mouths and stay indoors.
David Lochbaum, who worked at three US reactors with designs similar to Daiichi, said that judging by photographs of the stricken plant, the explosion appeared to have occurred in the turbine hall, not the reactor vessel or the containment that surrounds the vessel.
The Daiichi reactor is a boiling-water reactor. Inside the containment, the reactor sends its steam out 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, Lochbaum 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. New York Times News Service with reports from Reuters and Associated Press