TOKYO—Japan’s nuclear crisis showed signs of spinning out of control Tuesday, after officials reported a third explosion and warned of possible damage to a critical part of the cooling system at the troubled Fukushima Daiichi nuclear-power complex.
Tuesday’s explosion at the No. 2 reactor in the Fukushima complex, 150 miles north of Tokyo, for the first time raised the possibility that the key containment structure of the unit, which protects the reactor vessel and keeps dangerous radioactive materials from leaking out, had been damaged.
The telltale sign, Tokyo Electric Power Co. officials said, was that pressure had dropped in the “suppression pool”—the part of the reactor that converts steam to water at the bottom of the container. Meanwhile, radiation levels outside the reactor building rose sharply.
Tepco evacuated some of the workers from the unit as a precaution.
Prime Minister Naoto Kan gave a brief address to the nation saying, “We will do our utmost to prevent further spreading of radiation leaks. I sincerely urge everyone in the nation to act calmly.”
Mr. Kan, wearing the blue jumpsuit uniform for emergency workers, added that “the risk of further increases in radiation leaks to surrounding areas is rising.” However, he only changed modestly the existing government recommended restrictions for nearby residents. He said that anyone within 18 miles of the plant should stay indoors. Previously the government had said people living within 12 miles should evacuate.
Concern rippled around the world, fueled by a seemingly slow initial response by Japanese authorities. Executives from Tepco, which operates the complex, looked visibly shaken as they acknowledged in a press conference that they still weren’t sure how severe the damage was, or how much radiation could leak out. Japan’s government only established a task force to oversee the problem shortly before 6 a.m. Tuesday morning. At least one airline, Skymark Airlines, said it was considering diverting flights from the skies over Fukushima, depending on developments.
Japan’s meteorological agency said the wind Tuesday morning was blowing south, toward metropolitan centers. Measurements at another nuclear-power complex in the prefecture of Ibaraki, halfway between Fukushima and Tokyo, showed elevated radiation readings, officials said.
The explosion at the Daiichi No. 2 reactor is the latest setback in a long battle to keep reactor cores from overheating in the wake of Friday’s massive earthquake, which knocked out cooling systems and backup generators. Authorities had been pumping in water to cool the reactors. When those pumps failed, they switched to seawater in a last-ditch attempt to keep temperatures down.
The No. 2 explosion followed explosions in recent days at the No. 1 and No. 3 reactors. In the case of the previous two events, the reactor vessels themselves weren’t damaged, and officials said only limited radiation leaked out. The explosions were apparently the result of gases that are produced when nuclear fuel reaches extremely hot temperatures and begins to break down.
Even before the reported explosion, officials were becoming increasingly worried about No. 2 reactor, due to a problem in the cooling system that hasn’t occurred in the others. Now all three appear to be experiencing a degree of melting of the fuel rods in their cores—a condition that, if it worsens, could lead to dangerous radiation leaks.
In a surprise turn of events, Monday saw a sharp deterioration in the condition of the No. 2 reactor, which had previously been stable. By late Monday night, No. 2 had become the most problematic of the three reactors, with all its fuel rods fully exposed, making them highly vulnerable to significant damage. Tepco had been pumping water to raise the level in the No. 2 reactor.
The latest problem is in the suppression pool, a concrete container of water that is part of the reactor cooling system. It sits below the reactor, is an emergency source of coolant, and also is designed to be a barrier if molten fuel burns through the steel and concrete above it.
Mr. Kan said Tuesday that the government and Tepco will set up a joint task force to exchange real time information on the Fukushima Daiichi nuclear complex in northeastern Japan, which was damaged by Friday’s massive earthquake and tsunami.
“We are taking every possible step to prevent the spread of damages, but worrying conditions are continuing,” Mr. Kan said. “I want to spearhead the efforts to overcome this crisis.”
The Japanese government also stressed they still had matters under control.
At a news conference late Monday, Chief Cabinet Secretary Yukio Edano said “Even in the worst-case scenario, we won’t have a repeat of Chernobyl.” He added, “At this point, we can say we are moving in the direction of stabilizing the situation in a certain managed manner.” The reactor at Chernobyl plant in the former Soviet Union exploded in 1986, resulting in widespread radioactive contamination.
Tests performed on local residents who have been asked to evacuate found slight exposure to radiation on over 20 people. So far, roughly 70,000 to 80,000 people living within in a radius of about 12 miles of the two plants have been asked to leave the area; the 500 or so who remained were evacuated following the latest explosion.
Japan had some earlier warning that its nuclear plants might not withstand a mammoth earthquake and its after-effects.
In July 2007 the Tepco avoided disaster when an earthquake rattled its Kashiwazaki-Kariwa Nuclear Plant in western Japan. A team of International Atomic Energy Agency inspectors found that 6.6 magnitude earthquake “significantly exceeded the level of the seismic input taken into account in the design of the plant,” according to an IAEA assessment of damage to the plant.
A spokesman for the IAEA declined to comment on the details of the 2007 report.
Already, comparisons are being made to the 1979 accident at the Three Mile Island power plant in Pennsylvania and the 1986 accident at the Chernobyl power plant in the Ukraine. But both reactors were very different from the units in Japan.
Experts say the key to preventing the current crisis from turning into a full-blow disaster is to protect the reactors’ containment structure, so radiation is kept inside, even after significant nuclear fission occurs due to damage to the fuels. In the 1979 Three Mile Island incident in Pennsylvania, the containment structure remained intact, minimizing health effects on local residents.
“If you can continue to cool the reactor, and if the containment vessel is not broken, we will be able to eventually put this under control,” said Masashi Goto, a former plant engineer for Toshiba Corp., part of a team that designed the containment structure for the Fukushima Daiichi reactors. “But the current situation is still very unstable,” he added.
In a worst-case scenario, fuel pellets from the damaged fuel rods settle to the bottom of a reactor pressure vessel, fuse into a molten, radioactive mass and begin to burn their way through the essential steel encasement.
If they burn through the reactor pressure vessel—akin to a giant, suspended steel thermos container—they are released into the containment structure, like a thermos inside a steel lunch pail. If the integrity of that structure is violated, radiation can escape to the outside in large amounts.
Thus, the containment structure, which is built of steel and concrete, is the last line of defense in terms of isolating radioactive material.
No nuclear plant of this design has ever reached that worst-case scenario, but the problems at Fukushima Daiichi bring the industry closer to that pass than most engineers thought likely.
“It’s been designed to contain almost anything,” said James F. Stubbins, head of the nuclear engineering department at the University of Illinois. He said so far the containment structure appears to be working.
Murray Jennex, a nuclear power expert at San Diego State University, agreed.
“This is not a Chernobyl. The difference is Chernobyl did not have a containment structure, which these plants do,” Mr. Jennex said.
But Edwin Lyman, a physicist at the Union for Concerned Scientists, an environmental group that has been critical of nuclear power in the past, said containment vessels such as the one at Daiichi are known to be vulnerable to failure if melted nuclear fuel reaches the bottom of the reactor.
“That would essentially mean a large radiological release to the environment,” he said
Write to Yuka Hayashi at yuka.hayashi@wsj.com and Phred Dvorak at phred.dvorak@wsj.com
