WASHINGTON — Energy Secretary Steven Chu said Wednesday that U.S. officials believe at least one Japanese nuclear power reactor is in "partial meltdown," and the top federal nuclear power regulator said that radiation is so high it warrants a much wider evacuation zone.
Gregory Jaczko, the chairman of the Nuclear Regulatory Commission, said the U.S. embassy in Japan has advised American citizens to move at least 50 miles from the earthquake-devastated Fukushima Daiichi Nuclear Power Station. The Japanese government has so far ordered evacuations in a 20-kilometer, or 12-mile, radius of the plant, and urged people within 20 to 30 kilometers merely to take shelter.
Their frank assessment stood in contrast to Japanese officials, who continue to downplay the threat posed by the damaged plant. However, since the disaster began to unfold over the weekend, each reassuring statement was followed by a new setback.
Testifying back-to-back at what was slated to be a mundane House budget hearing, Chu and Jaczko described the event as the worst nuclear calamity in a quarter century and perhaps ever, based on reports from a team of 39 U.S. technicians dispatched to monitor the situation.
Unlike the 1979 Three Mile Island incident in Pennsylvania and the deadly 1986 disaster at the Chernobyl nuclear plant in the former Soviet Union, the Fukushima crisis entails at least four of the six reactors at a single plant. The events raise the specter that a meltdown of one reactor could spew so much radiation as to hobble already impaired attempts to avoid disasters at the others.
Chu told a joint hearing of two House Energy and Commerce subcommittees that the Japanese incidents "actually appear to be more serious than Three Mile Island," the worst-ever U.S. accident.
Questioned later by Rep. Doris Matsui, D-Calif., he noted that the Pennsylvania reactor also had a partial meltdown, but its containment vessel didn't fail and was able to contain the radiation. Chu said he "wouldn't want to speculate on exactly what will happen" in Japan.
Jaczko was measured as he described the predicament in the Fukushima plant's No. 4 reactor, which was actually off line when last week's 9.0 earthquake hit the region and triggered a towering tsunami that leveled property and knocked out electric power. In that reactor, however, a cooling system failed for nearly as dangerous spent fuel.
"What we believe at this time is that there has been a hydrological explosion in this unit due to an uncovering of the fuel in the fuel pool," he said. "We believe that the secondary containment has been destroyed, that there is no water in the spent fuel pool.
"And we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures."
Absent water, nuclear fuel rods can rapidly heat toward their 2,200-degree melting point, beginning a meltdown, in which uranium forms a puddle and begins to melt anything beneath it. If it reaches critical mass, it could explode, sending a white cloud of highly radioactive dust into the air.
Jaczko said the three reactors that were operating when the earthquake hit were shut down following normal procedures.
"We believe that, in general for these three reactors, they have suffered some degree of core damage from insufficient cooling from loss of offsite power and inability of diesel generators to operate successfully following the tsunami," he said.
While pumping of seawater has provided some cooling, he said, the No. 2 reactor core "is not stable," though its thick reactor containment vessel appears to be intact. Water levels in the spent fuel pools of both that reactor and reactor No. 3 appear to be decreasing, heightening the risks further, he said.
U.S. crews took 1,700 pounds of monitoring equipment, including aerial measurement systems, with them, Chu told the panels.
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