The chief U.S. nuclear regulator told a congressional subcommittee on Wednesday that a new failure at one of the six reactors at the Fukushima Dai-ichi power plant may make it impossible to keep the crippled plant from deteriorating further.
"We believe radiation levels are extremely high, which could possibly impact the ability to take corrective measures," Jaczko said.
However, Jaczko's dire assessment was based on limited information. And the Tokyo Electric Power Co., which operates the plant, denied his report.
If the new problems at the No. 4 reactor turn out to be true, it could severely hamper workers' efforts to prevent further melting in four of the plant's six nuclear reactors.
The reported problem with the No. 4 reactor wasn't the only new failure Wednesday. Japanese authorities said a crack has appeared in the steel-and-concrete containment vessel of the plant's No. 3 reactor, which now seems to be leaking radioactive steam.
Meanwhile, about 170 workers at the plant, wearing protective suits and rotating in and out of radiation hot spots, frantically tried to prevent further damage to exposed nuclear fuel rods.
The International Atomic Energy Agency, which is monitoring the situation, could not confirm the new report about the No. 4 storage pool. But the agency's director-general, Yukiya Amano, acknowledged that "the situation is ... very serious."
Amano, speaking from Vienna at the agency's daily briefing, said conditions at the plant have clearly deteriorated over the past two days.
Edwin Lyman, a physicist with the Union of Concerned Scientists, agrees that the situation at the Fukushima Dai-ichi power plant "is deteriorating, and it's unclear how long workers can stay in that environment without risking grave bodily injury. If they must be permanently evacuated, it's unclear how the extent of the damage that's now occurred can be contained."
One sign of the worsening situation was a change in Japan's law governing the radiation limit for nuclear power workers. International guidelines say a higher limit is appropriate only when workers are saving lives or protecting large populations.
Meanwhile, the U.S. Embassy on Wednesday told American citizens within 50 miles of the troubled plant to evacuate or take shelter indoors.
Radiation levels at the plant rose so high early Wednesday, Japan time, that all workers had to take shelter in a shielded bunker for several hours. That led to incorrect reports that the workers had been completely withdrawn from the site.
Plant operators are still pumping seawater through damaged reactors, but more desperate measures are still planned. Japanese Defense Minister Toshimi Kitazawa announced Wednesday that military helicopters will try once again to dump water onto the No. 3 reactor. That unit was severely damaged in an explosion on Monday, developed the new crack on Wednesday, and has sustained extensive melting of its fuel rods.
Earlier plans to use helicopters were scrapped because radiation levels above the damaged reactors were too hazardous for flight crews. It's not clear when radioactive steam clouds will clear enough to permit the helicopters to fly.
Officials are also preparing to spray water from high-pressure hoses onto the No. 4 reactor, and onto the No. 3 unit as well, once some explosive debris is cleared away. The buildings housing both reactors are badly damaged and radiation is leaking out.
A Clear Path For The Release Of Radiation
U.S. nuclear experts say the gravest threat comes not from the radioactive fuel inside the plant's reactors but from the pools containing used fuel rods. High radiation levels released from reactor No. 3 Wednesday halted workers' efforts to add water into the cooling ponds in reactor No. 4.
"Probably the biggest concern is the spent fuel pools, because there is a clear pathway for the release of radionuclides [radioactive material] into the environment," says Lyman of the Union of Concerned Scientists.
The storage pools are on an upper floor of the reactor building and are not shielded within heavy containment barriers, as nuclear reactors are. Lyman said the situation in reactor No. 4 is especially hazardous.
"The danger with spent fuel is that if the water level decreases and there's an increase in temperature, then the spent fuel in the pool can reach a condition not unlike reactor meltdown," Lyman said.
Because of explosions and fires in the buildings that house the No. 3 and 4 reactors, their spent fuel pools "are essentially open to the air," he says.
Much of the water that is supposed to cover the stored fuel rods — both to keep them from reaching a melting point and to buffer their radioactivity — has boiled away in the past few days, as circulating pumps failed, backup power was lost, and equipment was damaged by explosion and fire.
Meanwhile, radioactive gases released from the power plant have caused radiation levels to increase as far away as Tokyo — although authorities say the levels are not a threat to public health.
More troubling is an increase in radiation levels in Ibaraki prefecture, just south of Fukushima prefecture, where the power plant is based. Levels in Ibaraki rose to around 30 times the usual background rate. While this may pose no immediate danger to people, experts say, it was measured outside the 12-mile evacuation radius.
U.S. regulators recommend a 50-mile evacuation zone in the event of a similarly serious nuclear power accident.
