Scientists point to several factors. On Thursday, the Japan nuclear crisis took a hopeful turn as engineers installed a cable to connect the Fukushima I nuclear power plant to the utility grid.
As workers at Japan's quake- and tsunami-stricken Fukushima I nuclear power plant struggle to bring a seven-day-long crisis under control, engineers Thursday finally installed a cable to connect the plant to the utility grid. This would enable workers to reactivate massive pumps that can provide high volumes of cooling water needed to keep damaged reactors and exposed assemblies of spent fuel cool.
According to a statement from the International Atomic Energy Agency, Japanese officials alerted the organization to the development and said they would hook Unit 2, one of six reactors at the plant, to the line first as soon as workers finish spraying water on Unit 3.
The development offers hope that the jury-rigged cooling system that workers have been forced to use in the Japan nuclear crisis will be replaced by the system originally designed to cool the reactors and spent-fuel pools.
This comes as concerns have mounted along America’s west coast over a plume of radiation from the plant that could reach the skies over California as early as Friday. The prospect reportedly has triggered a rush to drug and health-food stores to buy potassium iodide tablets, which public-health officials say can reduce some, but not all, of the health risks posed by excessive amounts of radiation.
The concerns have largely arisen among the general public – not among many public-health officials and emergency management officials.
If an accident at one of California's two operating nuclear power plants led to a release of radiation, potassium iodide pills would go out to people living within a 10-mile radius of the plant, according to Kelly Huston, assistant secretary of the California Emergency Management Agency, in an interview with Bay Area affiliates of NBC News.
By contrast, the plume from Fukushima has had to travel 5,000 miles. Along the way, the plume – carried along by high-altitude winds – disperses as it broadens. Radioactive material in the plume, which survives the journey, has had time to decay.
If the plume were to remain corralled in a relatively small volume for the entire trip, it might pose problems along a similarly narrow corridor below it, some researchers say. But "the idea of transporting something thousands of miles so it would be contained would be a miracle in itself," says William Marlow, a nuclear engineering professor at Texas A&M University at College Station.
Still, reports are emerging that global supplies of potassium iodide pills are beginning to vanish as Americans buy them up, notes Edwin Lyman, a senior scientist specializing in nuclear energy at the Union of Concerned Scientists in Washington.
"Given the fact that Japan is thousands of miles from the United States, it is highly unlikely that Americans would be exposed to radioactive iodine from direct inhalation of a plume from the Fukushima nuclear complex," he said in the statement, adding that potassium iodide is used to counter inhaled radioactive iodine.
In the region surrounding the stricken plant, however, high radiation levels remain a deep concern.
On Thursday, pilots with Japan's Self Defense Forces as well as police temporarily joined workers at the plant in efforts to douse reactors and spent-fuel pools with seawater to keep them cool. Four of the six nuclear reactors at Tokyo Electric Power Co.'s Fukushima I plant have been damaged by explosions and fire following last Friday's magnitude 9 earthquake and ensuing tsunami.
Spent-fuel pools associated with the damaged reactors appear to have lost some or all of their water, allowing the highly radioactive spent-fuel assemblies in the pools to spew radiation into the environment and to heat up. If the fuel gets too hot, the cladding that encases the uranium fuel burns, releasing additional radioactive byproducts into the air.
The overwhelming geophysical disaster left the plant, whose reactors automatically shut down during the quake as they were designed to, with no electricity from the wider grid to keep cooling systems running. The tsunami neutralized the plant's emergency generators and the stored fuel they need to operate, leaving the plant without emergency power.
Japanese authorities have evacuated people living within 12 miles of the power plant and have posted an additional six-mile zone where people must stay indoors if they elect not to evacuate. A 36-mile-diameter no-fly zone has been set up with the plant at the center. The US ambassador to Japan, John Roos, has urged Americans living within 50 miles of the plant to remain indoors, hunkering down in a shelter-in-place mode.
Although a transpacific plume of radioactive material triggers images of the globe-circling plume from the explosion and fire at the Chernobyl nuclear plant in 1986 in what is now Ukraine, that event differs considerably from the Fukushima crisis.
The risk of contamination beyond the region around the Fukushima reactors – especially as far away as the US – "has got to be extremely small," says Peter Caracappa, an assistant professor and radiation safety officer at Rensselaer Polytechnic institute in Troy, N.Y. "The biggest reason for that is in thinking about how that material is being released into the atmosphere," he says.
At Chernobyl, the reactor, made of graphite, exploded and burned in a building with no shielding surrounding the reactor. The force of the explosion and extreme heat from the burning graphite sent plumes of highly radioactive smoke and gas some three miles into the sky, where upper-level winds could begin transporting the substances over long distances.
At Fukushima, damaged reactors remain encased in their steel-and-concrete containment structures. The radioactivity is being released as workers vent steam from inside the reactors. Thus the plume initially travels at the whim of lower-altitude weather patterns, which can disrupt and disperse the plume more quickly.
So far, the Japanese haven't released detailed information on the relative abundance of radioactive elements that the plant has been releasing. Still, the components that raise the most concerns include radioactive iodine, a gas; radioactive forms of nonreactive gases such as krypton or xenon; and radioactive cesium, Dr. Caracappa says.
Weather patterns provide an opportunity to loft some of the plume to altitudes where prevailing winds can ship it eastward. As low-pressure systems move across the plume, they can loft the material to altitudes more favorable for long-range travel.
Meanwhile, each day the material remains in transit, the radioactive elements have been decaying – if rainfall or even so-called dry deposition haven't removed the material first.
Thus, specialists say, the long journey and dispersion of the plume, along with radioactive decay, so significantly reduce the risk of increased radiation levels at ground level and at thousand-mile distances that while radiation from the incoming plume will be measurable, it is highly unlikely to rise to the level of an additional public-health risk.