A leaked U.N. model showing how theoretical radioactive particles would cross the Pacific Ocean from the Fukushima nuclear plant's imperiled reactors doesn't change the overall conclusion that the crisis is largely of concern to plant workers and Japanese citizens.
The precautionary forecast, prepared by the Comprehensive Test Ban Treaty Organization (CTBTO), does not contain actual data about radioactive particles released at Fukushima. Rather, for the sake of simplified prediction, it projects a continuous stream of particles released from the plant, which would then migrate into a diffuse cloud in the atmosphere.
The chart is simply a guide for U.S. EPA and other operators of radiation detection equipment. The forecast, for example, allows EPA to predict that it may detect small levels of radioactive particles in Alaska's Aleutian Islands and, eventually, California, while the prevailing winds would cause the hypothetical plume to avoid Hawaii.
Yesterday, while warning U.S. citizens to remain 50 miles from the Fukushima reactors, the Nuclear Regulatory Commission confirmed that it saw no U.S. health threat from the ongoing accident.
"All the available information continues to indicate Hawaii, Alaska, the U.S. territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity," NRC said.
Similar, though not exactly analogous, models were passed around widely during the Gulf oil spill last year, when fears were high that oil would slip into the Loop Current and spread to the Atlantic seaboard. One model, by the National Center for Atmospheric Research, showed hypothetical oil particles stretching almost all the way to Europe -- a fate that did not come to pass.
The primary public health concern stemming from the Fukushima reactors are the release of two radioactive particles, variants of iodine and cesium. Given the vast distances and times required to migrate from Japan to the United States, experts continue to advise that the particles will become so dilute and washed out of the atmosphere that they will not pose any health concerns.
"These short-lived isotopes are dangerous, but only for a few days," said Brian Toon, head of the atmospheric science department at the University of Colorado, Boulder. "If you lived in Japan, you could easily have some of this blow on you in a day or so. ... [But] you really have to be near the reactor to get a significant dose."
Currently, the hydrogen explosions at Fukushima have been small in scale, and the scattered fires even smaller. The limited force of these events directly influences how radioactive particles spread, Toon said. The particles are likely to remain low in the atmosphere, where they will be frequently exposed to rainfall and more likely to wash out of the air, he said.
"One would be more worried if there were a large explosion or the meteorology were producing big thunderstorms, or a lot of mixing carrying [air] higher into atmosphere," he added.
The only nuclear reactor accident to release significant amounts of radioactive particles to the environment, Chernobyl, had exactly the dynamic needed to project the particles high into the atmosphere. A furious fire, fed by flammable graphite, raged in the reactor's core for several days, projecting waste high into the sky.
During Chernobyl, fears gripped the world, and especially Northern Europe, about a plume of particles that spread from the reactor's uncontained fire. (Despite the week's drama, there has been no strong evidence that the Fukushima plant has released particles close to Chernobyl's levels.) Scandinavia was especially exposed, and some radioactive particles were deposited on the ground there. Similar, if limited, deposits could occur in Japan's neighbors, Toon said.
Chernobyl's plume did spread to the United States, but in such minimal levels that it triggered no public health precautions, according to EPA. The dilute particles were well within the normal amount of radiation that U.S. citizens are exposed to on a yearly basis, called background radiation.
Despite the worldwide panic caused by Chernobyl's plume and misinformation -- exacerbated by a secretive Soviet Union -- the ultimate direct health disaster of Chernobyl, beyond the nuclear workers killed taming the plant's fire, was a rise in thyroid cancer in local children.
These children were fed milk contaminated by radioactive iodine, which migrated into cows after landing on their forage. Overall, more than 4,000 children contracted cancer but some 99 percent of these children were successfully treated, according to NRC.
It is highly unlikely that such a health risk could occur in the United States, even if radioactive particles did spread in high enough concentrations into the country. Regulators are well aware of the importance of limiting thyroid exposure to radioactive iodine and base much of their protective guidelines on this threat. For example, NRC's extended evacuation radius was based on hypothetical dose projections that focused on whole body and thyroid exposure.
In addition to the CTBTO projections, several groups inside the United States are responsible for tracking any release of radioactive particles, including the long-time nuclear experts at the Department of Energy's Lawrence Livermore National Laborartory.
"If there were a significant release, they would probably know where it's going," Toon said.
Detected radiation levels continue to spike and subside at the Fukushima plant, at points reaching concentrations that could cause a small increased cancer risk to the nuclear workers trying to cool the facility.
One Fukushima worker has suffered from "significant" radiation exposure, according to the International Atomic Energy Agency (see related story). Meanwhile, levels detected farther from the plant have not reached consistently threatening heights, but have risen in recent days.
CTBTO has previously released similar models for North Korea's secret testing of a small atomic bomb earlier this decade. That bomb, however, was detonated underground, allowing only short-lived radioactive noble gases to seep out of the earth and spread on atmospheric current. Such gases are not typically considered a public health threat.