The radiation is quite small but bears the signature of damaged nuclear fuel

By Janet Raloff

Web edition: March 19, 2011

Late Friday, March 18, the U.S. Environmental Protection Agency and Department of Energy jointly announced that a network of federal radiation-monitoring stations had begun picking up traces of radioactivity attributable to the earthquake-damaged Fukushima reactor complex in Japan.

The finding was hardly a surprise. Radioactive contaminants can ride the winds. They easily cross continents and oceans, as has been witnessed since the 1950s following nuclear tests — and, of course, the Chernobyl nuclear accident.

On Friday, a federal radiation monitoring station in Sacramento, Calif., “detected miniscule quantities of the radioactive isotope xenon-133,” the EPA said. “The origin was determined to be consistent with a release from the Fukushima reactors in Northern Japan.”

Sensors detected approximately 0.1 disintegration (a measure of radioactive decay during which an unstable element releases energy into the environment) per second per cubic meter of air. That radiation is “approximately one-millionth of the dose rate that a person normally receives from rocks, bricks, the sun and other natural background sources,” the EPA said. The Sacramento reading, it added, was consistent with ones observed on each of the previous two days in Washington State.

That was the first formal U.S. recognition that fallout from Japan’s crippled reactors had crossed the Pacific.

At Berkeley: A plume or no plume?
One day earlier, a report of the Fukushima plume’s arrival appeared in several California news organizations. Their story, by a San Jose Mercury News reporter, quoted a University of California, Berkeley professor as saying: “We see evidence of fission particles – iodine, cesium, barium and krypton, a whole dog’s breakfast of radiation.”

That “implies that I’m the first one to observe radiation from Japan in America,” says Ed Morse, the nuclear engineer to whom that quote was attributed. But he was misquoted, he claims. He said he told the reporter that those isotopes are what he would expect to see if a radioactive plume from Fukushima had reached California. In fact, he emphasizes, to date the small rooftop air-radioactivity sampling system that his students have set up on campus has detected nothing.

That’s not surprising, he adds, since it filters less than a cubic meter of air per day. His group would need to concentrate particles from a much bigger volume, he says, if they’re to have any hope of detecting evidence of very diffuse pollution.

Across town, radiochemist Al Smith at the Energy Department’s Lawrence Berkeley National Laboratory has been searching for radioactive emissions from airborne particles using a substantially more robust air sampling system. “And,” he told me, “I have found a signature of isotopes that seems to me can only have come from Japan.”

One diagnostic isotope is iodine-131. Another is tellurium-132. “If I’d only seen iodine, I couldn’t be sure whether it came from Japan or a local hospital,” he says. After all, many nuclear medicine departments use radioactive iodine for imaging the thyroid and to treat thyroid diseases, including cancer.

“But I see the tellurium also,” Smith notes. And in his most recent sample, he’s seen “little hints of cesium-137 and -134,” he adds. “So that’s pretty conclusive evidence that it came from the Japanese reactors.”

Because these radioactive species are produced as uranium in the reactor fuel fissions (or splits), and because many of the isotopes have fairly short half-lives (periods over which half of their quantity decays into another isotope), their collective presence offers a fingerprint of pollution vented from a recently operating reactor, Smith says. He points out that these fission products would not come from used-fuel storage depots – one of which recently caught fire at the Fukushima complex and released a plume of radioactive debris – because the fuel in them underwent its last fission too long ago for these short-lived isotopes to still be present.

Plumes are not highly radioactive
Most of the fission products that Smith has been measuring for the past half-century don’t move through air as naked elements, but instead hitchhike on dust or other airborne pollutant particles. Research has shown that if tiny enough, dust and other pollutant particles can fly around the world, falling back to Earth largely as a result of rainfall.

Some of the radioactive materials spewed during the Chernobyl accident entered the jet stream, Smith notes, where 150 to 300 mile-per-hour winds rapidly shuttled the pollution around the world, much of the time at high altitudes. “The stuff from Japan, I don’t know if it has enough thermal energy to get it up that high,” Smith says. If it stays closer to ground level, he notes, it will travel considerably slower and be more vulnerable to raining out.

The radiation he’s been measuring “is not as high intensity as I saw after Chernobyl," Smith says. And even the Chernobyl fallout, he points out, didn't induce any known health impacts.

Earth is a radioactive environment. Rocks, soil and certain minerals release radiation, to which everyone is exposed daily. Cosmic rays from space also bombard people, especially those living at high altitude or flying on planes.

The amount of radiation from Japanese pollution that Smith measured on Thursday, March 17, would not boost the background exposures of people in San Francisco’s bay area with even as much as they’d get from spending a week camping at Yosemite National Park. There, Smith says, granite and other rocks emit radioactive thorium, uranium and potassium. The Japanese plume would also deliver only “a tiny fraction” of the extra radiation that would come from flying on a plane, he adds.

All of this could, of course, change at any time – depending on winds and the condition of the Japanese reactors.