By Devin Powell, Science News
Radioactive particles wafting eastward over the Pacific Ocean from Japan have been spotted in Seattle and used by a forensic team of physicists as a window into recent events inside the crippled Fukushima Daiichi nuclear plant 5,000 miles away. Working backward from these nuclear byproducts, the physicists have confirmed that contaminated steam is the source of this radiation, not spent fuel rods or material ejected directly from the reactor core.
“We haven’t seen any of the heavier stuff that would come right from the core, which people saw 30 years ago during the Chernobyl accident,” says Andreas Knecht, a nuclear and particle physicist at the University of Washington in Seattle who published the new data online March 24 at arXiv.org.
Starting March 16, Knecht and his colleagues saved and analyzed the air filters that clean 100 million liters of air every day in the ventilation system of the University of Washington’s physics and astronomy building. Using a detector originally designed to spot neutrinos coming from outer space, the researchers searched for gamma rays originating in the by-products of nuclear fission. On March 18, the first nuclear isotopes arrived from Japan.
The mixture of elements found in the Seattle filters drives home the differences between Chernobyl and Fukushima. The total meltdown of the Chernobyl reactor in 1986, which exposed the core, belched tons of radioactive material from fuel rods directly into the atmosphere. At the time, scientists in Paris detected 20 different isotopes. The partial meltdown of the Fukushima plant, in contrast, released only five isotopes measurable by the Seattle team’s equipment: iodine-131, iodine-132, tellurium-132, cesium-134 and cesium-137.
The complete absence of iodine-133, an ephemeral isotope that breaks down in days, confirmed that the radiation spotted by Knecht had been traveling for at least a week or so.
The presence of tellurium-132, a by-product of nuclear fission that degrades over weeks, suggests that the wind-blown radiation came from a material that had recently seen fission inside a nuclear reactor. This rules out older, spent fuel rods kept on the premises of the power plant and points to the fuel rods that were generating power until the earthquake struck.
The lack of heavier elements ruled out the possibility that the material in these fuel rods was tossed directly into the atmosphere after the earthquake. Instead, radioactive cesium and iodine—which dissolve easily in water as the compound cesium iodide—likely contaminated steam released by Japanese utility company TEPCO to control pressure inside the damaged reactors.
“This is what we expected to see,” says Knecht. “But obviously it doesn’t hurt to check.”
These particles tell the same story repeated by groups of scientists at the Environmental Protection Agency, the University of California, Berkeley, and other institutions monitoring the West Coast: Only minute amounts of radiation are reaching the United States. Levels of radioactive iodine, a cause for concern in Japan itself, were in Seattle a mere hundredth of the safety level set by the EPA.
“We’d like to confirm that what’s coming over here is at a level which is tolerable,” says Ed Morse, a nuclear engineer at the University of California, Berkeley. “So far that’s consistent with what we’re seeing.”
The scientists have analyzed only the first five days of data so far but will continue to monitor the air above Seattle. They’ve seen some evidence of day-to-day fluctuation in radiation levels and hope to explain these changes by looking to changes in local weather patterns or events in Japan.