"What that means is that they were successful in shutting down the reactors at the time of the earthquake," Robertson said. "The lack of iodine 133 indicates that the chain reaction was shut down."
The researchers speculate that, because they see only three of the many possible products of nuclear fission, the material that arrived in Seattle came from the evaporation of contaminated steam released from the reactors. Similar tests following the Chernobyl nuclear reactor meltdown in 1986 found a much broader spectrum of elements, indicating that material from actively burning fuel was being sent into the atmosphere.
While the radioactivity is arriving in the United States at levels far lower than are considered harmful to humans, it can raise havoc with sensitive physics experiments. That includes one called Majorana, in which the UW physicists are deeply involved, that is being planned for a lab nearly 1 mile down in the proposed Deep Underground Science and Engineering Laboratory in the old Homestake Mine in Lead, S.D.
The experiment is designed to determine the precise mass of subatomic particles called neutrinos, and any radioactive dust particles that make it into the lab could wreck the experiment, Miller said. Increased atmospheric radioactivity could cause problems for experiments in other laboratories as well, he said.
"This work helps us to understand filtering efficiency, how well the filters keep the radioactive materials out of the lab," he said.
The findings are contained in a paper the scientists posted on an open-access website called arXiv.org. Besides Robertson and Miller, authors are graduate students Jonathan Diaz and Alexis Schubert and research associates Andreas Knecht and Jarek Kaspar, all with the UW experimental
|Contact: Vince Stricherz|
University of Washington