February 21, 2012
As the first plumes of radioactive dust began crossing the Pacific from Japan’s damaged nuclear reactors, physicists at the Pacific Northwest National Laboratory joined an urgent and meticulous tracking effort.
The team’s anxiety wasn’t aroused so much by health risks. Levels of radioactivity reaching the U.S. are vanishingly small – thousands of times lower than people routinely face from natural sources. But minute amounts of radioactive pollutants could easily ruin expensive and delicate equipment and months of work by scientists searching for exotic subatomic particles and so-called dark matter, the invisible stuff that makes up much of the universe.
In this line of work, “A banana would be a relatively hot source of radioactivity,” says Craig Aalseth, a senior research scientist at the lab in Richland, Wash. The abundant potassium in bananas includes a radioactive isotope that occurs naturally in soils.
While the contamination reaching the West Coast remains minimal, scientists are using it to refine their ability to predict the long-distance spread of airborne radioactive pollutants. They’re also gleaning clues about how much danger the accident may pose to those nearby.
“It’s a huge human tragedy in Japan,” says Harry Miley, a physicist at the lab. “We don’t want the scientific information to go to waste.”
The first known traces to reach the West Coast arrived on Wednesday, March 16. Miley and Aalseth’s group in Richland detected radioactive xenon, a gas.
Less than a day after the xenon gas reached Richland, small amounts of three other radioactive elements showed up: iodine, cesium, and tellurium. In Seattle at nearly the same time, University of Washington physics professor R.G. Hamish Robertson and colleagues found the same three elements in air filters in a building on the UW campus.
“The fact that we see only those three, not a much broader spectrum, tells us something about how the material was transported from the reactors to the atmosphere,” Robertson says. In the 1986 Chernobyl meltdown, an explosion ejected hot nuclear fuel into the environment. Within days, monitoring stations in Seattle and Richland detected 30 or more radioactive elements. In the current crisis, detection of only three elements suggests hot fuel itself is not escaping from the Japanese reactors.
“This probably points to steam carrying most of the radioactivity out of the reactors,” Robertson says. The presence of relatively short-lived iodine and tellurium isotopes suggests the primary source is fuel rods in the damaged reactors, he says, not the spent fuel in storage pools that emergency crews have struggled to keep cool.
Efforts to forecast the long-distance spread of radioactivity proved largely reliable. As expected, the first traces took about five days to cross the Pacific. Computer simulations developed by the National Oceanic and Atmospheric Administration predicted arrival times within about 12 hours at stations along the West Coast.
“This was a real test of our abilities,” Miley says. “It was an intense week or so as we tried to get all these arrival times correct.”
Further analysis of monitoring data and computer simulations will allow scientists to determine just how sensitive the international monitoring network is to covert bomb tests. The national lab developed the radiation monitoring systems used by the Comprehensive Test Ban Treaty Organization at 30 stations around the world to look for covert nuclear bomb tests.
The plume of radioactivity hasn’t ruined any physics experiments in the Northwest. Aalseth’s team was in the middle of making a batch of ultru-pure copper to be used in instruments for detecting neutrinos, the strange subatomic particles that carry no electrical charge and can pass through matter without being affected by it. The scientists stopped work and sealed the facility as a precaution.
After assessing the level of radioactivity and their ability to keep it out of their underground laboratory, they restarted production on Monday.