When the Fukushima nuclear disaster began to unfold after the 11 March earthquake and tsunami, it quickly became clear that anything downwind was in for a sprinkling of radioactivity. So Jonathan Diaz Leon and pals at the University of Washington in Seattle were ready.

These guys began removing air filters from the intake to the ventilation system of the Physics and Astronomy building at the University of Washington and then measuring the levels of radiation they were emitting. Initially, the filters contained nothing out of the ordinary. Then, sometime between 12pm on 17 March and 2pm on 18 March, the radiation levels began to rise.

By measuring the energy of the gamma rays from the filters, these guys have identified exactly which fission products have made their way across the Pacific. And this in turn allows them to make a number of interesting inferences about what has gone wrong at Fukushima. Today, they post the results of the first five days of monitoring on the arXiv.

What they found was small amounts of iodine-131, iodine-132, tellurium-132, iodine-133, cesium-134 and cesium 137.

First things first: the levels of all of these substances were all well below the limits set by the US Environmental Protection Agency. The levels of iodine-131, for example, were at least 100 times lower than the EPA's limit. "We note that the observed radioactivity levels are well below alarming limits at our location," say Diaz Leon and buddies.

Having got that out of the way, they draw a number of interesting conclusions from the data.

The first comes from the amount of iodine-131 and tellurium-132 which are both short-lived with half lives of 8 and 3 days respectively. That indicates that they must have come from fuel rods that were recently active rather than from spent fuel.

Second, they could find almost no iodine-133. This has a half life of just 20 hours. Since there is about twice as much iodine-133 as iodine-131 in a steadily burning reactor, Diaz Leon and co estimate that about 8 days must have passed since the fuel had stopped burning regularly. That roughly matches the time between the accident and the date this stuff reached Seattle, which was 7 days.

Finally, there are a huge number of possible breakdown products from nuclear fission in a reactor and yet the Seattle team found evidence of only three fission product elements--iodine, cesium and tellurium. "This points to a specifific process of release into the atmosphere," they say.

Cesium Iodide is highly soluble in water. So these guys speculate that what they're seeing is the result of contaminated steam being released into the atmosphere. "Chernobyl debris, conversely, showed a much broader spectrum of elements, reflecting the direct dispersal of active fuel elements," they say.

That's reassuring, as far as it goes. But things could still change. Their report covers only the first five days of monitoring after the first detection of fission products. They're continuing to study their air filters and have promised to release the data as they get it. We'll be watching.

Ref: arxiv.org/abs/1103.4853: Arrival Time And Magnitude Of Airborne Fission Products From The Fukushima, Japan, Reactor Incident As Measured In Seattle, WA, USA