A lovely little scientific study has just been published by UW Applied Physics Scientists Knecht, Miller, Robertson and Schubert on the radiation from Fukushima that is detectable here in Seattle.
For the tl;dr crowd, the data and analysis in this scientific paper support the suspicion that at least one of the reactors at Fukushima has lost primary containment, and has been leaking (at least at times) steam laden with specific radioactive isotopes only found as a byproduct of nuclear fission. Tonight, TEPCO is acknowledging a partial meltdown of Reactor 2 probably occurred. If so, this reactor could be the source of the isotopes being detected. Finally, the isotopes are barely detectable here, and at levels of no concern for human health.
Technology review has a concise and readable summary:
...sometime between 12pm on 17 March and 2pm on 18 March, the radiation levels began to rise....
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.