On 28 March, scientists got a whiff of something strange in the air off a pier in San Diego, California. The atmosphere had suddenly become flush with radioactive sulfur atoms. That sulfur, it turns out, had traveled across the Pacific from a nuclear power plant in Fukushima, Japan, that was shaken by the 11 March earthquake and the tsunami  and aftershocks that followed. Now the same team has studied those radioactive winds to come up with the first estimate of damage to the plant's cores at the height of the disaster.
To cool fuel rods and spent fuel while stanching a total meltdown, responders pumped several hundred tons of seawater into three reactors at the Fukushima Dai-ichi nuclear power plant. The white-hot rods fizzled off steam, which had to go somewhere. So workers vented it into the air.
Meanwhile, across the Pacific, atmospheric scientist Antra Priyadarshi of the University of California, San Diego (UCSD), remembered a study she had read a while back: Following underwater nuclear bomb tests in the 1950s and '60s, physicists noticed that a heavy form of sulfur—sulfur-35—had mushroomed. Nuclear reactions spit out lots of fast and therefore "hot" particles called neutrons, which can then bang into abundant chloride ions in saltwater, converting them to sulfur-35. Priyadarshi and her colleagues were already tracking tiny traces of radioactive sulfur to study how layers of air mix in the atmosphere, so all they had to do was wait.
They didn't have to wait long. The sulfur was already swirling over Fukushima, where it had combined with oxygen to form sulfur dioxide gases and fine particles of sulfates called aerosols. Soon, strong winds pushed them east. Sulfur-35 does occur naturally—cosmic rays zap argon atoms in the upper atmosphere, or stratosphere, to make radioactive sulfur. But little of it makes its way down to the lowest slice of atmosphere, called the marine boundary layer. On a normal day, Priyadarshi sees between 180 and 475 sulfur-35 atoms as sulfates per cubic meter of air, but on the 28th, her team recorded about 1500. "No one has ever seen such a high percentage of the stratospheric air coming into the marine-bound layer," she says.
The UCSD team ran a computer simulation to trace the path of the gases and aerosols from Fukushima to the West Coast. Most sulfur-35 atoms likely dispersed or rained down into the sea before hitting San Diego, but Priyadarshi estimates that about 0.7% completed the trip, too few to become harmful. Based on the simulation, about 365 times the normal levels of radioactive sulfates had gathered over Fukushima during the disaster , Priyadarshi and colleagues report online today in the Proceedings of the National Academy of Sciences (PNAS).
And because the researchers knew how many neutrons it would take to make that much sulfur, they could estimate how many were expelled during the disaster: For each square meter of reactor space doused by saltwater, the nuclear material ejected 400 billion neutrons before 20 March. And that, in turn, may give scientists a good look at the damage done to the cores during the disaster, says study co-author Mark Thiemens, an atmospheric scientist who is also at UCSD. If unchecked, these particles can heat up fuel rods and stores of spent fuel to the point of causing disastrous meltdowns like the one that rocked Chernobyl in 1986.
But Andreas Stohl, a scientist at the Norwegian Institute for Air Research in Kjeller, isn't convinced. Trying to figure out what happened to Fukushima's sulfur-35 as it was buffeted by haphazard winds on its nearly 10,000 kilometer journey to San Diego requires a lot of guesswork, he says: "The uncertainties must be huge."
Karl Turekian, an atmospheric geochemist at Yale University who edited Priyadarshi's paper for PNAS, agrees. But he adds the San Diego researchers did their best to account for that atmospheric chaos. And scientists haven't yet come up with any other way to estimate neutron "leaks" from nuclear fuel. "Somebody didn't have a neutron thermometer in Fukushima," he says.
Now that Fukushima's reactors have cooled back down, the biggest challenge facing scientists will be to contain radioactive elements that escaped during the disaster. Thiemens will be working with Japanese researchers to follow sulfur-35's path through soil and streams near Fukushima to find where even more harmful elements may have hidden.
Correction: A previous version of this article stated that winds pushed sulfur over Fukushima west. It has been corrected to say east.