Just how bad were things at Fukushima? So far, painting a clear picture is difficult, as all we know for sure is how much radioactivity has been spotted at specific sites of contamination. Now, researchers have put together a largely independent estimate of the neutron flux that occurred during the meltdown, pieced together from an unlikely source: a long-term monitoring experiment being run in San Diego. Nearly two weeks after the earthquake and tsunami, some equipment on the Scripps Pier picked up a surge in radioactive sulfur that has enabled a rough estimate of the radioactivity released at Fukushima.

Normally, a radioactive form of sulfur (35S) is produced in the atmosphere when cosmic rays react with argon in the upper atmosphere. In San Diego, this produces fairly steady levels of 35SO 2 and 35SO 4 -2, except during seasonal periods when winds shift more material down from the stratosphere, where cosmic ray exposures are highest. But, on March 23rd of this year, levels of radioactive sulfur suddenly spiked, reaching over seven times normal background by the end of the month. With no indications of an atmospheric disturbance, the researchers focused across the Pacific, on Fukushima.

35S can also be formed by neutron bombardment of 35Cl; that isotope is normally present in sea water, which was dumped on the ailing reactors in copious quantities. As the seawater reached the reactor core, it would receive a healthy dose of neutrons. A decent fraction of the 35S that formed would then boil off with the water, reacting with oxygen and forming sulfates. The uncharged form would fall out of the atmosphere over the next few days as a solid, but the 35SO 4 -2 could potentially stay airborne for an entire trip across the Pacific. By modeling the flow of winds prevalent at the time of the reactor meltdown, the authors were able to demonstrate that some portion of the air that was reaching San Diego probably had originated above the reactor site.

Given the half life of 35S (87 days) and the amount of radioactivity registered on the Scripps Pier, the authors could work backwards to estimate what was going on at the reactor site. Based on the 1,500 atoms of 35S in each cubic meter of air in San Diego, they calculate that there were 2 x 105 atoms that reached the atmosphere's boundary layer above Fukushima. That translates to 4 x 1011 neutrons per square meter from the Fukushima reactors being "leaked" into the environment.

A few things that are important to note: the levels of radioactive sulfur that actually reached California are pretty minimal, and, in the grand scheme of things, 35S is a relatively benign isotope. But these estimates also include some features that suggest the leak at Fukushima was larger than these numbers might suggest; for example, the authors assumed 100 percent of the sulfur that was formed through neutron bombardment ended up in the atmosphere, which is probably unrealistic.

In the paper, the authors steadfastly refuse to describe what this might mean in terms of the environment at Fukushima, focusing instead on how the event has shed light on atmospheric sulfur dynamics. One of the authors, however, told Nature News that normal levels of neutrons within a reactor are much higher, suggesting that this does not indicate a total failure of containment. Still, as it may be years before we'll get to see the conditions inside the reactor housing, this rough estimate will be one more piece of the puzzle that others can use to understand the scope of the problems.

PNAS, 2011. DOI: 10.1073/pnas.1109449108 (About DOIs).