Fukushima: Radioactive sulfur came to California

Researchers determine sulfur formation and neutron emission from damaged reactors

Pier of the Scripps Instance of Oceanography at La Jolla, California, where the radioactive sulfur compounds were measured. © Kimberly Prather
Read out

In the first days after the Fukushima reactor accident, the air over the coast of Japan contained 365 times more radioactive sulfur than normal. The report US researchers in the journal "Proceedings of the National Academy of Sciences". They traced the trail of radioactive sulfur atoms from California to the Japanese coast. The sulfur was created when the seawater used for cooling reacted with the neutron radiation in the reactor cores, say the scientists. How exactly this happened and how much neutron radiation was released in the process has now been quantitatively calculated for the first time.

The study started with air pollutants in La Jolla, California, by researchers from the University of California at San Diego. There, on 28 March 2011, the measuring instruments registered record levels of 1, 501 radioactive sulfur atoms per cubic meter of air. "That was the highest activity ever measured in this location, " say the scientists. Using a model of the air currents over the Pacific, Priyadarshi and her colleagues identified the source of the contaminated air masses: the coast of Japan. The strong westerly winds had once pushed the radioactive cloud across the ocean at a height of around 900 meters.

Most of the sulfur particles were already washed out, decayed or carried away during the journey. Only 0.7 percent of them reached California, the model calculations showed. From these values, the researchers determined the original concentration of radioactive sulfur in the air over Fukushima.

Bombardment of seawater with neutrons

As the radioactive sulfur in Fujkushima reactors, the researchers reconstructed so: On March 13, 2011, the Japanese nuclear engineers began to cool the damaged reactors with seawater. Of the partially molten fuel elements emitted strong neutron radiation. The neutrons collided with the chloride ions of the sea salt and converted them into radioactive sulfur (35S). "You know how much seawater they used in Fukushima, how far the neutrons penetrate the seawater, and how big the chloride ion is, " says Antra Priyadarshi of the University of California at San Diego.

Model of air mass movements in the days before March 28, 2011, marked in red the path of the radioactive sulfur atoms from Fukushima to La Jolla in California. Gerardo Dominguez, UC San Diego

400 billion neutrons per square meter

From this information one can calculate how many neutrons must have reacted with the chloride to produce the radioactive sulfur. Their result: By March 20, 2011, nearly 400 billion neutrons per square meter of spent nuclear fuel escaped into the reactor cores and sinking basins of the Fukushima Daiichi nuclear power plant. Neutron radiation is considered to be particularly harmful to biological tissues. The biological effectiveness of high-energy neutrons is rated 10 to 20 times greater than that of gamma radiation. display

Even in the reactor core, the sulfur oxidized within seconds to sulfur dioxide and sulfur aerosols. With the evaporating cooling water, these particles came into the air above the reactor and were blown across the sea. (PNAS, 2011, DOI: 10.1073 / pnas.1109449108)

(UC San Diego / PNAS, 16.08.2011 - NPO)