Abstract

A recent earthquake and the subsequent tsunami have extensively damaged the Fukushima nuclear power plant, releasing harmful radiation into the environment. Despite the obvious implication for human health and the surrounding ecology, there are no quantitative estimates of the neutron flux leakage during the weeks following the earthquake. Here, using measurements of radioactive ³⁵S contained in sulfate aerosols and SO₂ gas at a coastal site in La Jolla, California, we show that nearly 4 × 10¹¹ neutrons per m² leaked at the Fukushima nuclear power plant before March 20, 2011. A significantly higher activity as measured on March 28 is in accord with neutrons escaping the reactor core and being absorbed by the coolant seawater ³⁵Cl to produce ³⁵S by a (n, p) reaction. Once produced, ³⁵S oxidizes to and and was then transported to Southern California due to the presence of strong prevailing westerly winds at this time. Based on a moving box model, we show that the observed activity enhancement in is compatible with long-range transport of the radiation plume from Fukushima. Our model predicts that , the concentration in the marine boundary layer at Fukushima, was approximately 2 × 10⁵ atoms per m³, which is approximately 365 times above expected natural concentrations. These measurements and model calculations imply that approximately 0.7% of the total radioactive sulfate present at the marine boundary layer at Fukushima reached Southern California as a result of the trans-Pacific transport.