Evidence of neutron leakage at the Fukushima nuclear plant from measurements of radioactive 35S in California

摘要

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 4x10" neutrons per m2 leaked at the Fukushima nuclear power plant before March 20, 2011. A significantly higher 35SO42 activity as measured on March 28 is in accord with neutrons escaping the reactor core and being absorbed by the coolant seawater 3SCI to produce 35S by a (n, p) reaction. Once produced, 35S oxidizes to 3SSO2 and 35SO42~ 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 B5SO42~ is compatible with long-range transport of the radiation plume from Fukushima. Our model predicts that 35SO42~, the concentration in the marine boundary layer at Fukushima, was approximately 2x105 atoms perm3, 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.