Detection of radioactive 35S at Fukushima and other Japanese sites

The Fukushima nuclear power plant was severely damaged by an earthquake and concomitant tsunami during March 2011. An effect of this disaster was secondary formation of radioactive 35S via the 35Cl(n,p)35S reaction, when neutrons from the partially melted reactor cores activated the coolant sea water. Here we report the first measurements of 35S in sulfate aerosols and rain water collected at six Japanese sampling sites, Hokkaido, Tsukuba, Kashiwa, Fuchu, Yokohama, and Fukushima, during March‐September 2011. The measured 35SO42‐ concentrations in aerosols vary significantly. The Kashiwa (AORI) site shows the highest 35SO42‐ concentration (6.1 × 104 ± 200 atoms/m3) on 1 April 2011, which is nearly 100 times higher than the natural background activity. Considering the percentage loss of 35SO42‐ resulting from dry and wet deposition and dilution of the radiation plume in the boundary layer during transport, it was determined that the surface air concentration of 35SO42‐ at the Fukushima would have been 2.8 × 105 atoms/m3 during the week after the earthquake, which is in agreement with the model prediction [Priyadarshi et al., ]. 35SO42‐ activity in rain water collected during March‐May 2011 at Tokyo Tech Yokohama varies from 1.1 × 105 to 9.8 × 105 atoms/liter, whereas stream water collected near Fukushima was found to have 1.2 × 105 atoms/liter during April. Even after 6 months, 35SO42‐ activity remains very high (9.9 × 104 ± 770 atoms/m3) in the marine boundary layer in the Fukushima region, which implies that the reactor core was producing radioactive sulfur. Key Points First measurement of 35SO4 at Fukushima 35S at Fukushima and other site is significantly higher than the background 35S used to calculate the aerosol rain scavenging coefficient