Depositional fluxes and residence time of atmospheric radioiodine (I-131) from the Fukushima accident

Abstract

Activities of radioiodine (I-131) along with Pb-210 and Po-210 in time series precipitation samples were measured to determine the depositional fluxes of I-131 in the Southern United States and its removal rate and residence time in the atmosphere during the Fukushima nuclear accident. Radioiodine released from the Fukushima accident reached the Southern United States within 11 days, giving rise to a concurrent I-131 peak and anomalous Po-210/Pb-210 ratios in the precipitation samples. The cumulative I-131 depositional flux was 4.6 +/- 0.2 Bq m(-2) during the maximum fallout. The removal rate of I-131 out of the atmosphere, derived from a definite I-131 integral model, ranged from 0.03 to 0.14 d(-1) with an average of 0.08 +/- 0.02 d(-1), which corresponds to a residence time of I-131 in the atmosphere of 12 +/- 3 days, consistent with the resident timescale constrained by the Po-210/(210)pb disequilibrium technique. These results support our hypothesis that radioiodine was removed from the atmosphere by precipitation within two weeks. It seemed that regions reachable by I-131 transport within two weeks from Fukushima Japan would receive much more fallout, whereas places outside that distance would be relatively less polluted with radionuclides. (c) 2012 Elsevier Ltd. All rights reserved.