Estimation of 137Cs source term from Fukushima nuclear accident

Abstract

Objective To study the 137Cs source term from the Fukushima Daiichi nuclear power plant, based on inverse modeling, so as to provide reference for accident assessment and radiation protection. Methods The 137Cs source term was estimated by means of inverse modeling of nuclear accidents based on variational data assimilation combined with truncated total least squares (TTLS-VAR). The environmental monitoring data was balanced, and the dispersion model operator and monitoring data vector were corrected, in order to reduce the influence from atmospheric dispersion model error, and then improve the accuracy of inverse modeling of source term. Results The total amount of 137Cs released was estimated to be 1.74×1016-3.73×1016 Bq, with the highest peak of release rates estimated appearing on March 18, 2011 and the average release rate in exceed of 1.00×1012 Bq/s. The estimated total amount of 137Cs was close to the data published by IAEA and UNSCEAR. Also, the estimated release sequences were in good consistent with Japanese analytical results of source terms and sequence of events. The highest peak of the estimated release rate curve corresponds to the leakage incident of unit 3. Conclusions In this study, the 137Cs source term from the Fukushima Daiichi nuclear power plant is estimated by using TTLS-VAR inverse modeling, which could provide the basis for accident assessment and radiation protection. Key words: Fukushima nuclear accident; Inverse modeling; Source term for nuclear accidents; Variational data assimilation; Truncated total least squares