Detailed source term estimation of the atmospheric release for the Fukushima Daiichi Nuclear Power Station accident by coupling simulations of an atmospheric dispersion model with an improved deposition scheme and oceanic dispersion model

Abstract

Abstract. Temporal variations in the amount of radionuclides released into the atmosphere during the Fukushima Daiichi Nuclear Power Station (FNPS1) accident and their atmospheric and marine dispersion are essential to evaluate the environmental impacts and resultant radiological doses to the public. In this paper, we estimate the detailed atmospheric releases during the accident using a reverse estimation method which calculates the release rates of radionuclides by comparing measurements of air concentration of a radionuclide or its dose rate in the environment with the ones calculated by atmospheric and oceanic transport, dispersion and deposition models. The atmospheric and oceanic models used are WSPEEDI-II (Worldwide version of System for Prediction of Environmental Emergency Dose Information) and SEA-GEARN-FDM (Finite difference oceanic dispersion model), both developed by the authors. A sophisticated deposition scheme, which deals with dry and fog-water depositions, cloud condensation nuclei (CCN) activation, and subsequent wet scavenging due to mixed-phase cloud microphysics (in-cloud scavenging) for radioactive iodine gas (I2 and CH3I) and other particles (CsI, Cs, and Te), was incorporated into WSPEEDI-II to improve the surface deposition calculations. The results revealed that the major releases of radionuclides due to the FNPS1 accident occurred in the following periods during March 2011: the afternoon of 12 March due to the wet venting and hydrogen explosion at Unit 1, midnight of 14 March when the SRV (safety relief valve) was opened three times at Unit 2, the morning and night of 15 March, and the morning of 16 March. According to the simulation results, the highest radioactive contamination areas around FNPS1 were created from 15 to 16 March by complicated interactions among rainfall, plume movements, and the temporal variation of release rates. The simulation by WSPEEDI-II using the new source term reproduced the local and regional patterns of cumulative surface deposition of total 131I and 137Cs and air dose rate obtained by airborne surveys. The new source term was also tested using three atmospheric dispersion models (Modèle Lagrangien de Dispersion de Particules d'ordre zéro: MLDP0, Hybrid Single Particle Lagrangian Integrated Trajectory Model: HYSPLIT, and Met Office's Numerical Atmospheric-dispersion Modelling Environment: NAME) for regional and global calculations, and the calculated results showed good agreement with observed air concentration and surface deposition of 137Cs in eastern Japan.

Updating source term and atmospheric dispersion simulations for the dose reconstruction in Fukushima Daiichi Nuclear Power Station Accident

“…The new scheme deals with dry and fog-water deposition, cloud condensation nuclei (CCN) activation, and subsequent wet scavenging due to mixed-phase cloud microphysics (in-cloud and below-cloud scavenging) for radioactive iodine gas (I2 and CH3I) and other particles (CsI, Cs, and Te) as shown in Fig. 2 [9]. …”

“…For this purpose, the source term estimated by Katata et al [9] was the most appropriate one in the published source terms, and was used in this study. The temporal change in release rate of 131 I from 12 to 24 March 2011 estimated by Katata et al [9] is shown in Fig. 1.…”

“…The spatiotemporal distribution of air concentration and surface deposition of radioactive materials can be reproduced by applying the source term to the dataset of each ATDM simulation. In this study, the new source term [9] was used as the most appropriate one in the published source terms. Since this source term was estimated by using the meteorological model MM5 and dispersion model (new GEARN) [9], it is considered to provide the optimum result by applying to the dataset of ATDM simulation with MM5 and new GEARN.…”

“…-The estimation method with coupled simulations of atmospheric and oceanic dispersion models using concentrations of 134 Cs and 137 Cs in marine surface over the Pacific Ocean observed during the period from April 2 to May 17, 2011 [13] was applied to correct the release rate during the period when the plume flowed toward the Pacific Ocean. -The atmospheric dispersion model was modified to include sophisticated deposition scheme [9]. …”

“…To improve our source term, we recently made detailed source term estimation by using additional monitoring data near the FDNPS and WSPEEDI including new deposition scheme [9]. The simulation of modified WSPEEDI and the new source term successfully reproduced the local and regional deposition pattern of 131 I and 137 Cs derived from airborne monitoring by the United States Department of Energy (US-DOE) and Ministry of Education, Culture, Sports, Science and Technology (MEXT) [10,11].…”