Source Term Estimation of 131I and 137Cs Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere

Abstract

It is urgent to assess the radiological dose to the public resulting from the month-long discharge of radioactive materials into the atmosphere from the Fukushima Daiichi Nuclear Power Plant accident in Japan in March 2011. To do this task, computer simulations on the dispersion of radioactive materials in the environment are useful. However, the source term essential to computer simulations was not available. Thus, the Japan Atomic Energy Agency has been trying to estimate the source term of iodine and cesium discharged to the atmosphere. As the first step, the source term was preliminary estimated by coupling environmental monitoring data with atmospheric dispersion simulations. The release rates and total amounts of 131 I and 137 Cs discharged into the atmosphere were estimated for the period from 12 March to 5 April, 2011. Then, detailed analysis on the local atmospheric dispersion around the Fukushima Daiichi Nuclear Power Plant was carried out, revealing the formation process of high dose rate zones in a northwest direction from the plant. With this and further analyses for the early phase of the accident, the release rates and total amounts of 131 I and 137 Cs discharged into the atmosphere were reestimated for the period from 12 to 15 March 2011. Finally, the validity of the revised source term was examined by comparing daily and monthly surface deposition (fallout) over land in eastern Japan between measurements and outputs from the regional-scale atmospheric dispersion simulation.

Predictability of the dispersion of Fukushima-derived radionuclides and their homogenization in the atmosphere

“…Emission data (emission rate and amount) was obtained from Katata et al . 8 and their earlier work 7 .…”

“…The latest emission estimate of Katata et al . 8 (WSPEEDI-II) was compared to their earlier results 7 (WSPEEDI) to demonstrate the sensitivity of global results on the improved local scale deposition scheme used in WSPEEDI-II. Peak 131 I concentration results improved in all regions as the larger total emission of WSPEEDI-II reduced the general underestimation of concentrations ( Fig.…”

“…Modelling efforts focused on inverse methods that provided more accurate estimates of 65–400 PBq for total 131 I 1 6 . Among others, the WSPEEDI (Worldwide System for Prediction of Environmental Emergency Dose Information) model has been used with data obtained from the Japanese measurement network to estimate a more detailed emission inventory and emission rates, resulting in a total 131 I emission of 124 PBq and 137 Cs emission of 8.8 PBq 7 . Recently, the improved WSPEEDI-II model has provided an updated emission data and a detailed reconstruction of the events leading to the accidental release 8 .…”

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry-3

“…It is recognized that there were several events of radioactive Cs release in the accident at FDNPP, and there was some extent of bias in the contribution of radioactive Cs from each reactor in these events [35][36][37]. Nishihara et al reported that the radioactive Cs in each reactor would be reflected in the variation of the 134 Cs/ 137 Cs activity ratio and the 135 Cs/ 137 Cs isotopic ratio [29], suggesting that the source of the radioactive Cs in the environmental samples could possibly be evaluated from the isotopic data such as the isotopic ratio and the activity ratio of radioactive Cs.…”

Estimation of the Intensity of the Successive Instantaneous Emissions Released by an Air Pollution Point Source

“…This makes it possible to carefully assess the risk to the population and take the necessary measures in advance to combat air pollution [13,15]. As a consequence, the development of methods for assessing the intensity of emissions of pollutants is of paramount importance for emergency assessment and response to the impact of hazardous substances accidentally released into the atmosphere as a result of accidents at industrial enterprises and nuclear power plants [1,9]. It is important to note that the estimation of emission rates is often an ill-posed problem for which there is no general method for solving, and each developed method solves the estimation problem taking into account certain simplifications in the system [2].…”