VI Source Estimation

Investigation of Uncertainty in the Release Rates of 131I and 137Cs from Fukushima Dai-ichi NPS Estimated from Environmental Data

Shigekazu Hirao, Hironori Hibino, Takuya Nagae, Jun Moriizumi, and Hiromi Yamazawa

Abstract It is an urgent task to estimate the release rate of airborne radionuclides and its uncertainty for the assessment of internal and external dose to the public from the Fukushima Dai-ichi Nuclear Power Station accident. We estimated the release rates of 131I and 137Cs by using a method commonly used in the previous studies for the source term estimation. This study investigated the sensitivity of the estimated release rate to the deposition parameters. It was found that the dry deposition velocity had only minor significance on the estimated release rate, predominately because of the inherently small contribution to total deposition when wet deposition occurred. The scavenging coefficient, on the other hand, showed a substantial effect on the estimated release rate. The release rate estimated with the small scavenging coefficient could be larger than that with the large scavenging coefficient when the calculated deposition was small because of weak rainfall intensity and the short travel time of the plume in the rainfall area. In contrast, the large scavenging coefficient also resulted in a large estimated release rate when the calculated deposition was small because heavy rainfall caused substantial depletion of the plume before the plume reached sampling sites. Adoption of a scavenging coefficient three times larger could result in increases of 10 and 21 fold in the estimated release rates of 131I and 137Cs, respectively, at maximum.

Keywords Release rate estimation • 131I • 137Cs • Sensitivity analysis • Atmospheric dispersion model • Environmental monitoring data

Introduction

During the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), the released airborne radionuclides spread out over the Kanto and Tohoku regions and consequently caused land surface contamination. Assessment of radiological dose to the public requires information on the spatial and temporal evolution of the atmospheric transport of the radionuclides, land surface contamination, and radioactive composition. The most reliable information is environmental monitoring data. Although in the early phase of the accident atmospheric concentrations were measured, the spatial coverage of the measurements was too coarse to delineate the evolution of the radioactive plume and its radioactive composition.

Airborne measurements were made by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in collaboration with the U.S. Department of Energy [1]. This work provided the distribution of land surface contamination for the mediumand long-lived radionuclides of 134Cs and 137Cs. The atmospheric dispersion and deposition of short-lived radionuclides, such as 131I, which are important to evaluate radiological doses, still have a large uncertainty. In this situation, atmospheric dispersion simulations can play an important role of evaluating the atmospheric transport of radioactive plumes when source term information, such as release rates of radionuclides, is available.

To estimate the release rates, environmental monitoring data have been used with atmospheric dispersion models in several studies [2–5]. All studies with inverse methods indicate similar temporal variations in the atmospheric release rate of 131I and 137Cs despite of the use of different sets of monitoring data and atmospheric dispersion models. According to these studies, the release began in the morning of 12 March 2011, the largest release occurred on 15 March, and during other periods the releases were one to two orders of magnitude smaller than that on 15 March. Although this similarity implies the reliability of the estimated release rates, the uncertainties in the estimated release rate have not been discussed sufficiently. In our previous study, the uncertainty was preliminary estimated to be approximately a factor of 3 because of errors in the modeling of deposition processes [5]. In the absence of extensive environmental monitoring data, evaluation of the uncertainty of the source term is a difficult task. In these cases, the uncertainty of the source term can typically be evaluated by a sensitivity analysis of parameters that might affect the source term [6]. The purpose of this study is to estimate the release rates of 131I and 137Cs and their uncertainties. We investigated the sensitivity of the estimated release rates to the deposition parameters, that is, the dry deposition velocity and the washout coefficient. The analysis was conducted first with a simplified meteorological condition, and second with an underrealistic meteorological condition during the period of March 2011 that was calculated by a meteorological model.

 
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