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

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.