Release Rate Estimation of Radioactive Noble Gases in the Criticality Accident at Tokai-Mura from Off-Site Monitoring Data

A method of release rate estimation of radioactive noble gases from an accidental nuclear facility was proposed. This method was applied to the criticality accident case at Tokai-mura to evaluate an uncertainty of results. The release rate is estimated by combining the gamma dose rates measured at off-site monitoring posts and those calculated using an atmospheric dispersion model. To analyze the dependence of uncertainty of estimated release rate on that of the wind field, two wind field data were used; one from the meteorological model MM5 and the other from the analysis of measured wind data. The results of comparison that the analyzed wind field was in better agreement with the measured wind data than the MM5 wind field led to the smaller uncertainty in the analyzed wind case than that in the MM5 wind case. A gradual decrease in release rate with time was clearly shown in the analyzed wind case, while it was obscured by a large scatter in the MM5 wind case. The total activities of the released radioactive noble gases evaluated by the present method were 3:0 × 102 and 2:1 × 102 TBq for the cases with the retention times of noble gases, that is, the time elapsed before the noble gases are released into the atmosphere, of 5 and 10 min, respectively. The retention time of 10 min was reasonably compared with that evaluated from the total number of the fission reactions with the assumption of the same retention time.