Investigation on distribution of radioactive substances in Fukushima, 8; Improvement of the MERCURY model and its application to the Fukushima

Sakuma, Kazuyuki   ; Machida, Masahiko  ; Yamada, Susumu  ; Kurikami, Hiroshi  

In order to estimate the amount of radioactive materials discharged into the ocean via rivers more accurately, a GIS-based parameter setting method and parameter optimization method were incorporated into the radionuclide discharge estimation model MERCURY. The elevation (at 100 m intervals), slope (at 10-degree intervals), land use, soil, and surface geology area ratios in the target river basins were calculated using GIS, and multiple regression equations were developed using the five parameters in MERCURY as objective functions and the area ratios for each basin as explanatory variables. The parameters were determined from the multiple regression equations created for the Maeda, Kuma, Ukedo, and Abukuma Rivers and the area ratio obtained for the Takase River, and calculations were conducted. Four optimization methods, the Newton, the PSO, the SCE-UA, and the Bayesian optimization, were implemented as automatic calibration functions and applied to the above five rivers. The results of the verification for the Takase River showed that the relative squared error RSE was about 0.44. Although the number of rivers used in the multiple regression equation was only four, the results were good. On the other hand, for the automatic calibration function, the RSE ranged from 0.29-1.5 for the Newton method (0.29-0.5), 0.28-0.56 for the PSO method, 0.18-0.39 for the SCE-UA method, and 0.29-0.42 for the Bayesian optimization method.