Development of a transfer model for radionuclides in a brackish lake

This study was intended to develop a transfer model for radionuclides in Lake Obuchi, which is a brackish lake adjacent to the first commercial nuclear spent-fuel reprocessing plant located in Rokkasho, Aomori Prefecture. The reprocessing plant is now under the final test using actual spent nuclear fuel. A small amount of radionuclides is released into the surrounding environment during the test and will be also discharged in the commercial operation in future. The released radionuclides may enter the lake through various routes: from the ocean by tides, from the land by river water, and from the atmosphere by fallout. Radionuclides incorporated into the lake are transferred not only by physical advection and diffusion but also by biological activities. The transfer model of radionuclides was developed taking into account the effects of the low trophic level ecosystem on the transfer of radionuclides as well as the hydraulic movements in the lake. The hydraulic movement was described with a 3D-hydrodynamic model. Transfer of radionuclides in the ecosystem including a phytoplankton species and an animal plankton species was also modeled. The combined model connecting these two models well described the salinity distribution and temporal variation of the plankton numbers in the brackish lake. Concentrations of fallout 3H and 137Cs in lake water calculated with the model showed good agreement with the observed data. However, the depth distribution of 137Cs in lake sediment did not well agree with the data actually observed, which indicated that the input of 137Cs from the main river to the lake should be more precisely evaluated. (author)