Development of a radionuclide transport model in ecosystem of brackish lake Obuchi

This study was intended to develop a computer code for a radionuclide transport model in Lake Obuchi, which is a brackish lake that is adjacent to nuclear fuel cycle facilities, including the first commercial spent-nuclear-fuel reprocessing plant in Rokkasho, Aomori Prefecture. Radionuclides incorporated into the lake are transferred not only by physical advection and diffusion, but also by bio-chemical activities. The model was planned to include the effects of the low trophic level ecosystem on the transfer of radionuclides as well as the hydraulic movements in the lake. We have already developed a code of a transport model of radionuclides (3H, 14C and 137Cs), combined with the hydraulic and ecosystem model. In fiscal year 2005, we systematized input and output interface for the code constructed so far. A graphical user interface to input various parameters, such as quantities of radionuclides introduced to the lake and their locations, was added to the system. Output of the calculations was depicted graphically on a PC. The test runs of the system confirmed that it was operational and easy to use. Wetlands are a possible ecosystem that affects the behavior of radionuclides in the lake. However, our current model did not include wetlands. For assessing the role of the wetlands in the radionuclides' behavior, total quantities of natural plants in the wetlands were estimated using area sampling. The dominant plant in the wetlands was Phragmites australis. The total weight of the plant community in the wetlands was measured as 270, 650 and 360 t in May, August and October 2005, respectively. The annual average carbon quantity in the plants was estimated as 3800 kg C, which is a similar level to that of plankton (3300 kg C) in the lake. Results indicated that the wetlands around Lake Obuchi are important for describing the behavior of radionuclides, including that of 14C. (author)