order to evaluate the transport of 3H and 137Cs
radionuclides in semi-closed brackish Lake Obuchi, Japan, bordered by nuclear
fuel cycling facilities, a 3D-lower-trophic eco-hydrodynamic model has been
developed and validated. In a short-term prediction, 3H and 137Cs
activity levels in water should be in an agreement with field measurements. It
became clear that the results depended on the mixing of fresh water and
seawater in the model. Moreover, a short-term simulation estimated that most 3H
and 137Cs flowed to the ocean rather than remaining in the lake.
Based on calculations over the past 50 years, a peak of 137Cs in
sediment was in 1963, when the maximum 137Cs fallout was observed.
The calculation showed a rapid decrease after that peak, however, the field
measurement data gradually declined. This suggested that the process by which 137Cs
accumulated from the watershed to the lake was actually slower in the field
than in the model calculations. The model may be successfully applied to a
variety of different environmental situations as a generic tool for evaluating
the concentration and migration of 3H and 137Cs in a