At Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant (1F), ALPS-treated water—purified with the Advanced Liquid Processing System (ALPS)—is being discharged into the ocean to enable decommissioning work to proceed. This treated water contains trace amounts of tritium. Because tritium behaves similarly to water, it is necessary to evaluate its transfer to marine organisms with a proper understanding of its behavior in the environment. National authorities and the operator conduct monitoring of seawater and marine products to ensure safety. However, due to the limited number of observations, it is difficult to capture fine-scale temporal and spatial variations. 

To address this, JAEA and IES numerically estimated the amount of tritium that moves from seawater through the food chain (plankton → small fish → Japanese flounder) and into Japanese flounder, and evaluated the dose impact when consumed as food. 

Figure 1. Overview of the study  

The evaluation combined two models. First, an oceanic dispersion model calculated the distribution of tritium in seawater along the Fukushima coast based on coastal ocean conditions. Next, a tritium transfer model for the marine food web (ecosystem transfer model) considered transfer from seawater as well as transfer through the food chain to estimate time series of tritium concentration in Japanese flounder. The calculation period was January–December 2023, using the actual release conditions. 

Figure 2. Conceptual diagram of the simulation models 

Model validity was confirmed by comparing calculated and observed values of water temperature (surface and bottom), a representative indicator of ocean conditions; they agreed well. This demonstrates that the model sufficiently reproduces the ocean conditions that govern the dispersion of tritium in seawater. 

Figure 3. Comparison of calculated and observed surface water temperature (March 2023) and bottom water temperature (annual, 2023) off Fukushima 

As a result, for “average” Japanese flounder inhabiting an area approximately 5–60 km from 1F during 2023, the maximum concentration of organically bound tritium (OBT) in the fish was estimated to be 0.033 Bq/L. This is about one-tenth of the minimum detectable concentration in monitoring (0.21 Bq/L) and lower than the tritium concentrations observed in precipitation and river water around Fukushima (0.24–0.81 Bq/L).