How Much Radioactive Cesium Was Discharged from Rivers to the Ocean?
—Methods for Estimation of the Total Outflow of Radioactive Cesium Have Been Finally Developed by a Combination of Observation Results and Models—

Radioactive cesium discharge through rivers into the ocean was considered to be the third release route, which comes after the direct release route from TEPCO HD Fukushima Daiichi Nuclear Power Station (1F) into the ocean and the atmospheric radioactive fallout route. However, since it was a time-consuming effort to observe necessary parameters for each river, including water level, turbidity and cesium concentration, the early period after the accident suffered from a lack of monitoring data. This continued even after that period, resulting in the availability of only fragmented assessment periods and discreet observation points. At that time, a certain calculation model that allows for long-term assessment of radioactive cesium levels (for instance, during rainfall) was not available.

JAEA developed a simulation model named “MERCURY”, which is designed based on observation results, and used the model to calculate the radioactive cesium discharge through rivers into the ocean after the accident of 1F. As a result, it was revealed that the amount of radioactive cesium discharged for 6 months after the accident was lower by about two digits, as compared to other release routes, and also that the amount accounted for 60% of the total discharge by the end of 2017 from rivers into the ocean.

Utilizing this model enables us to evaluate the amount of radioactive cesium discharged into the ocean not only in the early stage after the accident but also for a long period of time, and furthermore, to predict how the radioactive cesium discharge during every rainfall and/or cesium concentration in river water will change with time. It is highly expected that the model may be applied to, for example, pollutant migration associated with the runoff of water and soil.