Understanding H-3/Cs-137 Behavior to Track Dispersion Sources in Fukushima’s Marine Environment

Thirteen years have passed since the accident at the Fukushima Daiichi Nuclear Power Plant, but the levels of Cs-137 and H-3 near the power plant have not returned to the levels they were at before the accident. In addition, the levels of Cs-137 and H-3 near the power plant continue to be higher than in other areas of the sea. This suggests a persistent leakage from the power plant or the surrounding sea area. However, the mechanism of the leakage is unknown. As the estimated rate of leakage outside the port is higher than the estimated rate of leakage inside the port, it is also possible that the leakage is not via the port.

The ratio of radioactive material concentration does not change in seawater over a short period of time, so it is useful for estimating the source of leakage. Here, we focused on the ratio of H-3 and Cs-137 and analyzed the data.

After 2016, the concentration of Cs-137 near the power plant has hardly decreased.

The concentration of Cs-137 and H-3 is high in the port area and low outside the port area. However, the H-3/Cs-137 ratio is small in the port area and large outside the port area. This suggests that the concentration of Cs-137 and H-3 in the port area does not necessarily affect the concentration outside the port area, and there is a possibility that there is another source.

The H-3/Cs-137 ratio fluctuates greatly over time. This may be due to the presence of multiple sources with different H-3/Cs-137 ratios, or it may be a sampling issue due to large fluctuations in concentration over time and space.

We have now started analyzing the H-3/Cs-137 ratio in nearby river water and groundwater and will discuss the relationship between these at the time of the presentation.