Radionuclide Dynamics in the Coastal Ocean off the Fukushima Daiichi Nuclear Power Plant Using Radioactivity Ratios.

Fifteen years after the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident, ¹³⁷Cs and ³H activities in coastal waters near the plant remain elevated compared to surrounding regions, indicating persistent radioactive inputs. While concentrations within the port are highest, recent estimates suggest that the leakage rate outside the port exceeds that inside, implying the presence of an additional or previously unrecognized source outside the F1NPP site. However, the mechanisms governing these releases remain unclear.

The ³H/¹³⁷Cs activity ratio is a useful tracer for identifying contamination sources, as it remains relatively stable in seawater over short timescales. Since approximately 2016, ¹³⁷Csconcentrations near the FDNPP have shown little decline, while spatial contrasts in the ³H/¹³⁷Cs ratio have become more pronounced. Although both radionuclides’ concentrations peak within the port, the ratio is consistently lower there and higher offshore. This suggests the potential existence of sources outside the harbor governing the distribution pattern of the radionuclides.

To investigate these patterns, we applied a color-classified ³H/¹³⁷Cs ratio analysis and conducted release-rate estimations for the port and adjacent coastal waters. In addition, we collected independent samples of seawater, river water, groundwater, and spring water near the F1NPP. The ³H/¹³⁷Cs ratios of river water, groundwater, and spring water were used in an end-member mixing analysis to evaluate potential terrestrial and subsurface contributions. Preliminary results indicate that the end members for groundwater and spring water (excluding river water) show trends similar to the ³H/¹³⁷Csratio in seawater, potentially explaining the observed increase in the ratio offshore.

This integrated analysis improves constraints on radionuclide sources and transport pathways in the F1NPP coastal environment and contributes to a better understanding of long-term radioactive contamination dynamics.