Simulation of anthropogenic tritium discharge into the ocean from the Fukushima Daiichi Nuclear Power Plant

Following the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, large quantities of radioactive materials were released into the atmosphere and ocean. Since the FDNPP nuclear accident, Tokyo Electric Power Company (TEPCO) operators have been implementing measures to reduce groundwater inflow into the FDNPP damaged reactor buildings while pumping water to cool the nuclear reactors and fuel debris. The resulting huge water volume began the discharge into the ocean from August 2023, after being treated by an Advanced Liquid Processing System (ALPS) to remove radionuclides for acceptable discharge levels except tritium. Since then, tritium concentrations in seawater and aquatic ecosystems near the FDNPP site are continuously monitored and disseminated publicly. It is essential to assess the long-term safety threshold of ALPS-treated water discharge procedure in terms of tritium concentration in coastal areas of Japan and the Pacific Ocean. However, there is no global oceanic simulation with tritium concentration and, by extension, no projection of tritium concentration at Pacific Ocean scale.

In this study, we used the TEPCO ALPS treated water release plan as an input to the ocean general circulation model (OGCM) COCO4.9, which is the ocean component of the Model for Interdisciplinary Research on Climate, version 6 (MIROC6 [1]). This approach allowed us to simulate the anthropogenic tritium concentration in the ocean due to ALPS treated water release in the forthcoming decades. The spatial distribution and temporal evolution of the projected tritium concentrations in different parts of the Pacific Ocean, as well as the impact of global warming on them, were analyzed. Moreover, the anthropogenic tritium concentration following the FDNPP accident was modeled to evaluate how large the tritium concentrations due to current treated water release are compared to the accidental one in 2011. Finally, given that oceanic tritium concentrations are mainly controlled by ocean mixing, our study represents a valuable opportunity to evaluate the impact of the Kuroshio current representation in COCO4.9 on tritium concentrations at non-eddy-resolving and eddy-resolving horizontal resolutions.

[1] Tatebe et al., Geosci. Model Dev., 12, 2727–2765, doi:10.5194/gmd-12-2727-2019, 2019.