Long-term simulation of ¹³⁷Cs behavior in the ocean around Fukushima considering multiphase interaction processes

The accident at the Fukushima Daiichi Nuclear Power Plant caused a large-scale migration of radionuclides, including cesium-137 (¹³⁷Cs), into the marine environment. Cesium-137 in the ocean exists in four phases (dissolved, suspended, seabed sediment, and biota), and the interactions between these phases have a great impact on its long-term behavior. Therefore, this study developed an oceanic dispersion model considering multiphase behavior and evaluated the impact of interphase interactions on the long-term behavior of ¹³⁷Cs during the decade after the accident. The model sufficiently reproduced the oceanographic conditions and the behavior of planktons, suspended particles, and ¹³⁷Cs. The results were consistent with previous studies, suggesting that the dissolved phase and seabed sediment phase played a central role in the ¹³⁷Cs behavior within the first three months after the accident and thereafter, respectively. Within 10 years after the accident, the concentrations of all phases in offshore waters (deeper than 25 m) returned to pre-accident levels, but those in coastal waters (shallower than 25 m) were still at high levels due to the long-term source from seabed sediment to other phases, which is attributed to the retention of high concentrations in the seabed sediment phase. The quantitative analysis of interphase interactions made it possible to confirm that this retention was caused by a large proportion of fine-grained particles (bay area), riverine inputs (northern Fukushima), and high accumulation in the surface layers in the early stage of the accident and vigorous vertical mixing (southern Fukushima).