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

Area of Science:

• Marine environmental science
• Radiological impact assessment
• Oceanographic modeling

Background:

• The Fukushima Daiichi Nuclear Power Plant accident released significant amounts of cesium-137 (¹³⁷Cs) into the marine environment.
• Cesium-137 exists in dissolved, suspended, seabed sediment, and biota phases in the ocean, with interphase interactions influencing its long-term fate.

Purpose of the Study:

• To develop and utilize an oceanic dispersion model incorporating multiphase behavior.
• To evaluate the impact of interphase interactions on the long-term behavior of ¹³⁷Cs over a decade post-accident.

Main Methods:

• Development of a multiphase oceanic dispersion model for ¹³⁷Cs.
• Validation of the model against oceanographic conditions and plankton/particle/¹³⁷Cs behavior.
• Quantitative analysis of interphase interactions and their contribution to ¹³⁷Cs retention.

Main Results:

• The model accurately reproduced oceanographic conditions and the behavior of ¹³⁷Cs.
• Dissolved and seabed sediment phases were key for early and later ¹³⁷Cs behavior, respectively.
• Offshore ¹³⁷Cs levels returned to pre-accident levels within 10 years, while coastal levels remained elevated due to seabed sediment sources.

Conclusions:

• Seabed sediment acts as a long-term source of ¹³⁷Cs in coastal waters.
• Factors like fine-grained particles, riverine input, early surface accumulation, and vertical mixing influence ¹³⁷Cs retention in sediments.
• Understanding interphase dynamics is crucial for predicting the long-term marine radiological impact.