Lagrangian trajectories of Fukushima Daiichi NPP originated water, transported by large-scale circulation in the North Pacific Ocean, and reached the Sea of Okhotsk

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident contaminated large areas of the Pacific Ocean with different radionuclides. However, not all areas are studied equally. For example, the Sea of Okhotsk is one of the least studied regions, with almost no measurement data available. In the current study, we applied the Lagrangian particle tracking model Parcels V3.0 to simulate the trajectories of virtual particles containing radionuclides in the Pacific Ocean. Here, the output from the KIOST-MOM circulation model is used. It includes monthly mean climatic data for 3D currents (U, V, W components of water velocity) and vertical diffusivity coefficients. Coefficients for horizontal diffusion are calculated using the Smagorinsky formula.Virtual particles were emitted at the location of the FDNPP during 31 days (26 Mar to 25 Apr 2011), when 96.6% of the total amount of radionuclides was released directly to the ocean. Each particle initially contained a certain activity of radionuclides (137Cs, 134Cs, 90Sr, 3H, 129I) proportionally to the estimated total release of each radionuclide. The activity of each radionuclide inside the particle decreased according to radioactive decay with the corresponding half-life. The atmospheric deposition of radionuclides on the sea surface was not considered here.Model results were validated on the 134Cs concentrations in the Northeastern Pacific in areas with measurement data after 2012, when the impact of atmospheric deposition decreased. For the Sea of Okhotsk, the concentrations of 5 radionuclides were calculated and analyzed. For particles that reached the Sea of Okhotsk, we calculated statistical characteristics based on Lagrangian trajectories: visitation frequency, mean age, and representative trajectory, which demonstrated the pathways of water masses transporting radioactivity from FDNPP to the Sea of Okhotsk.