Short-term dispersal of Fukushima-derived radionuclides off Japan: modeling efforts and model-data intercomparison

Abstract. The Great East Japan Earthquake and tsunami that caused a loss of power at the Fukushima nuclear power plants (FNPP) resulted in emission of radioactive isotopes into the atmosphere and the ocean. In June of 2011, an international survey measuring a variety of radionuclide isotopes, including ¹³⁷Cs, was conducted in surface and subsurface waters off Japan. This paper presents the results of numerical simulations specifically aimed at interpreting these observations and investigating the spread of Fukushima-derived radionuclides off the coast of Japan and into the greater Pacific Ocean. Together, the simulations and observations allow us to study the dominant mechanisms governing this process, and to estimate the total amount of radionuclides in discharged coolant waters and atmospheric airborne radionuclide fallout. The numerical simulations are based on two different ocean circulation models, one inferred from AVISO altimetry and NCEP/NCAR reanalysis wind stress, and the second generated numerically by the NCOM model. Our simulations determine that > 95% of ¹³⁷Cs remaining in the water within ~600 km of Fukushima, Japan in mid-June 2011 was due to the direct oceanic discharge. The estimated strength of the oceanic source is 16.2 ± 1.6 PBq, based on minimizing the model-data mismatch. We cannot make an accurate estimate for the atmospheric source strength since most of the fallout cesium had left the survey area by mid-June. The model explained several key features of the observed ¹³⁷Cs distribution. First, the absence of ¹³⁷Cs at the southernmost stations is attributed to the Kuroshio Current acting as a transport barrier against the southward progression of ¹³⁷Cs. Second, the largest ¹³⁷Cs concentrations were associated with a semi-permanent eddy that entrained ¹³⁷Cs-rich waters, collecting and stirring them around the eddy perimeter. Finally, the intermediate ¹³⁷Cs concentrations at the westernmost stations are attributed to younger, and therefore less Cs-rich, coolant waters that continued to leak from the reactor in June of that year.