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

[摘要] The Great East Japan Earthquake and tsunami that caused a loss of power atthe Fukushima nuclear power plants (FNPP) resulted in emission of radioactiveisotopes into the atmosphere and the ocean. In June of 2011, an internationalsurvey measuring a variety of radionuclide isotopes, including ¹³⁷Cs,was conducted in surface and subsurface waters off Japan. This paper presentsthe results of numerical simulations specifically aimed at interpreting theseobservations and investigating the spread of Fukushima-derived radionuclidesoff the coast of Japan and into the greater Pacific Ocean. Together, thesimulations and observations allow us to study the dominant mechanismsgoverning this process, and to estimate the total amount of radionuclides indischarged coolant waters and atmospheric airborne radionuclide fallout. Thenumerical simulations are based on two different ocean circulation models,one inferred from AVISO altimetry and NCEP/NCAR reanalysis wind stress, andthe second generated numerically by the NCOM model. Our simulations determinethat > 95% of ¹³⁷Cs remaining in the water within~600 km of Fukushima, Japan in mid-June 2011 was due to the directoceanic discharge. The estimated strength of the oceanic source is16.2 ± 1.6 PBq, based on minimizing the model-data mismatch. We cannotmake an accurate estimate for the atmospheric source strength since most ofthe fallout cesium had left the survey area by mid-June. The model explainedseveral key features of the observed ¹³⁷Cs distribution. First, theabsence of ¹³⁷Cs at the southernmost stations is attributed to theKuroshio Current acting as a transport barrier against the southwardprogression of ¹³⁷Cs. Second, the largest ¹³⁷Cs concentrations wereassociated with a semi-permanent eddy that entrained ¹³⁷Cs-rich waters,collecting and stirring them around the eddy perimeter. Finally, theintermediate ¹³⁷Cs concentrations at the westernmost stations areattributed to younger, and therefore less Cs-rich, coolant waters thatcontinued to leak from the reactor in June of that year.

[学科分类] 地球化学与岩石