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

The 129-iodine content of subtropical Pacific waters: impact of Fukushima and other anthropogenic 129-iodine sources

“…Such estimates are very likely to be model dependent and require independent knowledge of the relative influence of atmospheric and direct discharge of 129 I. An ocean model forced with National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis and satellite altimetry as described in Rypina et al (2013) and using the KOK sampled region 2 PBq excess inventory of cesium (Buesseler et al, 2012) implies that ∼82.5 % of the direct oceanic discharge and ∼95 % of the atmospheric deposited Cs had been advected out of the region the KOK sampled by the time of the cruise. The convolution of the two source terms and export indicate that more than 95 % of the excess cesium in the region sampled by the KOK came from direct discharge (Rypina et al, 2013).…”

“…3). The North Pacific is only actively ventilated to 2000 m or less, coincident with the base of the wind driven circulation, with deeper portions forced remotely by the large-scale buoyancy-thermohaline circulation (e.g., Rommenich and McCallister, 1989). Turbulence along the NPSG 4842 T. P. Guilderson et al: The 129-iodine content of subtropical Pacific waters boundary and within the Kuroshio confluence zone, where isopycnals are compressed, will mix tracers more efficiently than the interior of the NPSG where transport is slower (e.g., Kelley and Van Scoy 1999).…”

“…Turbulence along the NPSG 4842 T. P. Guilderson et al: The 129-iodine content of subtropical Pacific waters boundary and within the Kuroshio confluence zone, where isopycnals are compressed, will mix tracers more efficiently than the interior of the NPSG where transport is slower (e.g., Kelley and Van Scoy 1999). In general the interior circulation at ∼1000 m, with much slower velocities (1-2 cm s −1 ), has a similar pattern to the overlying surface current along its northern and western boundaries, although its eastward return flow is compressed poleward and the eastern boundary is diffuse to non-existent (e.g., Reid and Mantyla, 1978).…”

Cesium-134 and 137 activities in the central North Pacific Ocean after the Fukushima Dai-ichi Nuclear Power Plant accident

“…The area of interest is dominated by east-flowing warmer waters of Kuroshio and Kuroshio extension currents at a latitude of about 35 • N (for general circulation patterns, see, for example, Qiu, 2001; for dominant current directions in June 2011, see Rypina et al, 2013). The contaminated water from F1-NPP was discharged north of the Kuroshio current, which acted as a southern boundary for transport of this oceanic source Rypina et al, 2013). The atmospheric transport models (e.g., Stohl et al, 2012) predicted that some radionuclides from the atmospheric source were spread south of the Kuroshio current.…”

“…From these, 134 Cs (half-life 2.07 a) and 137 Cs (halflife 30.08 a) are important F1-NPP-derived radionuclides in the ocean because of their radioactive-conservative behavior and large discharged quantities. It is estimated that up to several tens of PBq (PBq = 10 15 Bq) of 134 Cs and 137 Cs entered the environment (e.g., IAEA, 2011;Bailly du Bois et al, 2012;Estournel et al, 2012;Stohl et al, 2012;Charette et al, 2013;Rypina et al, 2013).…”