Research and Development Relating to the Accident at the TEPCO's Fukushima Daiichi NPS

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Fig.1-6 Concentration of Cs-137 at the sea surface predicted using a simulation assuming direct ocean release andatmospheric deposition
The simulation results illustrate that Cs-137 was deposited on the sea surface along the Fukushima and Miyagi prefectures and theoffshore area northeast of Miyagi prefecture via atmospheric release immediately after the accident. Additional Cs-137 was also
directly released into the ocean from 1F. Cs-137 in the ocean was transported by ocean currents, and the mixing process of the ocean
currents diluted its concentration.

A large amount of radionuclides was discharged into theatmosphere by the TEPCO s Fukushima Daiichi NPS (1F)
accident. Many of them were transferred toward the east anddeposited into the ocean. Moreover, some water used forcooling of the damaged nuclear reactors leaked from theplant and flowed into the ocean. It is important to understandthe actual conditions of the release of radionuclides into theocean in order to assess the influence of the accident on themarine environment. Therefore, an ocean diffusion
experiment based on a numerical simulation was performed
using a general ocean circulation model developed at Kyoto
University and a diffusion model for radionuclides in theocean (SEA-GEARN) developed by JAEA.
To perform a radionuclide migration simulation in theocean, it is necessary to predict the ocean currents. In thisstudy, the accuracy of ocean current data was improved byapplying the four-dimensional variation method, which is ahighly efficient data assimilation technique that assimilates
observational data to a numerical model.
Determining the amount of released radionuclides
presented the most difficulty, because information about thesource term, e.g., the released radionuclides, amounts, andduration, which is essential for the numerical experiments,
was not available. Thus, the source term for the radionuclides
released directly into the ocean from 1F was estimated using
the concentrations of radioactive materials in the ocean
reported by TEPCO. Regarding the atmospheric deposition ofradionuclides, the results calculated with WSPEEDI wereused.
Fig.1-6 shows the concentration of Cs-137 at the sea
surface, as determined using the simulation. A peakatmospheric release occurred on March 15. Cs-137 wasdeposited on the sea surface along the Fukushima and Miyagi
prefectures and offshore of the area northeast of Miyagi
prefecture. The concentration of the radionuclides depositedon the sea was diluted by the Oyasio water and then they
moved southward. The radionuclides that arrived in theKuroshio region were quickly carried eastward by theKuroshio extension, whereas the radionuclides separated
from the Kuroshio extension showed a tendency to stagnatein that region for a while.
A numerical simulation of the long-term dispersal ofradionuclides in the Pacific Ocean will be performed in order
to assess their impact on the marine environment.

Reference
Kawamura, H., Kobayashi, T. et al., Preliminary Numerical Experiments on Oceanic Dispersion of 131I and 137Cs Discharged into the Ocean because of theFukushima Daiichi Nuclear Power Plant Disaster, Journal of Nuclear Science and Technology, vol.48, no.11, 2011, p.1349-1356.
JAEA R&D Review 201213