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Journal Articles

Development of regional downscaling capability in STEAMER ocean prediction system based on multi-nested ROMS model

Kamidaira, Yuki; Kawamura, Hideyuki; Kobayashi, Takuya; Uchiyama, Yusuke*

Journal of Nuclear Science and Technology, 56(8), p.752 - 763, 2019/08

Oceanic regional downscaling capability was implemented into Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) developed by Japan Atomic Energy Agency to enable us to predict more realistically the oceanic dispersion of radionuclides at higher spatiotemporal resolutions for broader applications. The system consisted of a double-nested oceanic downscaling circulation model with tidal forcing and an oceanic radionuclide dispersion model. This system was used to comparatively examine downscaling and tidal effects on the dispersion of radionuclides hypothetically released from the Fukushima Daiichi Nuclear Power Plant in the colder season. The simulated dissolved $$^{137}$$Cs distribution was different from that obtained using coarser-resolution models because downscaling enhanced both horizontal and vertical mixing. The suppression of horizontal mixing and the promotion of vertical mixing by tidal forcing synergistically reduced offshore $$^{137}$$Cs transport. In addition, the submesoscale effects strengthened the three-dimensional $$^{137}$$Cs fluctuations by $$<$$10 times, while the tidal effects promoted slightly increased the intensity of three-dimensional $$^{137}$$Cs fluctuations by approximately 3%. This indicated that the submesoscale effects substantially surpassed tidal forcing in oceanic mixing in the coastal margin off Fukushima in the colder season.

Journal Articles

Development of a short-term emergency assessment system of the marine environmental radioactivity around Japan

Kobayashi, Takuya; Kawamura, Hideyuki; Fujii, Katsuji*; Kamidaira, Yuki

Journal of Nuclear Science and Technology, 54(5), p.609 - 616, 2017/05

 Times Cited Count:2 Percentile:42.02(Nuclear Science & Technology)

The Japan Atomic Energy Agency has, for many years, been developing a radionuclide dispersion model for the ocean, and has validated the model through application in many sea areas using oceanic flow fields calculated by the ocean model. The Fukushima Dai-ichi Nuclear Power Station accident caused marine pollution by artificial radioactive materials to the North Pacific, especially to coastal waters northeast of mainland Japan. In order to investigate the migration of radionuclides in the ocean caused by this severe accident, studies using marine dispersion simulations have been carried out by JAEA. Based on these as well as the previous studies, JAEA has developed the Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) to immediately predict the radionuclide concentration around Japan in case of a nuclear accident.

Journal Articles

Development of SPEEDI-MP and its application to a hypothetical accident of a nuclear submarine in the Japan Sea

Kobayashi, Takuya; Nagai, Haruyasu; Chino, Masamichi; Togawa, Orihiko

Proceedings of International Symposium on Radioecology and Environmental Dosimetry, p.500 - 504, 2003/00

The possibility of radionuclide release to the environment is on the increase due to a growth of nuclear facilities in Asia, military problems, nuclear terrorism, etc. The released radionuclides circulate among the atmospheric, oceanic and terrestrial environment, influencing human and natural environment through complex processes. In order to reduce the damage from these pollutants, it is necessary to estimate their migration behavior in detail. A software system SPEEDI-MP (SPEEDI Multi-model Package) is under development to resolve such environmental problems by simulating the 'inclusive and successive' behavior of pollutants in the atmospheric, oceanic and terrestrial environment. As code verification, the system has been applied to a hypothetical accident of a nuclear submarine if it sinks in an offshore region around Japan. The predicted results are examined, and the applicability and validity of the system are discussed.

Journal Articles

Marine environmental assessment system of radionuclides around Japan

Kobayashi, Takuya; Togawa, Orihiko

Proceedings from the International Conference on Radioactivity in the Environment (CD-ROM), 4 Pages, 2002/09

A marine environmental assessment system STEAMER is developing for predicting the short-term (30days) dispersion and assessing the collective dose to the Japanese population due to radionuclides released to the ocean. The computer code system for short-term predictions of radionuclide dispersion is a combination of the Princeton Ocean Model (POM) for predicting ocean currents and a particle random walk model SEA-GEARN for oceanic dispersion of dissolved radionuclides. The system has been applied to a hypothetical accident of a nuclear submarine if it sinks in an offshore region around Japan, by using measured currents, temperature, salinity and meteorological regional objective analysis data (RANAL). Another computer code system DSOCEAN is also applied to the same hypothetical accident in order to compare the results of radionuclide dispersion in the ocean and the collective dose to the Japanese population. An equidistant-grid compartment model combined with a model of the geostrophic current analysis is used in DSOCEAN.

Oral presentation

Surface mixing and dispersion of radioactive tracer due to submesoscale eddies off the northeastern Pacific coast of Japan

Kamidaira, Yuki; Kawamura, Hideyuki; Kobayashi, Takuya; Uchiyama, Yusuke*

no journal, , 

A multiple nesting technique enables us to examine submesoscale eddies and their effects on surface mixing and associated material transport (e.g., Kamidaira et al., 2016). In this study, downscaling simulations for the coastal margin off Fukushima are conducted by using ROMS with a horizontal resolution of 1 km. The oceanic initial and boundary conditions are given by an oceanic data assimilation system MOVE developed at Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA). Energy conversion analysis exhibits that surface eddy potential energy to eddy kinetic energy (EKE) conversion through baroclinic instability is substantial in vorticity generation in this area. A further simulation is performed on oceanic dispersion of a radioactive tracer released from the Fukushima Dai-ichi Nuclear Power Plant to investigate eddy-induced mixing using an offline, oceanic tracer dispersion model SEA-GEARN developed at Japan Atomic Energy Agency (JAEA) (Kobayashi et al., 2007). A spatial moment analysis with respect to the center of gravity of the concentration reveals that tracer dispersion occurs with two distinct regimes leading to anisotropic transport in the zonal and meridional directions. The first regime lasting for about 5-8 days after the tracer release are mainly caused by coastally-trapped alongshore jet and submesoscale eddy-mixing. The subsequent regime occurs after the tracer is sufficiently dispersed offshore where mesoscale eddies play more dominant roles. Moreover, time series of zonal and meridional tracer variances fluctuate with enstrophy, area averaged density anomaly and EKE. These results clearly illustrate that fluctuating submesoscale and mesoscale eddy field induced by baroclinic insatiability evidently affects the tracer dispersion off Fukushima.

Oral presentation

Submesoscale eddy-induced mixing and dispersion of radioactive tracer off the northeastern Pacific coast of Japan

Kamidaira, Yuki; Kawamura, Hideyuki; Kobayashi, Takuya; Uchiyama, Yusuke*; Furuno, Akiko

no journal, , 

no abstracts in English

Oral presentation

Development of an emergency assessment system of the marine environmental radioactivity around Japan and its utilization

Kobayashi, Takuya; Kawamura, Hideyuki; Fujii, Katsuji*; Kamidaira, Yuki

no journal, , 

The Fukushima Dai-ichi Nuclear Power Station (FNPS1) accident in Japan in March 2011 led to the release of large amounts of radionuclides into the atmosphere as well as direct discharges into the ocean. Various environmental assessments have been performed by many research groups using marine dispersion simulations after the FNPS1 accident. It was recognized from these woks that the risk of radionuclide release from nuclear facilities to the environment was very high and the establishment of emergency ocean dispersion forecasting systems was necessary. Then, the Japan Atomic Energy Agency (JAEA) has developed the Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) to immediately predict the radionuclide migration for a nuclear accident in ocean around Japan, by integrating previous study results. In this study, structure, performance test results, and utilization of the system are described.

Oral presentation

Study on marine pollution due to the Fukushima Daiichi Nuclear Power Plant accident

Kobayashi, Takuya

no journal, , 

no abstracts in English

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