Takuya Kobayashi

Anthropogenic radionuclides in the Japan Sea: their distributions and transport processes

The anthropogenic radionuclides, (90)Sr, (137)Cs and (239+240)Pu, were measured in the water column of the Japan Sea/East Sea during 1997-2000. The vertical profiles of radionuclide concentrations showed: exponential decrease with depth for (90)Sr and (137)Cs, and surface minimum/subsurface maximum for (239+240)Pu. These results do not differ substantially from results reported previously. The area-averaged concentrations of radionuclides in the Japan Sea are higher than those found in the Northwest Pacific Ocean below surface layer showing the accumulation of the radionuclides in the deep waters in the Japan Sea. Concerning spatial distributions, the area of high (137)Cs inventory extends from the Japan Basin into the Yamato Basin. It is suggested that wintertime convection of water, occurring mainly in the Japan Basin, causes the radionuclides to sink. The nuclides then advect into the Yamato Basin after detouring around the Yamato Rise.

Anthropogenic Radionuclides in Seawater of the Japan Sea The Results of Recent Observations and the Temporal Change of Concentrations

Between 1996 and 2002, a wide-area research project on anthropogenic radionuclides was carried out in an area covering the Japanese and Russian Exclusive Economic Zones of the Japan Sea, through a collaboration of Japanese and Russian institutes. The aim was to investigate the migration behavior of anthropogenic radionuclides (90Sr, 137Cs, and 239+240Pu) in the sea. Four expeditions conducted in the Japan Sea between 2001 and 2002 found that the observed concentrations and distributions of radionuclides were similar to those found in previous investigations. Inventories estimated from the concentration data indicate that larger amounts of these radionuclides accumulate in the Japan Sea seawater (by a factor of 1.5–2.1) than are supplied by global fallout in the same latitude belt. Further, we found that the 90Sr and 137Cs concentrations in the intermediate layer show temporal variations with time scales of 1 to several years. The results of cross-analysis using the data of 137Cs and dissolved oxygen suggest that the distribution and variation of radionuclide concentrations in the intermediate layer may reflect water mass movement in the upper part of the Japan Sea.

Development of Ocean Pollution Prediction System for Shimokita Region Model Development and Verification

A three-dimensional model system has been developed to predict oceanic dispersions of radionuclides released into the off Shimokita region. This prediction system is a combination of the Princeton Ocean Model (POM) for predicting ocean currents and a particle random walk model for oceanic dispersion of radionuclides. The model has been verified by using measured currents, temperature and salinity at the coastal area of Shimokita, Aomori-ken, Japan, where a nuclear fuel reprocessing plant is under construction. The results obtained from simulations are as follows; (1) Sea surface wind and the Tsugaru Warm Current (TWC) entering into the objective region through the Tsugaru Strait significantly affect the structure of current field over the region. (2) POM can represent seasonal variations of the TWC well with the input of monthly inflow data of the TWC. The calculation succeeded in reproducing the coastal mode from winter to spring and the gyre mode from summer to autumn.

Ocean Model Coupling with Atmospheric Model for Prediction of Radioactivity Contaminant Dispersion in Emergency

In order to predict the atmospheric and oceanic dispersion of radionuclides due to a nuclear accident, a coupling model between mesoscale atmosphere and ocean were proposed. And we have carried out several numerical experiments under different atmospheric conditions. The results are summarized as follows: (1) Wind stress transferred through the sea surface does not always have an influence on every area in the ocean, but the influence of wind stress on mean current variation is not negligible, especially over shallow water. (2) Sea surface winds tend to have a greater impact on the dispersion of radioactivity contaminants than normal sea currents. (3) The calculation time for the CPU is about 15% less for this combination system than for systems without the coupling library Stampi.

Estimates of Collective Doses from a Hypothetical Accident of a Nuclear Submarine

The collective dose to the Japanese population has been estimated from a hypothetical accident of a nuclear submarine if it sinks in an offshore region around Japan. A computer code system DSOCEAN has been used for assessing the collective dose due to radionuclides released to the ocean from a sunken nuclear submarine. The estimated collective effective dose commitment from all of the radionuclides released after the break of the fuel pellets is estimated to be 2.5 x104 man-Sv. The contribution of 241Am to the total collective effective dose commitment is the highest, followed by 137Cs, 238Pu, 240Pu, 239Pu and 241 Pu. The maximum of the estimated collective effective dose by the annual intake of marine products after radionuclide releases for one year is approximately 0.3% of the annual average dose by the natural radiation that is reported by UNSCEAR.

Submesoscale Mixing on Initial Dilution of Radionuclides Released From the Fukushima Daiichi Nuclear Power Plant

This study developed a submesoscale eddy-resolving oceanic dispersal modeling system comprising a double-nested oceanic downscaling model and an offline oceanic radionuclide dispersion model. This was used to investigate the influences of submesoscale coherent structures (SCSs) and associated ageostrophic secondary circulations (ASCs) on the three-dimensional (3-D) dispersal of dissolved cesium137 (Cs) released from the Fukushima Daiichi Nuclear Power Plant (FNPP1). Extensive model-data comparison demonstrated that the innermost high-resolution model, with a lateral grid resolution of 1 km, could successfully reproduce transient mesoscale oceanic structures, the Kuroshio path and stratification, and spatiotemporal variations of Cs concentrations. Using an accompanying mesoscale eddy-resolving model (grid resolution: 10 km) as a guide, we showed that submesoscale dynamics are important for improved representation of both the eddy field and the resultant 3-D dispersal of Cs, with the temporal variability of surface Cs near the FNPP1 being equivalent to that in the coarse-resolution model. According to energy conversion and spectral analyses, SCSs and ASCs occur most intensively on the submesoscale, primarily because of shear instability. However, baroclinic instability serves as a secondary mechanism. SCSs have prominent seasonality, reflected by intensification in the colder months, which is when the FNPP1 accident occurred. Analysis of the vertical flux of Cs was performed by decomposition of the variables into eddy, mesoscale, and submesoscale components using frequency and wave number filters. It revealed that 42.7% of the FNPP1-derived Cs was transported downward below the mixed layer by eddies with the major contribution being from ASCs induced by submesoscale eddies.