Hideyuki Kawamura

Preliminary Numerical Experiments on Oceanic Dispersion of 131I and 137Cs Discharged into the Ocean because of the Fukushima Daiichi Nuclear Power Plant Disaster

It was reported that after the magnitude 9.0 earthquake and resulting tsunami that occurred on March 11, 2011, some radioactive material, including iodine and cesium, was discharged into the Pacific Ocean because of the Fukushima Daiichi nuclear power plant (NPP) disaster. High concentrations of I and Cs were found in the seawater and seabed sediment along the coastline of Fukushima Prefecture. In addition, radionuclides were detected in marine products caught near Fukushima Prefecture. In response to this situation, assessment of the distribution of concentrations of radioactive materials by means of computer simulation, as well as oceanic survey, is urgently required. The Japan Atomic Energy Agency (JAEA), in cooperation with the Japan Marine Science Foundation and Kyoto University, has been working on numerical experiments to predict the spread of I and Cs in the marine environment and to estimate how much of these materials were released. The numerical experiments were carried out using the oceanic dispersion model SEA-GEARN and the ocean general circulation model (OGCM) developed by Kyoto University and the Japan Marine Science Foundation. The numerical experiments took into account depositions calculated using the atmospheric dispersion model GEARN in WSPEEDI-II of I and Cs released into the atmosphere. The numerical experiments were performed from March 12 to April 30, 2011. This paper focuses on the amounts of I and Cs that were released during the above-mentioned period, as well as the concentrations of these materials in the seawater. II. Description of Numerical Experiments

Source term estimation of atmospheric release due to the Fukushima Dai-ichi Nuclear Power Plant accident by atmospheric and oceanic dispersion simulations

The source term of the atmospheric release of 131I and 137Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident estimated by previous studies was validated and refined by coupling atmospheric and oceanic dispersion simulations with observed 134Cs in seawater collected from the Pacific Ocean. By assuming the same release rate for 134Cs and 137Cs, the sea surface concentration of 134Cs was calculated using the previously estimated source term and was compared with measurement data. The release rate of 137Cs was refined to reduce underestimation of measurements, which resulted in a larger value than that previously estimated. In addition, the release rate of 131I was refined to follow the radioactivity ratio of 137Cs. As a result, the total amounts of 131I and 137Cs discharged into the atmosphere from 5 JST on March 12 to 0 JST on March 20 were estimated to be approximately 2.0 × 1017 and 1.3 × 1016 Bq, respectively.

The Formation and Circulation of the Intermediate Water in the Japan Sea

In order to clarify the formation and circulation of the Japan/East Sea Intermediate Water (JESIW) and the Upper portion of the Japan Sea Proper Water (UJSPW), numerical experiments have been carried out using a 3-D ocean circulation model. The UJSPW is formed in the region southeast off Vladivostok between 41°N and 42°N west of 136°E. Taking the coastal orography near Vladivostok into account, the formation of the UJSPW results from the deep water convection in winter which is generated by the orchestration of fresh water supplied from the Amur River and saline water from the Tsushima Warm Current under very cold conditions. The UJSPW formed is advected by the current at depth near the bottom of the convection and penetrates into the layer below the JESIW. The origin of the JESIW is the low salinity coastal water along the Russian coast originated by the fresh water from the Amur River. The coastal low salinity water is advected by the current system in the northwestern Japan Sea and penetrates into the subsurface below the Tsushima Warm Current region forming a subsurface salinity minimum layer.

Comparison of RIAMOM and MOM in Modeling the East Sea/Japan Sea Circulation

The seasonal variations in the circulation of the water mass in the East Sea/Japan Sea have been simulated using a free surface primitive ocean model, RIAMOM (RIAM Ocean Model), comparing the results from GFDL-MOM1 (Geophysical Fluid Dynamics Laboratory Modular Ocean Model, version 1.1, hereafter MOM) with the GDEM (Generalized Digital Environmental Model) data. Both models appear to successfully reproduce the distinct features of circulation in the East Sea/Japan Sea, such as the NB (Nearshore Branch) flowing along the Japanese coast, the EKWC (East Korean Warm Current) flowing northward along the Korean coast, and the NKCC/LCC (North Korean Cold Current/Liman Cold Current) flowing southwestward along Korean/Russian coast. RIAMOM has shown better performance, compared to MOM, in terms of the realistic simulation of the flow field in the East Sea/Japan Sea; RIAMOM has produced more rectified flows on the coastal region, for example, the narrower and stronger NKCC/LCC than MOM has. There is however obvious differences between the model results and the GDEM data in terms of the calculation of the water mass; both models have shown a tendency to overpredict temperature and underpredict salinity below 50m; more diffusive forms of thermocline and halocline have been simulated than noted in GDEM data.

Numerical simulation on the long-term variation of radioactive cesium concentration in the North Pacific due to the Fukushima disaster

Numerical simulations on oceanic (134)Cs and (137)Cs dispersions were intensively conducted in order to assess an effect of the radioactive cesium on the North Pacific environment with a focus on the long-term variation of the radioactive cesium concentration after the Fukushima disaster that occurred in March 2011. The amounts of (134)Cs and (137)Cs released into the ocean were estimated using oceanic monitoring data, whereas the atmospheric deposition was calculated through atmospheric dispersion simulations. The highly accurate ocean current reanalyzed through a three-dimensional variational data assimilation enabled us to clarify the time series of the (134)Cs and (137)Cs concentrations in the North Pacific. It was suggested that the main radioactive cesium cloud due to the direct oceanic release reached the central part of the North Pacific, crossing 170°W one year after the Fukushima disaster. The radioactive cesium was efficiently diluted by meso-scale eddies in the Kuroshio Extension region and its concentration in the surface, intermediate, and deep layers had already been reduced to the pre-Fukushima background value in the wide area within the North Pacific 2.5 years after the Fukushima disaster.

Estimation of Total Amounts of Anthropogenic Radionuclides in the Japan Sea

We estimated the total amounts of anthropogenic radionuclides, consisting of 90Sr, 137Cs, and 239+240 Pu, in the Japan Sea for the first time based on experimental data on their concentrations in seawater and seabed sediment. The radionuclide inventories in seawater and seabed sediment at each sampling site varied depending on the water depth, with total inventories for 90Sr, 137Cs, and 239+240Pu in the range of 0.52–2.8 kBq m−2, 0.64--4.1 kBqm−2, and 27-122Bqm−2, respectively. Based on the relationship between the inventories and the water depths, the total amounts in the Japan Sea were estimated to be about 1:2 ± 0:4PBq for 90Sr, 1:8 ± 0:7PBq for 137Cs, and 69 ± 14TBq for 239+240Pu, respectively; the amount ratio, 90Sr:137Cs:239+240Pu, was 1.0:1.6:0.059. The amounts of 90Sr and 137Cs in the Japan Sea were in balance with those supplied from global fallout, whereas the amount of 239+240Pu exceeded that supplied by fallout by nearly 40%. These results suggest a preferential accumulation of the plutonium isotopes. The data used in this study were obtained through a wide-area research project, named the “Japan Sea expeditions (phase I),” covering the Japanese and Russian exclusive economic zones.

Oil spill simulation in the Japan Sea

An assessment system of the marine environment in the Japan Sea is being developed at the Japan Atomic Energy Agency. This study aims to confirm the validity of the assessment system by reproducing the oil spill at the incident of Russian tanker Nakhodka in January 1997. As a main subject, one of the data assimilation techniques, an approximate Kalman filter, was applied to the assessment system by combining an ocean general circulation model with sea level measurements of satellite data. Using calculated ocean currents, simulations of the behaviour of spilled oil were performed with a particle random-walk model. A number of experiments with different parameters and situations showed that the assimilated daily ocean currents with wind drift were for the simulation for movement of spilled oil.

Oceanic dispersion of Fukushima-derived Cs-137 simulated by multiple oceanic general circulation models

To understand the concentration and amount of Fukushima-derived Cs-137 in the ocean, this study simulated the oceanic dispersion of Cs-137 by atmospheric and oceanic dispersion simulations. The oceanic dispersion simulations were carried out with an oceanic dispersion model and multiple oceanic general circulation models. The Cs-137 concentrations were sensitive to ocean currents in the coastal, offshore, and open oceans. The mean Cs-137 concentrations of the multiple models relatively well agreed with the observed concentrations in the coastal and offshore oceans during the first few months after the Fukushima disaster, and in the open ocean during the first year after the disaster. The Cs-137 amounts were quantified in the coastal, offshore, and open oceans during the first year after the disaster. It was suggested that Cs-137 actively dispersed from the coastal and offshore oceans to the open ocean, and from the surface layer to the deeper layers in the North Pacific.

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

ABSTRACT The Japan Atomic Energy Agency (JAEA) 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 oceanic circulation 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. Coupling the STEAMER with the emergency atmospheric dispersion prediction system, such as Worldwide version of System for Prediction of Environmental Emergency Dose Information version II (WSPEEDI-II), enables comprehensive environmental pollution prediction both in air and the ocean.

Assessment system of marine environment in the Japan Sea

An assessment system of marine environment in the Japan Sea is developed in Japan Atomic Energy Agency to calculate a movement of pollutants such as radionuclides and their effect on Japanese people. The assessment system consists of three models, ocean general circulation model, particle random-walk model, and dose assessment model. The assessment system was applied so far to a numerical experiment of an oil spill accident and a numerical experiment for reproduction of concentration of anthropogenic radionuclides in the Japan Sea. In addition, the lower trophic level ecosystem model is being developed to estimate a suspended solid in the Japan Sea.