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  • Author or Editor: J. Inaba x
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Summary  

In order to clarify the sedimentation rate of 210Pbexcess and 137Cs in brackish Lake Obuchi, bordered by nuclear fuel facilities in Rokkasho Village, Aomori, Japan, sediment core samples were collected at three points in the lake, and the rates were calculated by the chronological measurement method using 210Pb and 137Cs. The sedimentation rates were 0.29±0.04 g . cm-2 . y-1 in the estuary of the Futamata River (water depth of 1.2 m), 0.13±0.02 g . cm-2 . y-1 in the 3 m water depth area, and 0.11±0.02 g . cm-2 . y-1 at the center point (water depth of 4.5 m), respectively. The Futamata River estuary was found to be greatly affected by land erosion.

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Abstract  

The soil-to-grass transfer factors and grass-to-milk transfer coefficients were determined for 137Cs and stable Cs in soil, grass and milk samples collected in Aomori Prefecture, Japan. The concentrations of 137Cs in the soil and grass samples collected from 25 sampling sites were 13±12 Bq.kg-1 and 2.0±2.1 Bq.kg-1 dry wt., respectively. The geometric mean of soil-to-grass transfer factor of 137Cs was 0.13 and its 95% confidence interval was 0.017-0.98. The transfer factor of 137Cs was higher than that of stable Cs, and they had a positive correlation. The concentration of K in the soil affected both transfer factors. The concentration of 137Cs in milk samples collected from 16 sites was 76±43 mBq.kg-1 fresh wt. and had a good correlation with that of stable Cs. The geometric mean of grass-to-milk transfer coefficient of 137Cs was 0.0027, assuming that a cow's total daily intake was 20 kg of dry grass. The transfer coefficient of 137Cs was positively correlated with that of stable Cs.

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Summary  

A rapid method was developed using ultrafilters with a tangential flow filtering system for molecular size separation of naturally occurring 210Pb and 210Po in a freshwater sample. Generally, ultrafiltering of a large volume water sample for measuring the nuclides was too time consuming and not practical. The tangential flow filtering system made the filtering time short enough to adapt for in-situ ultrafiltering the large volume sample. In this method, a 20 liter water sample was at first passed through the 0.45mm pore size membrane filter immediately after sample collection to obtain suspended particle matter [>0.45mm particulate fraction (PRT)]. Two ultrafilters (Millipore Pellicon 2Ò) were used sequentially. The nuclides in the filtrate were separated into three fractions: high molecular mass (100 kDa-0.45mm; HMM), low molecular mass (10 k-100 kDa; LMM) and ionic (<10 kDa; INC) fractions. It took 80 minutes to process the sample after collection. The cut-off molecular size of each ultrafilter was confirmed by size exclusion chromatographs (SEC) of the LMN and the HMM fractions. Adsorption of the nuclides and organic compounds in the sample onto the ultrafilters was negligibly small. Good reproducibility of the nuclide concentrations in each fraction was confirmed by repeated experiments. The method was successfully applied to obtaine the molecular size distributions of 210Pb and 210Po in an oligotrophic lake, Lake Towada located in the northern area of Japan.

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Summary  

In order to evaluate the transport of 3H and 137Cs radionuclides in semi-closed brackish Lake Obuchi, Japan, bordered by nuclear fuel cycling facilities, a 3D-lower-trophic eco-hydrodynamic model has been developed and validated. In a short-term prediction, 3H and 137Cs activity levels in water should be in an agreement with field measurements. It became clear that the results depended on the mixing of fresh water and seawater in the model. Moreover, a short-term simulation estimated that most 3H and 137Cs flowed to the ocean rather than remaining in the lake. Based on calculations over the past 50 years, a peak of 137Cs in sediment was in 1963, when the maximum 137Cs fallout was observed. The calculation showed a rapid decrease after that peak, however, the field measurement data gradually declined. This suggested that the process by which 137Cs accumulated from the watershed to the lake was actually slower in the field than in the model calculations. The model may be successfully applied to a variety of different environmental situations as a generic tool for evaluating the concentration and migration of 3H and 137Cs in a brackish lake.

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Summary  

In order to identify the concentration of tritium (3H) in areas of fresh, brackish and sea water, bordered by nuclear fuel facilities at Rokkasho-Village, Aomori, Japan, water samples were collected from 2001 to 2004 at six points in those areas. Concentration ranges of tritium in fresh river water, brackish lake and seawater samples were 0.60 to 1.1 Bq. l-1(mean value 0.79 Bq. l-1), 0.20 to 0.87 Bq. l-1(mean value 0.41 Bq. l-1), and 0.08 to 0.25 Bq. l-1(mean value 0.15 Bq. l-1), respectively. Relationships between tritium concentrations and salinity in the samples showed a clear negative correlation. Moreover, the seasonal variation of tritium in water from Rokkasho-Village was high in spring and low in fall.

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Abstract  

The suspended particles floating in the seawater have the ability to biologically, as well as physically adsorb radionuclides and other elements dissolved in seawater. We have studied the distribution and composition of suspended particles, as well as the state of eluted of radionuclides in the decomposition process, in the coastal waters off Rokkasho Village, where radionuclides will be discharged from a nuclear fuel reprocessing plant in the near future.

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Abstract  

7Be deposition fluxes and atmospheric concentrations were measured at Rokkasho Village, Aomori Prefecture, Japan, from 2000 to 2005. It was confirmed that the 7Be deposition fluxes were minimum in summer, and the fallout maximizes in winter. The atmospheric concentration of 7Be was especially low in summer, and high in the other three seasons. A positive correlation was observed between the amount of precipitation and 7Be deposition. Clear seasonal differences were evident among the ratios of 7Be deposition flux to precipitation amounts in the four seasons. The ratios were especially high in winter, higher than those in the other three seasons. 7Be deposition flux was estimated by a simple simulation model using atmospheric 7Be concentrations and local meteorological data. As a result, the estimated deposition value was relatively lower than the measured value in winter.

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Abstract  

Atmospheric concentrations of 210Pb change with various factors such as meso-scale meteorological conditions. We have already reported the biweekly atmospheric 210Pb concentrations in Rokkasho, Japan for 5 years and found that they had clear seasonal variations: low concentrations in summer and high values in winter to spring. To study the reasons for the seasonal variations, the origins of the air mass flowing to Rokkasho were analyzed by 3-D backward air mass trajectory analysis. Routes of the calculated trajectories were classified into four regions: northeastern and southeastern Asian Continent, sea and other regions. The atmospheric 210Pb concentrations were well correlated with the frequency of the routes through the northeastern Asian Continent. A non-linear multiple regression analysis of the 210Pb concentrations and the relative frequencies of the four routes showed good fitting of the predicted values to the observed ones, and indicated that the atmospheric 210Pb concentrations in Rokkasho depended on the frequency of the air mass from the northeastern Asian Continent.

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Abstract  

We investigated the distribution of aquatic plants and the absorption of radionuclides by them in the brackish Lake Obuchi, Japan, which is bordered by nuclear fuel cycle facilities. We studied 5 species of submerged plants: Zannichellia palustris, Ruppia maritima, Potamogeton pectinatus, Zostera japonica, and Z. marina. The plants contained many elements, including radionuclides. The concentrations of 238U, 137Cs, and 90Sr in Z. marina were 11.3-12.4, 0.000-0.144, and 0.151-0.202 Bq.kg-1 dw, respectively. Those in Z. japonica were 5.2-8.8, 0.000-0.267, and 0.081-0.175 Bq.kg-1 dw, respectively. The concentrations of these radionuclides in the plants tended to be higher in higher-salinity regions than in lower-salinity regions of the lake. We found a close relationship between photosynthetic activity and the absorption of stable Sr by plants in the laboratory. Salinity, illumination, and water temperature influenced the photosynthetic activity of the plants and the consequent absorption of elements.

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