Masafumi Uchiyama
- Keywords: cesium, biological half-time, male, adult, Japanese
@In a previous study in the 1960's, an average cesium biological half-time of 86 days with 1 SD of 21 days was reported for 23 selected Japanese male adults. It was determined under a quasi-equilibrium condition by combining data from whole-body counting and urinalysis for 137Cs resulting from atmospheric nuclear weapon tests. Recently increased 137Cs body burdens were observed in returnees from the former USSR territories due to the Chernobyl accident. Measuring their body burdens repeatedly with an appropriate interval, the retention followed a linear line when 137Cs in the body was expressed as a logarithm. Biological half-times were obtained for 22 Japanese male adults in this way. An average of 100 days with 1 SD of 32 days was observed. When two individuals with extremely high half-times were excluded, an average of 91 days with 1 SD of 24 days was estimated. After a 20 year elapse in time, the difference in the biological half-time of cesium at a confidential level of 90 was not significant but had an increasing tendency. When the two Japanese groups that were measured in the 1960's and the 1980's were combined , the biological half-time of cesium was 93}27 days for 45 subjects (Fig.1) and 88}23 days for 43 subjects, without the 2 extremely large values, respectively. Insignificant differences in the Cs biological half-time between the 1960's and 1980's were indicated in Sweden, Germany and Russia for male adults. The reference biological half-time, thus, should be 90 days for Japanese male adults. @Cesium biological half-time for individual subjects might be subject to both differences and changes in physique and body constituents. Four subjects took part in both the studies in the 1960's and the 1980's. In the 1960's the four subjects were in their twenties. Their changes in biological half-time were various. One subject had lost 20 of his potassium but his body weight in the 1980's was similar to his 1960's weight. His biological half-time in the 1980' s was also similar to his value in the 1960's. Three other subjects had biological half-times in the 1980's that were 30 to 80 longer than during the 1960's. One subject among them had lost 14 of his potassium. The two other subjects had similar potassium contents for both times even though their body weight had increased by about 10 kg. This analysis suggests that the biological half-time of cesium increases with time despite the natural decrease in the potassium pool that occurs with aging.
[Publications] Fig.1 Distribution of biological half-times of cesium in 45 Japanese male adults.
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2.Three Decades' Study on 137Cs Body Burden and Its Consequent Internal Dose for Japanese Male Adults
Masafumi Uchiyama, Takeshi Iinuma, Tomio Ishihara, Tetsuo Ishikawa, Masaki Matsumoto
- Keywords : body burden, 137Cs, Japanese, male adults, internal dose
@The present study analyzing 137Cs body burden was commenced at the National Institute of Radiological Sciences (NIRS) in 1964. In the previous year, 1963, fallout from nuclear weapon tests was at its peak. The subjects measured were a group of Japanese adult male scientists, mainly from NIRS. The youngest were in their twenties and the oldest in their fifties. Periodical measurements of 137Cs body burden using a whole-body counter were performed. The man-year of the subjects measured ranged from 30 to 219. In the early period from 1963 to 1972, a whole-body counter with 2 sets of plastic detectors was usually used to measure the body burdens for 15 minutes in a stationary mode. For about 20 years, from 1972 onwards, a whole-body counter with two 8-in diameter by 4-in thick NaI(Tl) detectors was used. Measurements were carried out in a scanning mode of 5 cm per minute. To calibrate the measurements a block phantom of an adult Japanese male with an average physique for the early 1960's was filled with a known amount of 137Cs. The internal dose estimated in the present study was the total body dose. This was because external methodology needed to measure the organ weight in order to determine the effective dose had not and as yet, has not, been developed. The MIRD method was applied to estimate the internal dose from accumulated 137Cs after the absorbed fractions were modified according to the total body weight of each individual subject. @Average body burdens of 137Cs decreased linearly from a maximum of 520 Bq with one standard deviation (SD) of 43 Bq in 1964 to 132 Bq with 1 SD of 26 in 1967. Nuclear weapon tests by China decelerated this decreasing tendency by a factor of 1.9. From 1974 to 1985, 137Cs body burdens kept a constant level of around 20 Bq. The Chernobyl Accident brought about a small increase in the body burden. In 1987 there was a peak of 56 Bq. The consequences of this increase were detected for 5 years. @Internal dose due to 137Cs body burden changed with time as indicated in Fig. 33. The annual dose attained a maximum of 20ÊSv in 1964. The committed dose of 82Ê sv was estimated for internally deposited 137Cs resulting from unclear explosions in 1961 and 1962. The consequence from the Chernobyl accident to the dose of the present group was estimated to be 5.6ÊSv. The health effect of the dose on the group was evaluated to be insignificant. Before 1964, the increase in 137Cs body burden was estimated by both urinalysis and the deposition of fallout due to atmospheric nuclear weapon tests by the USA and USSR in the 1950's. Urinalysis data indicated that the body burden rose and fell symmetrically before and after 1964. Thus the internal dose from 137Cs could be estimated to be roughly 170 ÊSv in average for the whole period for the group measured.
[Publications] Fig.1 Three decades of changes in the annual averages of the internal dose due to 137Cs to an adult male group in Japan.
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3.Intestinal Absorption of 14C-chitosan in Rats
Yoshikazu Nishimura, Yoshito Watanabe, Jia Ming Hong, Hiroshi Takeda andMasae Yukawa (Institute of Nuclear Agricultural Sciences, China National Nuclear Corporation)
- Keywords: 14C-chitosan, intestinal absorption, distribution
@Chitosan is an insoluble polysaccharide consisting of À-(1,4)-linked N-acetylglucosamine(GlcNAc) units, and is abundant in the marine environment. This polysaccharaide, which is mainly derived from crustaceans, such as crabs and shrimps, has recently attracted attention as a useful biomass. Our previous research has proved that chitosan can reduce the bioavailability of radiostrontium in rats. However, the reduction mechanism and basic biokinetics are not clear yet. The purpose of this study was to investigate the absorption and the basic biokinetics of chitosan in rats. Carbon-14 labeled chitosan [(C8H13NO5)n] was synthesized by Tokai Research Laboratories and purchased from Dai-ich Chemical Company. It's radiochemical purity was assessed using High Performance Liquid Chromatography (HPLC) and was 98.7 . 14C-chitosan was administered orally to rats. It was observed that the radioactive material was distributed quite extensively in their tissues. The relative concentration (radioactivity per gram of wet tissue^radioactivity administered per gram of body weight) of radioactivity in the tissues, increased sharply for the first few hours and then decreased smoothly. From the sixth to the twelfth hour after administration, the relative concentration in most of the rat tissues increased, except in the intestine where it decreased. During the sixth to the twelfth hour, the sum of the total amount of radioactivity in the tissues except for the intestine was about 5 of the originally administered dose of 14C-chitosan. These results indicate that 14C-chitosan was probably absorbed from the gastrointestinal tract, and was metabolized and excreted quickly without re-bioavailability. The radioactivity in the serum, in extracts from the liver and in the contents of small intestine were separated using Gel Permeation Chromatography and measured for 14C. A great peak was found in high molecular weight region compared with the standard 14C-chitosan. This sharp peak had a retention volume the same as Bovine Serum albumin (Fig. 34). The sharp peak could be made to disappear if the proteins contained in the serum or liver were removed. In addition, a third peak was found in all 3 samples, regardless of whether the protein was removed or not. It followed the other peaks and was eluted with lower molecular weight range retention volume products, the same as chito-oligosaccharides. These results suggest that some 14C-chitosan is possibly absorbed into the blood, liver and other tissues, either directly or after being degraded to small molecular compounds.
[Publications] Fig.1 Analysis of serum and liver by GPC.
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4.Biokinetics and Dose Estimation of Radiocarbon in Rat
Hiroshi Takeda, Shoichi Fuma and Tetsuo Iwakura
- Keywords: radiocarbon, 14C-compounds, biokinetics, dose estimation, rat
@Radiocarbon (14C) is formed as a by-product of nuclear power generation, and a part of it is released to the environment. Although the estimated releases of 14C from presently operating nuclear power facilities are still quite small in comparison with natural and other man-made sources, the introduction of new nuclear technologies, coupled with reprocessing of spent nuclear fuel elements and the disposal of radioactive wastes, are virtually certain to raise the quantity of 14C that is produced and potentially released. In recent years, concern about its potential long-term hazard to man as a source of radiation exposure has prompted a number of assessments of proposed nuclear fuel cycles and facilities. 14C is often released into the environment in the forms of carbon dioxide or bicarbonate. A part of such inorganic 14C is considered to be transformed into organic 14C by photosynthsis in plants during food chain transfer. Human exposure to 14C is, therefore, occurs by intake of air and water containing inorganic 14C and by intake of food containing organic 14C. @The purpose of the present study is to estimate relative radiotoxicity of different chemical forms of 14C, which will be taken by ingestion of water or food. Sodium bicarbonate as inorganic 14C, and amino acids (leucine and lysine), fatty acids (palmitic acid and oleic acid), glucose and thymidine as organic 14C, were administered to rats by a single ingestion. After the ingestion, the rats were killed at various time intervals and dissected to obtain various tissue or organ samples. These samples were combusted in an oxidizer which automatically adds an aquatic scintillator and the radioactivities in the combustion water were determined with a liquid scintillation counter. The data were expressed in terms of relative concentration, defined as the percentage of radioactivity administered per g of body weight of individual rat. @The time-course of relative concentrations of 14C in the rat tissues after the ingestion of various 14C-compounds was compared. It was found that there were differences in the biokinetics among the ingested 14C-compounds. Lower incorporation and retention of 14C was observed after ingestion of 14C-bicarbonate, followed by 14C-thymidine ingestion. As shown in our previous study using RH-thymidine, the major part of ingested thymidine is decomposed rapidly in the gastrointestinal tract. This should explain the lower incorporation and retention of 14C-thymidine. Higher retention of 14C was observed after ingestion of 14C-amino acids and 14C-fatty acids. Based on these biokinetics data, radiation dose was calculated for individual tissues of rats due to ingestion of the same amount of radioactivity per g of body weight (Table 1). Tese results indicated that radiation doses depended upon the chemical form when 14C was ingested. Significant differences were observed between inorganic 14C and organic 14C, except for 14C-thymidine. The highest radiation doses to the majority of tissues were observed when 14C-amino acids were ingested. The doses from 14C-leucine and 14C-lysine were, respectively, 15-34 and 29-70 times higher than the doses from 14C-bicarbonate. The results of the present study indicated that 14C incorporated into protein through the food chain should be considered as an important source term of human exposure to environmental 14C.
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5.Application of a River Runoff Model Designed for the Kanto Plain to the Nuclear Site, Rokkasho Village
Kiriko Tanaka-Miyamoto, Yoshikazu Inoue, Tetsuo Iwakura and Takashi Iyogi
- Keywords:
Kanto Plain in 1954-1995 was compared with the observed values of river waters in Rokkasho Village in 1991-1995. @Fig.1 compares three response curves with the observed data of river waters in Rokkasho Village. The basic response curve (solid line) shows TSn for the Kanto Plain calculated using precipitation data in the Kanto Plain. The annual mean value of tritium concentration for precipitaion in Rokkasho Village for 1994-95 was 1.5 times as much as that for the Kanto Plain during the period of this study. The next response curve (dashed line) is based on the input data multiplied by this factor of 1.5 times for the period from 1954-1995. This curve only fits the observed data for river water whose tritium concentration is the lowest in this period. @To obtain a better fitting, the size of the second layer reservoir of groundwater in the Rokkasho Village was changed to "one third" of that in the Kanto Plain. The final response curve (cross-point line) shows a better fit to the observed data for river water whose tritium concentration is highest in this period. This two-stepped modification demonstrated that the river runoff model developed for the Kanto Plain is applicable to other local areas including Rokkasho Village. @There are several reasons why the amount of fallout tritium deposition by precipitation in Rokkasho Village was 1.5 times higher than that in the Kanto Plain in 1994-95. It is known that on a global scale there is gradient in the latitudinal distribution of tritium content in the atmosphere. The higher the latitude, the higher the tritium content, and this is especially evident in the northern hemisphere. Recently it was reported that the Japanese Islands are strongly influenced by atmospheric currents from the Asian Continent which contain greater levels of fallout tritium. This continental air mass has a greater influence on the northern part of the Japanese Islands than the southern. @Also, the areas under study vary in their geological scale. Water volume of the second layer in the Rokkasho Village was estimated to be one third of that in the Kanto Plain. Thus the volume of the second layer of groundwater is about twice that of annual precipitation in Rokkasho Village.
[Publications] Miyamoto, K., Kimura, K. and Hongo, S.: Fusion Technol., 28, 910-917, 1995. Fig.1 Tritium concentration in river waters observed in Rokkasho Village and comparison with the response curve TSn. The same symbols mean samples from the same locations at different sampling times. The solid line shows TSn for the Kanto Plain. The dashed line is based on 1.5 times higher concentration in precipitation than that of the Kanto Plain. The cross-point line is based on 1.5 times higher concentration in precipitation and the volume of the second groundwater being one third of that of the Kanto Plain.
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6.Influence of Microbial Activity on Technetium Behaviour in Soil under Waterlogged Condition
Keiko Tagami and Shigeo Uchida
- Keywords : technetium, immobilization, redox potential, soil, waterlogging, microbial activity
@The behaviour of Tc in soil depends upon its chemical forms. Technetium is known to exist in all valence states from {7 to -1. The most stable form of Tc in natural aqueous solutions in equilibrium with the atmosphere is the pertechnetate form, TcO |4. Therefore, under aerobic conditions, Tc is present as TcO |4, which has a high geochemical mobility and bioavailability. However, the form changes through a combination of factors such as redox conditions and microbial activity in soils. The anaerobic condition can be supplied in waterlogged rice paddy soil, which is common to Japan and other Southeast Asian countries. Since rice is main food in these countries, it is important to predict 99Tc behaviour in rice paddy field. @In this study, we have carried out radiotracer experiments to obtain the influence of microbial activity on 99Tc behaviour in soil under the anaerobic (waterlogged) condition by measuring Eh values. The microbial activity is expected to be controlled by addition of glucose (0.5w of soil) and the activity causes a change in the soil redox condition. @Air-dried soil and sterile soil samples with and without glucose (A-0, A-0.5, S-0 and S-0.5) were used to clarify the role of microorganisms in the Tc transformation. The soil samples, 40 g amounts of each, were placed in polystyrene vessels (120 mL) and the depths of the soil in the vessels were 4 - 4.5 cm. They were carefully waterlogged with 60 mL of 99Tc solution (JRIA, 370 Bq^mL). The vessels were sealed with parafilm to prevent entry by microorganisms from the air. At each sampling time, the surface solution was collected and its Eh value was measured. @The results for Eh and the relative concentrations (RC: C^CO) of 99Tc in the surface soil solution of AD-0, AD-0.5, S-0 and S-0.5 as a function of time are shown in Fig. 1. The RC was defined as "the activity in a solution at each sampling time (C)" divided by "the initial activity in the solution (CO)". The pH values of all soil samples were almost constant during the period. The RC of 99Tc in the surface solution of AD-0.5 decreased over time and at the end of the experiment, it was almost zero, however, those of AD-0, S-0 and S-0.5 were decreased about 20 to 40 by the end of the experiment. We found that Tc in the soil solution was adsorbed on the soil under the low Eh condition. It is, however, difficult to explain whether microbial activity affects only generation of low Eh or transformation of Tc in soil. From these results, it is clear, however, that the low Eh condition was generated not only by waterlogging, but also by microbial activity.
[Publications] Tagami, K. and Uchida, S.: In Scientific Basis for Nuclear Waste Management XVIII, Part 2 (Murakami, T. and Ewing, R. C. eds), Materials Research Society, Pittsburgh, 973-979, 1995. Fig.1 Relative concentration of 99Tc and Eh in the surface solutions of ADĦ0, ADĦ0.5, SĦ0 and SĦ0.5 as a function of time.
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7.Determination of Major and Trace Elements in Mushrooms, Plants and Soils Collected from Japanese Forests
Satoshi Yoshida and Yasuyuki Muramatsu
- Keywords : ICP-MS, trace elements, mushroom, plant, soil, forest
@Measurements of major and trace elements in biological and soil samples within a forest are needed to expand our knowledge of the elemental composition of the forest ecosystem and to predict migrations and effects of chemical elements. Most research efforts concerning the elemental migrations in forest ecosystems have concentrated on the major nutrient elements. Therefore, the distribution and transfer of many trace elements are still unknown. @In this study, inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) were used to measure many major and trace elements in plant, mushroom and soil samples collected in Japanese forests. Sample preparation and analytical conditions were investigated to set up a simple routine procedure for measuring a large range of elements. Fifty elements were determined for soil samples. For plant and mushroom samples, 25 elements were determined. @Concentrations of some trace elements such as Zn, Pb, Cd, Bi, Sn and Sb in forest soils tended to be the highest in the surface soil layer, indicating the importance of atmospheric deposition on the total contents in the soils of these elements. In comparison with the element contents of plants, the mushroom contents could be characterized by low Mg, Ca, Sr and Ba amounts. Transfer factors (TFs) were estimated from the ratio of "concentration in plant or mushroom on dry weight basis" to "concentration in the surface soil on dry weight basis" (see Fig. 1). The TFs of Co, Ba, lanthanide elements, Th and U were very low in all plant and mushroom samples. Mushrooms tended to accumulate Cu, Zn, Rb, Cd and Cs. The TFs of Cs for mushrooms were one or two orders higher than those for other plants growing in the same forest. High concentrations of radiocesium discharged through nuclear weapons testing and nuclear accidents have been reported in many countries. The high TFs for stable Cs obtained in this study indicated that mushrooms are important Cs accumulators and radiocesium is taken up from soils together with stable Cs. @Multi-element capability of ICP-MS can provide information on the distribution of many trace elements in forest ecosystems. In addition, analyses of both plants and soils provide in situ TFs. Species specific accumulation and plant-availability of the elements can be estimated by the TFs.
[Publications] Yoshida, S., Muramatsu, Y., Tagami, K. and Uchida, S.: Intern. J. Environ. Anal. Chem., 63, 195-206, 1996. Yoshida, S. and Muramatsu, Y.: Intern. J. Environ. Anal. Chem., 1997 (in printing). Fig.1 Transfer factors for plants and mushrooms collected from a pine forest in Tokai, Ibaraki.
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8.Dietary 232Th and 238U Intakes for Japanese and Contributions of Imported Foods to Internal Doses
Kunio Shiraishi and Masayoshi Yamamoto
- Keywords :
Furthermore, an attempt was made to calculate the effects of imported foods on internal dose by using the present analytical results. @Foodstuffs of 174 kinds were purchased from markets in Mito city, Ibaraki Prefecture. Statistical consumption data of 1987 were used for collection of the food samples. Standard Reference Material (SRM) 1571 Orchard Leaves was also obtained from the National Institute of Standards and Technology, and used for quality control. The foodstuffs purchased, approximately 74Ħ, were divided initially into 30 food groups and dryĦashed in a muffle furnace at a final temperature of 400. The ash obtained was collected, according to four final groups, as mixed ash samples. The four groups were as follows: group 1eggs, milk, and milk products; group 2bean, animal, and fish products; group 3fruits, vegetables, and potatoes; and group 4cereals, oil, and others. Statistical consumption data were also collected according to these four groups. An aliquot of the mixed ash samples was taken and completely decomposed with a mixture of concentrated nitric acid and perchloric acid. The sample solution was analyzed by inductivelyĦcoupled plasma mass spectrometry (ICPĦMS). Daily intakes of 232Th in the four respective food groups were found to be 0.047, 0.526, 1.05, and 0.599mBq per person. For 238U, intakes of groups 1, 2, 3, and 4 were 0.088, 2.96, 11.8, and 0.60 mBq, respectively. Total 232Th and 238U intakes were estimated to be 2.22 mBq and 15.5 mBq per person per day, respectively. @Using the present results, preliminary calculation was conducted. Calculation results are shown in Table 7. Six food groups were used: 1)milk products, 2) meat products, 3) fish products, 4) leafy vegetables, 5) roots and fruits, and 6) grain products, in accordance with the report of United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1993). First, one assumption must be made that all foodstuffs analyzed in the present work were selfĦsupplied in Japan (i.e. all foodstuffs taken were not imported ones). Next, suitable values obtained from the present results in the four food groups were arbitrarily assigned into the six food groups. Annual intakes of each food group (shown in the 2nd column in the Table) were referred to the data of the National Nutritional Survey. SelfĦsupply ratios of foods in Japan of 1990 (shown in the 3rd column), were used. Importation ratios were one minus the selfĦsupply ratios. The radioactivities of imported food groups (shown in the 6th column) were referred to UNSCEAR data. The activities were reported as averages collected from data of the northern hemisphere for a mild climate region. Radioactivities from selfĦsupplied food were obtained by multiplying the amounts of annual intakes (Ħ^y), selfĦsupply ratios, and radioactivities (mBq^Ħ) of^each food group selfĦsupplied in Japan. The share of the imported foods was calculated in the same way. In the case of 232Th, the annual intakes of selfĦsupplied and imported foods were 500 mBq and 556 mBq per person, respectively. Total annual intake was 1056 mBq^y. If all foods were selfĦsupplied in Japan, (i.e. ratios of selfĦsupply are all one), total annual intake would be 804 mBq. Owing to the ratio of B^A (about 1.3), a 30 increment would be obtained as the contribution of imported foods. If ratios of selfĦsupply were all 0.5 or all 0.3, the ratios of B^A would be higher, 1.8 or 2.1, respectively. Dietary intake of 238U would be about 1.6 times higher due to the importation of foodstuffs from foreign countries, as obtained by the same calculation method. Effects of imported food on internal exposures were estimated using the present analytical results and UNSCEAR data. But, in order to get more refined effects, radioactivities in many food groups and^or in each supplying country, must be clarified. After collecting these data, a good computer code should be established for the dose calculation concerning the effects by food importation.
[Publications] Shiraishi, K. and Yamamoto, M.: J. Radioanal. Nuc. Chem. Art., 196, 89Ħ96, 1995,
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9.239{240Pu and 137Cs Distributions in Seawater from the Japan Sea
Masatoshi Yamada, Tatsuo Aono, Shigeki Hirano
- Keywords: Japan Sea, 239{240Pu , 137Cs, activity ratio, inventory
@Artificial long-lived radionuclides have been spread worldwide through nuclear weapons tests making it necessary to understand the degree of artificial radioactive contamination in the marine environment. The purpose of our investigation was to determine concentrations of the artificial radionuclides, 239{240Pu and 137Cs, in seawater from the Japan Sea, and to discuss the behaviour of 239{240Pu by measuring distributions of 239{240Pu^ 137Cs activity ratios and 239{240Pu inventories in the water column. Seawater samples were collected at the Yamato Basin and at the Tsushima Basin of the Japan Sea during the N93-02 cruise of the R^V "Natsushima", Japan Marine Science and Technology Center. @239{240Pu and 137Cs are mainly delivered to the ocean by fallout derived from atmospheric nuclear weapons tests. The 239{240Pu^ 137Cs activity ratios can be used to evaluate the effect of scavenging on the 239{240Pu by comparing deviations from the global fallout ratio. The 239{240Pu^ 137Cs activity ratio versus water depth is plotted in Fig.1. The dotted line represents 239{249Pu^ 137Cs global fallout ratio of 0.023 corrected for 137Cs decay to 1993. At the Tsushima Basin station, the 239{240Pu^ 137Cs ratio increases gradually with depth from 0.005 at a 100 m depth to 0.035 at a 1400 m depth. The ratio in the upper 750 m depth is less than the global fallout value of 0.023. At the Yamato Basin station, the ratio has a minimum layer at a 500 m depth and increases largely with depth to 0.12 at a 1400 m depth. This ratio of 0.12 is five times that of the global fallout ratio. These results support the proposal that scavenging and removal of 239{240Pu relative to 137Cs take place in the upper layer of the Japan Sea. @The 239{240Pu inventories at the Yamato Basin station are estimated to be 12.5, 31.4, and 42.7 Bq^Ħ over the depth interval 0-500m, 500-1400m, and 1400m- bottom, respectively.@The inventories at the Tsushima Basin station are almost the same.@The 239{240Pu inventories at the Yamato Basin station are 14.5, 36.2, and 49.3 of those in the whole water column over the depth interval 0-500 m, 500-1400 m, and 1400 m - bottom, respectively. Half of the 239{240Pu is present in the deep water column over the depth interval 1400 m - bottom; this fact indicates that 239{240Pu relative to 137Cs is transported downward rapidly.@The 239{240Pu inventories in the whole water column are estimated to be 86.6 and 85.2 Bq^Ħ at Yamato and Tsushima Basin stations, respectively. These values are about two times greater than that of the estimated direct fallout input at the same latitudes of 30-40KN.@The 239{240Pu inventory in the water column is about 90 of that in the total (water column{sediment column). These results suggest that a large amount of 239{240Pu delivered to the Japan Sea still remains in the water column.
[Publications]
Fig1. The 239{240Pu^ 137Cs activity ratio versus depth at Stn. 1 in the Yamato Basin (circles) and at Stn. 2 in the Tsushima Basin (squares).
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