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To investigate the migration of anthropogenic 129I in the environment, we measured 129I concentrations at both subarctic (above 40oN) and subtropical (below 40oN) circulations in the surface seawater of the Japan Sea. The averaged concentrations of stations 193, 194, 201, 206 and 210 above 200 m were (2.1 ± 0.3) × 1010 atoms/m3, (2.0 ± 0.2) × 1010 atoms/m3, (1.6 ± 0.3) × 1010 atoms/m3, (1.4 ± 0.3) × 1010 atoms/m3 and (1.7 ± 0.3) × 1010 atoms/m3, respectively. The averaged concentration at the subarctic circulation in the Japan Sea above 200 m (1.9 × 1010 atoms/m3) was higher than that in the subtropical circulation (1.5 × 1010 atoms/m3). This latitudinal distribution pattern of 129I is not consistent with those of bomb-derived radionuclides such as 14C, 90Sr and 137Cs. Taking into account latitudinal location and the total amount of releases from reprocessing plants, this discriminating latitudinal distribution of 129I in the Japan Sea would indicate that a significant amount of 129I originating from active reprocessing plants in Europe is supplied to the surface of the Japan Sea.

A numerical experiment is carried out to reproduce distribution of concentration of 90Sr and 137Cs, estimate their total amount and verify their source in the Japan Sea. Model results are in good agreement with observational findings in the Japan Sea expeditions between 1997 and 2002 by the Japan Atomic Energy Agency. Vertical profiles of the concentration of 90Sr and 137Cs show exponential decreases with depth from the sea surface to the sea bottom. From the model and observational results, it is suggested that the concentration of 90Sr and 137Cs in the surface layer is approximately in the range of 1.0–1.5 Bq/m3 and 2.0–2.5 Bq/m3, respectively. On the other hand, it is found that the concentration in the intermediate and deep layer is higher than that observed in the northwestern Pacific Ocean, suggesting active winter convection in the Japan Sea. The total amount of 90Sr and 137Cs in the seawater is evaluated to be 1.34 × 1015 Bq and 2.02 × 1015 Bq, respectively, in the numerical experiment, which demonstrates an estimation by observational data obtained in the Japan Sea expeditions. The total amount of 90Sr and 137Cs changed during the second half of 20th century corresponding to deposition at the sea surface with the maximums of 4.86 × 1015 Bq for 90Sr and 7.33 × 1015 Bq for 137Cs, respectively, in the mid-1960s. The numerical experiment suggests that the main source of 90Sr and 137Cs has been global fallout, although there have been some potential sources in the Japan Sea.

We developed an extraction method for accurately and reproducibly determining dissolved organic radiocarbon in seawater by ultraviolet oxidation of dissolved organic carbon and subsequent accelerator mass spectrometry. We determined the irradiation time required for oxidation of the dissolved organic carbon. By modifying the experimental apparatus, we decreased contamination by dead carbon, which came mainly from petrochemical products in the apparatus and from the incursion of carbon dioxide from the atmosphere. The modifications decreased the analytical blank level to less than 1% of sample size, a percentage that had not previously been achieved. The recovery efficiency was high, 95±1%. To confirm both the accuracy and reproducibility of the method, we tested it by analyzing an oxalic acid radiocarbon reference material and by determining the dissolved organic carbon in surface seawater samples.

⋅ Carbon cycle, ⋅ organic carbon, ⋅ Japan Sea, ⋅ lateral transport, ⋅ vertical transport, ⋅ radiocarbon, ⋅ accelerator mass spectrometry (AMS)

The concentrations of 129I in seawater samples from two sites (off Sekine and the Toyama Bay) in the Japan Sea were determined by accelerator mass spectrometry. The observed concentrations exceed amounts expected from natural origin and globally distributed fallout due to nuclear weapons testing. Because the fraction of natural origin and global fallout is 2% and 8.9–13.8%, respectively, the residual more than 80% of the concentration must come primarily from nuclear fuel reprocessing plants. This result indicates a rapid distribution of 129I through atmospheric transport on a global scale. The depth distribution of 129I at the Toyama Bay in the Japan Sea shows that the 129I maximum is in the mixed layer and that concentrations decrease with depth. The inventory of 129I in water column is four times higher than that measured in the Gulf of Mexico which has almost the same depth at the Toyama Bay. This higher inventory probably reflects: (1) the rapid water sinking in the Japan Sea, (2) the difference of distance in sampling locations with respect to major 129I releasing plants and (3) the strong increase in emissions from nuclear fuel reprocessing plants after the profile of the Gulf of Mexico was taken.

In 1995 and 2000, the radiocarbon ratio (Δ14C) of total dissolved inorganic carbon was measured in the Japan Sea where deep and bottom waters are formed within the sea itself. We found that (1) since 1979, the Δ14C in bottom water below about 2000-m depth in the western Japan Basin (WJB) had increased by about 30‰ by 1995, and (2) the bottom Δ14C in the WJB did not change between 1995 and 2000. The former finding was due to penetration of surface bomb-produced radiocarbon into the bottom water owing to bottom ventilation, whereas the latter was caused by stagnation of the bottom ventilation there. In the eastern Japan Basin (EJB), the bottom Δ14C also increased by about 30‰ between 1979 and 2002. Recent stagnation of the bottom ventilation in the EJB is also suggested from analyses of constant bomb-produced tritium between 1984 and 1999. The temporal variations of Δ14C, tritium, and dissolved oxygen in the bottom waters indicate that: (1) new bottom water is formed south of Vladivostok in the WJB only in severe winters; and (2) the new bottom water then follows the path of a cyclonic abyssal circulation of the Japan Sea, which results in the increases in dissolved oxygen and the transient tracers in the bottom waters in the EJB and Yamato Basin with an approximate 3-to 6-year time lag. This process is consistent with the spatial variations of Δ14C, bomb-produced 137Cs, and chlorofluorocarbon-11 in the bottom waters of the Japan Sea.

A numerical simulation model system that consists of an ocean current model, Princeton Ocean Model (POM), and a particle random-walk model, SEA-GEARN, has been developed to describe the migration behavior of non-conservative radionuclides in a shallow water region. Radionuclides in the ocean are modeled in three phases, i.e., the dissolved phase in seawater, the adsorbed with large particulate matter (LPM) and the adsorbed with active bottom sediment. The adsorption and desorption processes between the dissolved and solid phases are solved by the stochastic method with the kinetic transfer coefficients. Deposition of the LPM and re-suspension from bottom sediment are also considered. The system was applied to simulate the long-term (24-year) dispersion of 137Cs actually released from the BNFL spent nuclear fuel reprocessing plant at Sellafield in United Kingdom. The calculation well reproduced the main characteristics of migration of dissolved 137Cs concentration in the Irish Sea.

Radiocarbon concentrations in the northernmost region of the Japan Sea were observed during the summer of 2002. The averaged surface δ14C (above 100 m depth) was 52 ± 8%, which is significantly higher compared with the values of the Pacific Ocean and Okhotsk Sea. The δ14C in the deep water decreased with density, and the minimum value was −70%. By analyzing 14C and other hydrographic data, we found that i) the Tsushima Warm Current Water reaches to the surface layer in the southern Tatarskiy Strait; ii) deep convection did not occur in the northernmost region, at least not after the winter of 2001–2002; and iii) the bottom water that was previously formed in this region may step down southward along the bottom slope and mix with the Japan Sea Bottom Water. Furthermore, a new water mass characterized by high salinity (>34.09 psu) was found in the subsurface layer in the area north of 46°N.

An iodine beam line has been set up at the AMS facility of the Japan Atomic Energy Agency. This beam line has a high mass resolution magnet and a high energy resolution electrostatic analyzer to remove the interfering peaks. Reproducibility of 0.53% and precision of 1.1% attained after solving some problems and realigning the equipments again are better than those of the acceptance test. It was demonstrated that this beam line has excellent measurement linearity between 10−10 and 10−12 iodine isotopic ratio by testing standard samples which have a variety of iodine isotopic ratios. Its detection limit is substantially below 10−13 iodine isotopic ratio. Evaluating these performance tests, it was found that this iodine line would be a potentially powerful tool not only for monitoring around a nuclear facility but also for geochemical studies such as dating and tracing in hydrogeologic and oceanographic research.

A dissolved radionuclide migration code system that consists of a ocean circulation model, Princeton Ocean Model, and the particle random-walk model SEA-GEARN has been developed. The oceanic migration of 137Cs discharged from a nuclear submarine in a hypothetical accident at the Tsushima Strait in the southwestern area of the Japan Sea was calculated as a model application. The calculations for instantaneous releases on different dates, every 10 days over one year, were carried out to study the seasonal differences of the migration process of the dissolved radionuclides. The migration tendencies of dissolved radionuclides were divided into two patterns. In the releases from January to September, all of the high concentration areas migrated to the northeast along the coastline of the Main Island of Japan from the release point. In the releases from October to December, some high concentration areas migrated to the west from the release point, and the concentration of 137Cs along the coastline of the Main I...

Under an IAEA's Co-ordinated Research Project "Worldwide Marine Radioactivity Studies (WOMARS)" 90Sr, 137Cs and (239,240)Pu concentration surface water time series in the Pacific and Indian Oceans have been investigated. The Pacific and Indian Oceans were divided into 17 latitudinal boxes according to ocean circulation, global fallout patterns and the location of nuclear weapons test sites. The present levels and time trends in radionuclide concentrations in surface water for each box were studied and the corresponding effective half-lives were estimated. For the year 2000, the estimated average 90Sr, 137Cs and (239,240)Pu concentrations in surface waters of the Pacific and Indian Oceans varied from 0.1 to 1.5 mBq/L, 0.1 to 2.8 mBq/L, and 0.1 to 5.2 microBq/L, respectively. The mean effective half-lives for 90Sr and 137Cs in surface water were 12+/-1 years for the North, 20+/-1 years for the South and 21+/-2 years for the Equatorial Pacific. For (239,240)Pu the corresponding mean effective half-lives were 7+/-1 years for the North, 12+/-4 years for the South and 10+/-2 years for the Equatorial Pacific. For the Indian Ocean the mean effective half-lives of 137Cs and (239,240)Pu were 21+/-2 years and 9+/-1 years, respectively. There is evidence that fallout removal rates before 1970 were faster than those observed during recent decades. The estimated surface water concentrations of 90Sr, 137Cs and (239,240)Pu in latitudinal belts of the Pacific and Indian Oceans for the year 2000 may be used as the average levels so that any new contribution from nuclear facilities, nuclear weapons test sites, radioactive waste dumping sites and from possible nuclear accidents can be identified.

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 sugge...

Spatial and temporal variations of particulate flux were observed by sediment trap experiments at three locations in the Japan Sea (western Japan Basin, eastern Japan Basin and Yamato Basin) during the period 1999–2002. Annual mean mass flux at 1 km depth was 455 mg/m 2 /day in the western Japan Basin, 252 mg/m 2 /day in the eastern Japan Basin and 147 mg/m 2 /day in the Yamato Basin. In the Japan Sea, the regional differences in mass flux were much larger than the change of mass flux at different depths in each area. At 1 km depth in the Japan Sea, more than 60% of settling particles consisted of lithogenic aluminosilicates and biogenic opal. In order to determine indicators of the origin of aluminosilicates, concentrations of 28 elements in surface sediment at six areas of the Japan Sea were compared. Regional differences in concentration were seen in manganese (Mn), cobalt (Co) and rare earth elements (REEs). The La/Yb and Mn/Al ratios were used as indicators of the origin of aluminosilicates and the “freshness” of particles, respectively. These indicators suggested three sources of lithogenic material for the Japan Sea, (1) atmospheric input of “fresh” Asian loess (i.e. Kosa), (2) lateral transport of “old” Asian loess from the East China Sea by the Tsushima Warm Current (TWC), and (3) lateral transport from the Island arc of the Sakhalin and the Japan Islands. In the western Japan Basin, particles originating from the Asian Continent were observed during winter and spring, and it comprised 84% of annual lithogenic fluxes at 1 km depth in the western Japan Basin. These particles are considered to have been supplied directly to the surface of the Japan Sea by the input of KOSA, because these particles showed the signature of “fresh” particles. On the other hand, “old” Asian loess were found in the Yamato Basin, suggesting that the TWC transported Asian loess from the East China Sea to the southern margin of the Japan Sea. A sporadic current in the southern Japan Sea is considered to play a role in bringing loess particles to the interior of the Yamato Basin. In the eastern Japan Basin, it was calculated that 87% of annual lithogenic flux originated from the Island-arc. These particles likely were transported laterally from the Japanese Islands through the water column.

Anticipating the application of 129 I as an oceanographic tracer in marine environment studies in the North Pacific, a new beamline has been set up at the AMS facility of the Japan Atomic Energy Research Institute for the measurement of iodine isotopic ratios ( 129 I/ 127 I). It is demonstrated that iodine isotopic ratios can be measured with a precision below 2%, and that the detection limit is substantially better than 10 −13 . The method of preparing AgI sample material from seawater is described.

The data stored in the IAEA's Global Marine Radioactivity Database (GLOMARD) developed in the framework of the IAEA's project "Worldwide Marine Radioactivity Studies (WOMARS)" have been evaluated for Pacific and Indian Ocean surface waters. Four anthropogenic radionuclides-- 3H, 90Sr, 137Cs and (239,240)Pu --have been chosen as the most representative of anthropogenic radioactivity in the marine environment, comprising beta, gamma and alpha-emitters which are the most frequently analysed in the marine environment and which have (with the exception of tritium) the highest potential contribution to radiation doses to humans via seafood consumption. For the purposes of this study, the Pacific and Indian Oceans were divided into latitudinal boxes for which average radionuclide concentrations were estimated for the year 2000. The highest concentrations have been observed in the Japan Sea/East Sea and the North-West Pacific Ocean, the lowest in the Southern Ocean.

In the framework of the Worldwide Marine Radioactivity Studies (WOMARS) project, water profile samples for radiocarbon measurements were taken during the IAEA'97 cruise at 10 stations in the southwestern North Pacific Ocean. While 14C concentrations were rapidly decreasing from the surface (Δ14C about 100‰) down to about 800 m at all visited stations (Δ14C about −200‰), the concentrations below 1000 m were almost constant. Some stations were in proximity to the GEOSECS stations sampled in 1973; thus, 14C profiles could be compared after a 24-yr interval. Generally, 14C concentrations had decreased in surface waters (by 50–80‰) and increased (by about the same amount) in intermediate waters when compared with GEOSECS data. In deep waters (below 1000 m), the observed 14C concentrations were similar to GEOSECS values. The bomb-produced 14C inventory had increased by more than 20% over the 24 yr from 1973 to 1997 and was estimated to be about (32 ± 5) 1012 atom m-2, with an annual 14C flux of (1.3 ± 0.3) 1012 atom m-2 yr-1. The results suggest that bomb-produced 14C has been advected northwards by the Kuroshio Current and the Kuroshio Extension and stored in the intermediate layer as North Pacific Intermediate Water.

An international sampling cruise, the IAEA’97 NW Pacific Expedition, was carried out in 1997 with the primary objective of contributing to better understanding of the present distributions of radionuclides in the open ocean and of the processes affecting their distributions in the water column, to study the sources which have introduced radionuclides to the Pacific Ocean, and to compare present results with historical data sets obtained from the previous expeditions (e.g., the Geochemical Ocean Sections Programme, GEOSECS). The observed temperature, salinity, dissolved oxygen, and silicate levels in water samples collected showed differences in the upper 1000 m layer in all studied profiles. Transuranics water profiles showed typical sub-surface maxima, with concentrations less by about a factor of 4 than the GEOSECS results, at depths greater by about a factor of 2 and a decrease in the water column inventory by about 20% over 24 years, which were less a function of particle-driven vertical processes and more in response to water mass circulation in the region. Increases in concentrations were observed in deep water near the sea floor. Transuranics inventories in the water column substantially exceeded those expected from global fallout. 90Sr and 137Cs data confirm that the observed changes in concentration profiles have resulted from physical circulation of the regional water masses and transport of radionuclides to deep waters. The IAEA’97 results present the most comprehensive recent study on the distribution of 239,240Pu, 90Sr and 137Cs in the NW Pacific Ocean with a description of the behaviour of these radionuclides in the water column and the effects of the physical forcing of water mass circulation over the last 24 years.

90 Sr, 137 Cs and 239+240 Pu data covering three decades were examined for temporal and spatial variations of their concentrations and inventories in waters of the Japan Sea/East Sea. The time trend of radionuclide concentrations in surface water showed a gradual decrease over the whole period and the effective half-lives of 90 Sr, 137 Cs and 239+240 Pu were estimated to be about 15, 20 and 15 years, respectively. The averaged concentrations of 90 Sr, 137 Cs and 239+240 Pu for the year 2000 were estimated to be 1.6, 2.8 Bq m −3 and 6.6 mBq m −3 , respectively. 90 Sr and 137 Cs concentrations in the south of the Japan Sea were higher than the concentrations in the north, whereas 239+240 Pu showed a reverse pattern. The radionuclide concentrations decreased with time in the surface layer and increased in the subsurface layer for 90 Sr and 137 Cs, or increased below the subsurface layer for 239+240 Pu. The averaged inventories of 90 Sr, 137 Cs and 239+240 Pu in Japan Sea waters in the 1990s were estimated to be 1.8 , 3.1 kBq m −2 and 63 Bq m −2 , respectively, and were 30–40% higher than in the Pacific Ocean. The area of high inventory intruded from the north of the Japan Sea into the south, and the low inventory region appeared in the central part of the Sea around the Yamato Rise. Distinctive changes and distributions of radionuclides found in this study are explained by the geographic and oceanographic characteristics of the Japan Sea.

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.

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.

Radiocarbon measurements in seawater samples taken at six stations in the southwestern North Pacific visited during the International Atomic Energy Agency (IAEA) 1997 Pacific Ocean Expedition were carried out at the accelerator mass spectrometry (AMS) facility of the Japan Atomic Energy Research Institute (JAERI). Three stations were located close to GEOSECS stations, and three were in the vicinity of Bikini and Enewetak Atolls, which may be influenced by former nuclear weapons testing. Compared with the GEOSECS data (1973), our results show an increase of 14C in intermediate waters. Furthermore, it is estimated that bomb-produced 14C inventories in the water column have increased by more than 20% during the last 24 years. However, vertical profiles of Δ14C at the stations near Bikini and Enewetak Atolls show a similar general trend to those found in other stations.

Radiocarbon in dissolved organic matter in seawater gives information on the dynamics and origin of the dissolved organic matter. Here, we present the vertical profile of Δ14C of dissolved organic carbon in the western North Pacific Ocean. The profile was almost parallel to that for Δ14C of dissolved inorganic carbon, indicating that the time scale of seawater circulation was the main influence on the Δ14C signature of dissolved organic carbon. On the basis of the Δ14C differences between dissolved organic carbon and dissolved inorganic carbon, and a comparison with Δ14C of dissolved organic carbon in the central North Pacific Ocean, we suggest that there may be a source of younger dissolved organic carbon in the western North Pacific Ocean.

Anticipating the release of 129 I from a new spent nuclear fuel reprocessing plant at Rokkasho in Japan, the levels of 129 I in seawater before its operation were measured at the western North Pacific Ocean and the Japan Sea from surface to bottom. Iodine isotopic ratios ( 129 I/ 127 I) in surface seawater at the offshore of Kushiro, the Japan Basin and the Yamato Basin were (7.1 ± 0.3) × 10 −11 , (5.8 ± 0.2) × 10 −11 and (4.6 ± 0.2) × 10 −11 , respectively. The 129 I/ 127 I in the deep layer at the offshore of Kushiro below 1500 m depth was constant and its ratio was (1.5 ± 0.4) × 10 −12 . Since this ratio agreed with the natural origin 129 I/ 127 I corresponding to (1.5 ± 0.015) × 10 −12 , the anthropogenic 129 I have not invaded the deep layer below this depth. On the other hand, 129 I/ 127 I in the bottom layer below 2000 m depth at the Japan Basin and Yamato Basin was also constant but their ratios were (7.1 ± 0.8) × 10 −12 and (9.1 ± 0.6) × 10 −12 , respectively. Taking account of the natural 129 I/ 127 I, anthropogenic 129 I was intruded into the basin bottom of the Japan Sea, completely. Furthermore, the inventory of anthropogenic 129 I for the Japan Basin and Yamato Basin was three times higher than that in the offshore of Kushiro. These results would be due to the characteristics for the ocean circulation and the bottom topography in the Japan Sea.

A new AMS facility has been set up at the Japan Atomic Energy Research Institute (JAERI) in 1997 and 14C as well as 129I will be used for marine environmental studies. The 3 MV AMS system is able to perform high-precision 14C as well as heavy element AMS. The 14C-AMS section of the system has been accepted and its precision is comparable to other high-precision 14C-AMS systems. The heavy element section is under preparation and close to acceptance. A sample preparation system for the extraction of dissolved inorganic carbon (DIC) from seawater has been built up and tested. An accuracy check of the sample preparation system and the 14C-AMS system has been carried out with internationally accepted 14C standard material.

IAEA-MEL has been engaged in an assessment programme related to radioactive waste dumping by the former USSR and other countries in the western North Pacific Ocean and its marginal seas. This paper focuses on the Sea of Japan and on estimation of collective doses from liquid radioactive wastes. The results from the Japanese-Korean-Russian joint expeditions are summarized, and collective doses for the Japanese population by the marine food pathway are estimated from liquid radioactive wastes dumped in the Sea of Japan and compared with those from global fallout and natural radionuclides. The collective effective dose equivalents by the annual intake of marine products caught in each year show a maximum a few years after the disposals. The total dose from all radionuclides reaches a maximum of 0.8 man Sv in 1990. Approximately 90% of the dose derives from 137Cs, most of which is due to consumption of fish. The total dose from liquid radioactive wastes is approximately 5% of that from global fallout, the contribution of which is below 0.1% of that of natural 210Po.

Assessment of contamination of anthropogenic radionuclides from past dumping of radioactive waste in areas of the Okhotsk Sea, NW Pacific Ocean and the Sea of Japan/East Sea has been performed. Two joint Japanese-Korean-Russian scientific expeditions were carried out in 1994-1995, where seawater and seabed sediments were samples from 22 sites. Results of sediment analysis are reported here, where concentrations of 90 Sr, 137 Cs, 238 Pu, 239,240 Pu and 241 Am in surface layer and bulk sediments showed on large spatial variations, ranging between < 0.04 and 0.75 Bq kg 1 dry wt., < 0.2 and 7.2 Bq kg -1 dry wt., < 0.001 and 0.08 Bq kg -1 dry wt., 0.006 and 2.0 Bq kg -1 dry wt., 0.03 and 1.8 Bq kg -1 dry wt., respectively. However, the concentrations are comparable with those found in reference sites outside the dumping areas and they generally fall within ranges previously reported for non-dumping areas of the investigated seas. Estimates of sediment inventories indicated differences in radionuclide load between shelf/slope and basin type sediments as well as dependence on water depth. Except for the shallow areas, most of the inventories of 90 Sr, 137 Cs and Pu isotopes are still to be found in the water column. Total inventories (in water + sediment) show a surplus of 137 Cs and Pu-isotopes compared to expected integrated global fall-out deposition, which is consistent with previous observations in non-dumping areas in the seas investigated. Analysis of sediment 238 Pu/ 239,240 Pu activity ratios showed values in accord with that of global fall-out. Analysis of radionuclide depth distributions in core samples from areas of the Sea of Okhotsk showed sedimentation rates of 0.2-0.4 g cm -2 year -1 and 0.03 g cm -2 year -1 for shelf and basin areas respectively, which is similar to values found in the Sea of Japan/East Sea. Depth profiles of 90 Sr, 137 Cs and Pu isotopes in cores of the basin area indicate a typical delay compared to the input records of global fall-out.

A dispersion of radionuclides ( 3 H, 90 Sr, 137 Cs, 239 Pu) potentially released from the Mururoa and Fangataufa atolls through the South Pacific Ocean has been studied by means of computer models. The models used consisted of three differently structured compartmental models for the regional field and a hydrodynamic world ocean model for the far-field simulations. The outcome of regional modelling is predicted activity concentrations with time in different regions of French Polynesia (over up to 10 000 years for plutonium). The far-field model simulates large-scale dispersion in the South Pacific Ocean over periods of up to 50 years. The overall result suggests that there will not be radioactive contamination of any radiological interest at inhabited sites in French Polynesia or anywhere else in the ocean at present or in the future.

Measurements of anthropogenic radionuclides in seawater samples near dumping sites of the East Sea/Japan Sea were conducted, as the first-stage Japanese–Korean–Russian Joint Expedition, in March–April 1994, to assess radioactive contamination of the East Sea/Japan Sea after the dumping of radioactive wastes by the former Soviet Union and Russian Federation. Concentrations of 137 Cs, 90 Sr and 239,240 Pu in surface waters of the East Sea/Japan Sea near dumping sites were in the ranges of 2.8–3.6,1.6–2.0, and 8–25 Bq kg -1 , respectively. Surface 137 Cs and 90 Sr concentrations were the same order of magnitude as those observed in the North Pacific, whereas surface 239,240 Pu concentration was significantly higher than those observed in the North Pacific. Vertical profiles of 137 Cs and 90 Sr, with surface maxima and decreasing with depth, showed higher depth gradients than those observed in the North Pacific, reflecting deep convection in the north central East Sea/Japan Sea. The higher 239,240 Pu concentration in the surface waters of the north central East Sea/Japan Sea may reflect rapid recycling of deep Pu. The results revealed that most of the recent radioactivity observed in the north central East Sea/Japan Sea was of global fallout origin from atmospheric nuclear testing and partly the Chernobyl fallout. In this survey, there was no clear evidence of an increase in radionuclide concentrations due to the dumping of radioactive wastes by the former Soviet Union and Russian Federation.

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.

Internal dose coefficients for ingestion and inhalation of radioiodine were preliminarily calculated considering the metabolic characteristics of Japanese. In the calculation, the fractional uptake by thyroid (f) of iodine entering blood and biological half-life in thyroid (Tb) were reflected among the data peculiar to Japanese concerning thyroid structure and iodine metabolism in thyroid. A metabolic model for iodine used here was based on the three-compartment recycle model described in the ICRP Publication 30. As to the parameters of f and Tb, the values of 0.2 and 40 days reported for Japanese were used instead of those of 0.3 and 80 days adopted by ICRP.The dose coefficients for Japanese calculated here are smaller than those for Reference Man of Caucasian. The author expects that the coefficients will be used for radiation protection purpose for workers and adults of the public in Japan.

The performances of assessment models for estimating the transfer and bioaccumulation of mercury in a lake ecosystem were tested by being applied to a test scenario proposed in an international cooperative study named BIOMOVS. Two kinds of models have been developed to estimate mercury concentration in fish. One uses a bioaccumulation factor approach which is applied to an ecosystem in equilibrium, whereas the other is a dynamic model in which both the change of the concentration in water and the metabolism in fish are taken into account. For this scenario, the former was not adequate but the latter could predict more accurately because the concentrations were not in equilibrium. The limitations of applications were suggested for the two models employed here. It would be important in the present case to take into account that equilibrium was not attained in the ecosystem and the metabolism of mercury in the fish was very slow.

A computer code TERFOC-N has been developed to calculate doses to the public due to atmospheric releases of radionuclides in normal operations of nuclear facilities. The code calculates the highest individual dose and the collective dose from four exposure pathways; internal doses due to ingestion and inhalation, external doses due to cloudshine and groundshine. A foodchain model, which is originally referred to the U. S. NRC Regulatory Guide 1.109, has been improved to apply to not only LWRs but also other nuclear facilities. This report describes the models employed and gives a sample run performed by the code. The parameters which were sensitive to ingestion dose were identified from the results of sensitivity analysis. The models which significantly contributed to the dose were identified among the models improved and extended here.

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.

[1] The variation of velocity and volume transport of the Tsugaru Warm Current (TWC) is investigated based on the data of continuous and cross-sectional current monitoring in the Tsugaru Strait. During November 1999–March 2000, the structure of the velocity transection across the strait was almost stable in which the TWC occupied the central part of the strait, while the returning flows existed in the northern and southern part of the strait, and the velocity of these flows decreases with time. The volume transport of the TWC varies from 2.1 to 1.1 Sv, with the mean value of 1.5 Sv, following the variation of the TWC. The temporal variation of the volume transport shows a linear correlation with that of the sea level difference between the Japan Sea and the Pacific Ocean. The contribution of the sea level difference to the volume transport is estimated to be roughly 70%.

[1] The newly formed bottom water in the Japan Sea was observed in the summer of 2001 after the severe winter 2000–2001. The new bottom water, which was observed in the northwestern Japan Sea, showed low temperature, high salinity, high dissolved oxygen and low nutrients concentration compared to the old bottom water. The distribution of the new bottom water indicates that the bottom water was formed in the south off Vladivostok, not in the northern Japan Sea north of 43°N, and was advected to the observation area. It is suggested that the formation event occurred in the late January-early February 2001, because strong flows of faster than 8 cms−1 appeared abruptly from mid-February 2001. This formation event may contribute not only the relaxation of the oxygen-decreasing trend in the bottom layer but also the spin-up of the thermohaline circulation in the Japan Sea.

A heavy element AMS system, based on a 3 MV Tandetron, has been put into operation at JAERI, Mutsu, Japan. The system uses sequential injection, designed for cycle frequencies of up to 1000 Hz. The high-energy section is unique in that the identification of the isotopes of interest is done in two successive steps, each using a separate foil combined with energy discrimination. This method allows for the detection of elements that suffer from problematic isobar interference like 36Cl and 41Ca. In that case the foils are chosen to be relatively thick in order to achieve the required energy dispersion. In order to cope with the large scattering caused by the foils, the applied TOF has a unique design that features the acceptance of extremely high divergent beams of up to 80 mrad. During the acceptance tests the precision was shown to be ∼1.1% for 129I measurements. The background was found to be below 10−13.

Large quantities of radioactive wastes have been dumped in the Far Eastern Seas by the former Soviet Union and the Russian Federation, and small amounts of radioactive wastes have been dumped by Japan and the Republic of Korea. In order to investigate the concentrations of anthropogenic radionuclides in the nine dumping areas, a second expedition was conducted in 1995 by Japan, the Republic of Korea, the Russian Federation and IAEA, following the first expedition in 1994. The results show that 137Cs, 90Sr and 239 + 240Pu concentrations in surface and bottom waters at dumping areas do not significantly differ from the values observed in background areas, and from historical values. There is no clear effect of possible contamination due to radioactive waste dumping. The concentrations and water column inventories of 137Cs, 90Sr and 239 + 240Pu in the Far Eastern seas are controlled by physical oceanic processes such as horizontal transport and biogeochemical processes such as scavenging.