Orihiko Togawa

Renewal of the bottom water after the winter 2000–2001 may spin‐up the thermohaline circulation in the Japan Sea

[1]xa0The 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.

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.

The AMS facility at the Japan Atomic Energy Research Institute (JAERI)

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.

Variation of velocity and volume transport of the Tsugaru Warm Current in the winter of 1999–2000

[1]xa0The 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%.

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.

Computer Code TERFOC-N to Calculate Doses to Public Using Terrestrial Foodchain Models Improved and Extended for Long-Lived Nuclides

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.

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.

A 3 MV heavy element AMS system using a unique TOF set-up

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.