Annual Report

58. Fate of Two Important Radionuclides in the Coastal Seas of Japan and Resultant Dose from Intake through Fishery Products

Teruhisa Watabe, Mitsue Matsuba and Setsuko Yokosuka

Keywords: 137Cs, 90Sr, radioactivity survey data, mathematical model, coastal sea, residence time, depth of mixing, effective dose commitment, collective effective dose commitment, fishery products

The fate of two important radionuclides, 137Cs and 90Sr, in the sea was investigated by analyzing radioactivity survey data, which had been collected since the 1960's. A simplified mathematical model was introduced to determine the relationship between the flux of the fallout radionuclides at the surface of the sea and their concentrations in seawater, and the parameter values determining the fate of radionuclides, namely the residence time and the depth of mixing of the radionuclides in the sea, were derived for three regions of the coastal sea of Japan by a regression analysis as reported previously in NIRS Annual Reports.

The residence time ranged from 5.3 y to 6.8 y and from 2.8 y to 5.7 y among the three regions in terms of effective half-life respectively for 137Cs and 90Sr, whereas the respective depth of mixing ranged from 46.6 m to 85.3 m and from 24.1 m to 44.1 m. It is likely that the effective half-lives observed in the present study were comparable to that of tritium observed in the early 1970's. This might reveal that both the radionuclides substantially dispersed in a manner comparable to the general water mass flow. The mathematical formulation of the fate of the radionuclides allows estimation of their time-integrated concentrations in seawater to infinity, which is basic information for assessments of dose to members of the public due to the consumption of fishery products. When a hypothetical release of 137Cs and 90Sr into the sea by unit deposition density (1MBq/km2) happens, resulting effective dose commitment of a member of the critical group would correspond to 2.6 X 10-3Sv to 4.1 X 10-3 Sv and 2.4 X 10-4 Sv to 6.3 X 10-4 Sv for 137Cs and 90Sr, respectively. The corresponding collective effective dose commitment could be determined, on the other hand, to be 8.9 X 10-4 to 4.5 X 10-3 manSv and 5.8 X 10-5 to 1.5 X 10-4 manSv, with total catch quantity for each region being taken into account. A release of 137Cs into the sea would result in an internal exposure of the population approximately one order of magnitude higher than that of 90Sr, although the time-integrated concentrations were not so greatly different between two radionuclides. This difference could be attributed entirely to the parameter values of the concentration factor of the radionuclides in marine organisms adopted. The consumption of fish obviously played an important role in the delivery of 137Cs from seawater to the human body, whereas more than 80 % of the total was delivered for 90Sr by consumption of sea weeds, which accounted for just approximately 6 % of the total consumption of fishery products on a weight basis. The results obtained in the present study will provide probable dose perspectives for an assessment of radiological impacts of the release of the radionuclides in liquid effluents from nuclear facilities.

Watabe, T., Matsuba, M and Yokosuka, S.: Proc. 10th Int. Cong. of Int. Radiat. Prot. Assoc., P-4-244, 2000.

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