** 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***:*** ^{137}Cs, ^{90}Sr, 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, ^{137}Cs and ^{90}Sr,
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 ^{137}Cs
and ^{90}Sr, 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 ^{137}Cs and ^{90}Sr into the sea by unit deposition
density (1MBq/km^{2}) happens, resulting effective dose commitment of
a member of the critical group would correspond to 2.6 X 10^{-3}Sv
to 4.1 X 10^{-3} Sv
and 2.4 X 10^{-4} Sv
to 6.3 X 10^{-4} Sv
for ^{137}Cs and ^{90}Sr, 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 ^{137}Cs into the sea would result in an internal
exposure of the population approximately one order of magnitude higher than that
of ^{90}Sr, 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 ^{137}Cs from seawater to the human body, whereas more
than 80 % of the total was delivered for ^{90}Sr 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.

Publication:

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