Transfer of Radiostrontium and Radioiodine to Marine Organisms
Shoko IMAI, Yoshio ISHIKAWA, Takashi TANI, Yasuhiro TAKO
Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
A commercial large-scale nuclear fuel reprocessing plant in Rokkasho, Aomori Prefecture, Japan is
now under final safety assessment by the Nuclear Regulation Authority. Radionuclides including
radiostrontium and radioiodine are discharged by the normal operation of the plant. In order to assess therealistic impact of those radionuclides, it is important to understand the processes of their accumulation fromseawater to marine organisms. In this research, we investigated the transfer of Sr and I from seawater andfeed to bastard halibut (Paralichthys olivaceus), a commercially important fish found in the coastal waters ofAomori Prefecture.
In order to investigate transfer of radiostrontium ingested as feed to bastard halibut, white leg shrimp(Litopenaeus vannamei) and Japanese medaka (Oryzias latipes) were kept in water with enriched 86Sr and87

Sr as tracers, respectively. The organisms were lyophilized, powdered, and dispersed in gelatin glue

derived from fish. The solidified glue was cut into a suitable size pieces and administered as feed to thehalibut. The concentrations of tracer Sr isotopes in the muscle and vertebra in the halibut were increased
depending on the dose.
To clarify the long-term excretion rate of Sr from the halibut, we transferred the halibut, which had
been kept in seawater with enriched 86Sr in advance, to normal seawater tanks. The fish were periodicallycollected up to 217 d after the transfer, and the concentrations of 86Sr in their muscle and vertebra weremeasured. The 86Sr concentration decrease in the muscle could be described by two exponential curves withhalf-times of 0.29 d and 1.7 x 102 d, while the decrease in the vertebra could be described by a curve withhalf-time of 2.2 x 102 d. The experiment will be continued in FY 2019, and all data obtained will be used forconstruction of a metabolic model of Sr in bastard halibut.
In FY 2018, we developed a short-term metabolism model of 125I in bastard halibut using retention
data after ingestion of 125I incorporated into Japanese medaka, which had been kept for 7 d in watercontaining 125I. Bastard halibuts aged >159 d after hatching were fed only 125I labeled medaka once a day for
1-6 d. The radioactivity of 125I in individual medaka was measured before feeding, and the medaka wasgiven whole to the bastard halibut. Bastard halibuts were collected 24 h after the last feeding and dissected,followed by the measurement of radioactivity in their tissues. In addition, certain fish in the group fed
medaka for 6 d were sampled 3 and 6 d after the last feeding. Using whole body retention data of 125I
excluding that in the gastro-intestinal (GI) tract, we constructed a single compartment model to simulate 125I
uptake and metabolism. The GI absorption ratio (f1=0.41) and elimination rate constant from the whole body(kel=0.010 h-1 corresponding to biological half-time of 2.9 d) were obtained using the least squares method.
The model adequately simulated the short-term behavior of 125I and indicated the rapid metabolism ofradioiodine in bastard halibut.

ｆ1Fig. 1kel
Whole body125I

kdecayScheme of compartment model of 125I ingested as biologically-incorporated 125I in bastard halibut.
f1, GI absorption ratio; kel (h-1), elimination rate constant; kdecay, decay constant of 0.00019 h-1.

4

Estimated radioactivity（kBq）

y = 0.94x
R² = 0.84
32

1

001

2

3

4

Measured radioactivity（kBq）
Fig. 2

Radioactivity of 125I estimated with the compartment model against the measured one in bastard
halibut after ingestion of biologically incorporated 125I.