Transfer of Radiostrontium and Radioiodine from Seawater to Marine Organisms
Yoshio ISHIKAWA, Shoko IMAI, Takashi TANI, Yasuhiro TAKO, Yuichi TAKAKU, Shun’ichi
HISAMATSU
Department of Radioecology
Abstract
In the safety review of the commercial spent nuclear fuel reprocessing plant in Rokkasho, a certain
amount of radiostrontium is assumed to be discharged into the Pacific Ocean. In addition, radioiodine wasdischarged to the ocean during the final test of the plant using actual spent fuel in 2006-2008. In order toassess the realistic impact of those radionuclides to the public, it is important to understand the processes of
their accumulation from seawater to marine organisms. In this research, from FY2016, the transfer rate ofradiostrontium in seawater to bastard halibut (olive flounder: Paralichthys olivaceus Temminck et Schlegel)has been studied by using stable 86Sr tracer instead of radiostrontium, while from FY 2018, it is planned tomeasure the transfer rate of radioiodine.
In FY 2016, we started exposure of one bastard halibut group, 284 d after hatching, to 86Sr-enrichedseawater with 86Sr/88Sr ratios of ~0.1495; we kept the fish at 15°C, giving then commercial bastard halibut
feed (EP-4). The accumulation of 86Sr in their muscle and bone was measured by using samples collectedsequentially up to 112 d after the start of the exposure in FY2016. We continued the exposure experiment up
to 553 d with sequential sampling and analysis in FY 2017. By using all data, we tentatively constructed amulti-compartment model for metabolism of Sr in bastard halibut, and confirmed that the model wellsimulated the measured data during the exposure period.
For validation and further tuning of the model, we planned an experiment for measuring excretion
86

rate of Sr from the bastard halibut, which had accumulated 86Sr in advance of the experiment. We started
exposure of two other groups of bastard halibut, 191 d and 276 d after hatching, to 86Sr-enriched seawaterwith 86Sr/88Sr ratios of ~0.1495 in FY 2017, and kept them under similar conditions as mentioned in thesecond paragraph. The excretion experiment will be started in FY 2018.
An air-tight acrylic tank system, which had 8 L seawater and 20 L air, was constructed for the shortterm125

I tracer experiment using bastard halibut and it was tested in the cold condition without the tracer.

Water quality level, which does not affect the survival of bastard halibut, could be maintained for 10 days inthe breeding density up to three fish per tank (5.85 g-fish weight L-1-seawater) with non-feeding. The actualtracer experiment will be done using the developed system in FY 2018.

Fig. 1Time dependency of 86Sr concentration in the muscle and bone of bastard halibut(Paralichthys olivaceus Temminct et Schlegal) during exposure to 86Sr-enriched seawater.
Circles show measured values and the solid lines show values estimated by themetabolism model. The dotted lines are the background 86Sr concentration in seawater.

Fig. 2

Scheme of 86Sr metabolism model in bastard halibut.

Table 1 Parameters of 86Sr metabolism model in bastard halibut.
Parameter

Unit

αk1
k2-1

dd-1

Value
0.049
0.037
0.0017

ConfidenceInterval of 95%
0.028 – 0.71
0.029 – 0.046
0.0012 – 0.0021