Transfer Parameters of Tritium from Seawater to Marine Organisms
Yoshio ISHIKAWA, Toshihiro SHIBATA, Yasuhiro TAKO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
The operation of the commercial spent nuclear fuel reprocessing plant in Rokkasho is accompanied by
the discharge of a certain amount of tritium into the Pacific Ocean. Although most of the tritium discharged
is diffused and spread widely throughout the ocean, some part will be transferred into marine organisms
living close to shore areas. Tritium in organisms is composed of two chemical forms: one is free watertritium (FWT) and the other is organically bound tritium (OBT) fixed by the organism metabolic activity. In
order to assess realistically the impact of tritium discharged into seawater to the public, it is important tounderstand the processes of transfer and accumulation of tritium from seawater to marine organisms,
especially OBT in them. In this research, the transfer rates of tritium in seawater to OBT in several marineorganisms are obtained. The OBT transfer through the food chain is also included in the scope of this study.
We have used the stable isotope of hydrogen, deuterium (D), as a substitute for tritium in our experiments.
In FY 2013, we investigated the biosynthesis and metabolism of non-exchangeable organicallybound deuterium (NxOBD) in olive flounder (Paralichthys olivaceus Temminck and Schlegel) before 1 yold. As the first experiment in FY 2014, olive flounder (1 y old) were kept in seawater containing HDO with
2.0 mmol D mol-1 H up to 530 d after starting the exposure, and NxOBD concentrations in their muscle andviscera were measured at the predetermined period. In the second experiment, olive flounder (2 y old) were
fed on a mixture of a commercial feed and a freeze-dry powder of D-labeled rockfish (Sebastes vulpesDoderlein), which was prepared by keeping in seawater with HDO. The exchangeable D in the rockfish
powder was removed by repeating freeze-drying after soaking the dry powder in water. The final mixture
feed contained NxOBD of 1.75  10-3 mol D mol-1 H. The flounder were given the pre-weighted mixture
feed once a day, and dissected at the pre-determined period. The NxOBD concentration in the muscle
increased depending on the weight of labeled feed given. In the third experiment, after olive flounder werekept in seawater containing HDO with 2.0 mmol D mol-1 H, they were cultivated for 210 d in seawater withthe background HDO to observe the excretion rate of deuterium. During the cultivation, the NxOBD
concentration in their muscle was periodically analyzed.
A compartment model for metabolism of NxOBD in olive flounder was constructed by using resultsfrom those three experiments. In the model, olive flounder consisted of a muscle NxOBD compartment anda free water deuterium (FWD) compartment which was assumed to be in equilibrium with seawater. The rate
constants of D transfer between each compartment were obtained by a least square fitting method using all
experimental data. The observed NxOBD concentrations were fairly well described by the model
calculation.

400

120

300

100

Relative NxOBD
concentration (%)

Excess NxOBD concentration(mol D mol H‐1)

500

200
10000Fig. 1

200
400
Exposed time (d)600ofolive

flounderexposure.

Open40200200400
Experiment time (d)(Paralichthys

olivaceus Temminck et Schlegal) during
HDO

600Time dependent NxOBD concentration in themuscle80diamondsshow

measured values and the solid line shows
estimated values by the metabolism model.
HDO concentration was kept to 0.2 % (mol D
mol H-1) during the experiment. BackgroundFig. 2

600

Time dependent NxOBD concentration in themuscle of olive flounder kept in seawater withthe background HDO after HDO exposure for
150 d. Open circles show measured values andthe solid line shows estimated values by themetabolism model. Horizontal axis is timeafter transferring olive flounders into seawater

NxOBD concentration was subtracted.

without spiking HDO. Vertical axis is NxOBD
concentration relative to that at the transferdate. Background NxOBD concentration wasEstimated NxOBD concentration(mol D mol H‐1)

subtracted.
1008060402000204060

80

100

Measured NxOBD concentration (mol D mol H‐1)

Fig. 3Comparison between measured NxOBD
concentration and that estimated by themetabolism model. Open squares and solid
triangles show results of 1 y and 2 y old olive
flounder, respectively. Background NxOBD
concentration was subtracted.

Fig. 4

Scheme of deuterium metabolism

model in olive flounder.