Transfer Parameters of Tritium from Seawater to Marine Organisms
Yoshio ISHIKAWA, Koichi ABE, Ryuji ARAI, 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. Inorder 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. However, information on the transfer of tritium from seawater to marine organisms
is quite limited. In this research, the transfer rates of tritium in seawater to OBT in several marine organisms
are obtained for the realistic dose estimation of tritium discharged from the reprocessing plant. The OBTtransfer through the food chain is also included in the scope of this study. In FY 2012, we investigated thebiosynthesis and metabolism of non-exchangeable OBD (Nx-OBD) in Ezo abalone (Haliotis discus hannai
Ino).
The stable isotope of hydrogen, deuterium (D), was used as a substitute for tritium in the following
three experiments. In the first experiment, Ezo abalones were kept in seawater containing HDO with 2000
mol D mol-1 H up to 160 d, and biosynthesis rate of Nx-OBD in their muscle and hepatopancreas wasmeasured. In the second experiment, after Ezo abalones were kept in seawater containing HDO with 2000
mol D mol-1 H for 100 d, they were cultivated for 100 d in seawater in which the HDO concentration was
the background level. During the cultivation, the Nx-OBD concentration in their muscle and hepatopancreas
were periodically analyzed. In the third experiment, Ezo abalones were fed on seaweed (Ulva. pertusa)
containing Nx-OBD with 580±86 (SD) mol D mol-1 H for 26 to 28 d. The consumed weight of theseaweed by each Ezo abalone was measured at every feeding, and the Nx-OBD concentrations in the muscle
and hepatopancreas were analyzed.
A compartment model for metabolism of Nx-OBD in Ezo abalone was constructed by using resultsfrom these three experiments. In the model, Ezo abalone consisted of hepatopancreas and muscle Nx-OBD
compartments connected to each other. A part of the OBD in food was assimilated into the hepatopancreas
OBD compartment, and the remaining part was promptly decomposed to FWD. Although a FWD
compartment in Ezo abalone was installed in the model, the HDO in the compartment was assumed to be inequilibrium with seawater. The rate constants of D transfer between each compartment were obtained by aleast square fitting method using experimental data. The observed Nx-OBD concentrations were welldescribed by the model calculation.

600
Nx-OBD concentration(mol D mol -1 H)

Nx-OBD concentration(mol D mol -1 H)

600

400200400200000306090120 150 180
Dayafterstarting
experiment(day)Days after starting experiment

0306090120
Day after starting experiment (day)Days after starting experimentFig. 1 Accumulation of Nx-OBD in Ezo abalone grown in
Fig. 2 Decrease in concentration of Nx-OBD in Ezo
seawater containing HDO withabalone grown in seawater, in which
2000 mol D mol-1 H .
concentration of HDO was background level.
（◇: hepatopancreas, ○: muscle, Line: model estimation）（◇: hepatopancreas, ○: muscle, Line: model estimation）

Fig. 3 Scheme of a compartment model for transfer and accumulation of deuterium in Ezo abalone.

Fig. 4Comparison between values measured and estimated using a model shown in Fig. 3,for Nx-OBD in muscle (right panel) and hepatopancreas (left panel).