Fig. 4-16 Environmental behavior of tritium after tritium gas was released to the atmosphere

Fig. 4-17 Time-variations of tritium gas (HT) and tritiated water (HTO) concentrations in air HTO was immediately detected in air after the start of HT release. The HTO concentration reached steady state about 10 days after the start of HT release. The HTO concentration was then two orders of magnitude smaller than the HT concentration.

Fig. 4-18 Time-variations of organically bound tritium (OBT) in plants OBT concentrations in plants did not attain steady state during the exposure period of 12 days. The production rate of OBT differed with different plant species and parts, and increased in the following order: fruits < roots < leaves.

Tritium will be used as a fuel in a future fusion reactor. From the viewpoint of environmental safety, it is important to predict and assess the radiation dose to the public due to tritium released to the environment. Information on the environmental behavior of tritium is necessary for precise dose assessment. In particular, tritiated water (HTO) is taken up by humans about 10,000 times more efficiently than tritium gas (HT). Therefore, in the event of an HT release, the oxidation of HT to HTO by microorganisms in the soil, followed by HTO transfer among air, soil, and plants, and organically bound tritium (OBT) formation in plants by photosynthesis should be taken into account (Fig. 4-16). In cooperation with the Atomic Energy Commission Limited of Canada, JAERI conducted a chronic HT release experiment in an open field to examine the environmental consequences of normal operation of a fusion reactor. The experiment has provided a wealth of unique information on the behavior of HTO and OBT in the environment, such as the time required to reach steady state, HTO concentration in air (Fig. 4-17) and the differences in OBT production rates by plant parts (Fig. 4-18).

References H. Noguchi et al., Tritium Behavior on a Cultivated Plot in the 1994 Chronic HT Release Experiment at Chalk River, Fusion Technol., 28, 924 (1995). H. Amano et al., Formation of Organically Bound Tritium in Plants during the 1994 Chronic HT Release Experiment at Chalk River, Fusion Technol., 28, 803 (1995).

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