Environmental Behavior and Biological Conversion of Tritium -Preriminary Release Experiment of Heavy water in a Greenhouse-

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Tritium is released from nuclear facilities into the environment during ordinary operation or by an accident. Tritium toxicity is markedly different depending on its chemical form in the environment, such as tritiated water (HTO), molecular tritium and organically bound tritium, and HTO is 25,000 times much radiotoxic than molecular tritium. Therefore, from a health physics view point, studies on tritium behavior in the environment are necessary for an accurate dose assessment, that is, actual HT oxidation in the environment, transfer of HTO to soil and environmental organisms and OBT formation and tritium elimination. However, field experiments on environmental behavior of tritium is difficult to do due to the public acceptance problem although such information under various local environmental conditions is needed to assess an accidental or chronic release of tritium. Thus heavy water (D_2O) vapor release experiments into a plastic greenhouse put on a field at Mito site of Ibaraki University were undertaken in November 1995 and August 1996, using D_2O as a substitute for HTO.The following studies were conducted : uptake kinetics of D_2O into leaves of vegetation including rice, Komatsuna, radish and so on, D_2O concentration ratios in edible vegetation to air moisture under steady-state, OBD formation in vegetation, deuterium elimination under the natural condition after D_2O release and effects of rainfall on deuterium elimination from vegetation, transfer of D_2O in water and aquatic organisms, deposition of D_2O into soil and its reemission, and formation of molecular deuterium and deuteriated methane.