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

氚在核设施的正常运行或事故中释放到环境中。氚的毒性取决于其在环境中的化学形式，如氚化水（HTO）、分子氚和有机结合氚，而HTO的放射性毒性是分子氚的25,000倍。因此，从健康物理学的角度来看，研究氚在环境中的行为对准确的剂量评估是必要的，即环境中的实际HT氧化，HTO向土壤和环境生物的转移以及OBT的形成和氚的消除。然而，由于公众接受问题，氚的环境行为的现场实验很难做到，尽管在各种当地环境条件下需要这些信息来评估氚的意外或慢性释放。因此，1995年11月和1996年8月，在茨城市大学水户基地的一个塑料温室中进行了重水（D_2O）蒸汽释放实验，使用D_2O作为HTO的替代品。进行了以下研究：水稻、小松、萝卜等植被叶片对D_2O的吸收动力学，稳定状态下可食植被中D_2O浓度与空气水分的比值，植被OBD的形成，D_2O释放后自然条件下的氘消除，降雨对植被中氘消除的影响，水和水生生物中D_2O的转移，D_2O在土壤中的沉积及其再排放；分子氘和氘化甲烷的形成。

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.