Tritium Behavior and Modeling in Man

INTRODUCTION Since, in future fusion reactors, the activity of tritium handled as the fuel is enormously large, i.e., about 10 TBq per a fusion reactor power of 1000 MW, it is necessary to develop more excellent handling technique of tritium and to store the research results related to the fusion reactor technology. In particular, it is expected that researchers and workers of the fusion reactor will be exposed to danger of tritium through tritium water, tritium gas and many kinds of tritiated compound. So it is important to investigate the tritium behavior in man and to estimate internal exposure dose of tritium, in order to avoid inflicting some damage on a person by intake of tritium. In the fusion research facility OKTAVIAN of Osaka University, Japan, utilizing 14 MeV fast neutrons produced by D-T reaction, studies on fusion nuclear technologies such as the measurement of double differential particle emission cross section (DDX) for various candidate structural materials in the fusion reactor, the blanket experiment, the shielding experiment, the material irradiation-damage experiment and so on are being carried out. The D-T neutron source requires tritium targets. At present, tritium of about 100 TBq is being kept against the maximum storage capacity of 888 TBq at the tritium target storage room in this facility. The tritium activity of a used target is usually 14.8 TBq or 370 GBq at a time, and use of a glove box restrains internal exposure due to handling of tritium target. To understand the dose distribution due to internal exposure following intake of tritium, the distribution of activity concentration and the excretion behavior of tritium after exposure in man are indispensable. However, since it is difficult to get them directly, they have to be estimated from tritium concentrations contained in bioassay samples that are urine, blood, exhaled water, feces, hair and so on. Tritium has various kinds of chemical forms in the body, therefore their distribution in the body and residence time vary, depending greatly on individuals and the ingestion path of tritium. If there is a simple model that can explain this complicated behavior of tritium in the body with a few parameters, we can estimate the internal exposure dose based on the simple method to measure the tritium activity such as an analysis of urine. The purpose of the present study is to make a simplified model so as to describe the complicated behavior of tritium in the body especially for the estimation of internal exposure dose on the occasion of ingestion of tritiated water, and for evaluation of sicknesses caused by radiation exposure. In order to analyze behavior of tritium in the body, tritium form is roughly divided into two kinds of chemical forms . One is tritiated water (HTO) called free water tritium (FWT), and the other is called organically bound tritium (OBT) of which the tritium is bound covalently to carbon in biomolecules. By assuming that tritium exists in two compartments depending on chemical forms (FWT, OBT), we can conduct the analysis with a model of the behavior of tritium in the body. However, to verify the metabolic model in man and to apply it to a case of an unplanned intake of tritiated water, it is necessary to examine various bioassay samples in man and to clarify their characteristics. In this study, we have investigated the behavior of tritium in the body through the continuous analysis for the tritium concentration of HTO and OBT forms using urine and exhaled water sample against some workers following multiple occupational exposures of tritium. Also a new model for tritium metabolism in man has been proposed and its model has been verified with the measured data.