Simplified oxygen combustion process in OBT analysis

Fujiwara, Kenso ; Kuwata, Haruka*; Misono, Toshiharu   ; Kitamura, Akihiro   

Generally, when tritium is produced or released in the environment, it is required to confirm the behavior of organically bound tritium (OBT) due to its longer biological half-life. However, sequential treatment processes are needed to measure the concentration of each type of tritium and it takes a month for measurement of Tissue Free Water Tritium (TFWT) and a few weeks for measurement of OBT. Therefore, simplification of the processes and reduction of the duration were required for the purpose of rapid analysis and obtaining more data on environmental monitoring, especially in case of accident that requires emergency response. We are applying the analytical system composed of the TFWT separation process by means of freeze-drying and the OBT separation process by means of oxygen combustion vessel. In the TFWT separation process, a heater was attached to the outside of the freeze-dried cell, and freeze-drying and heating-drying were continuously performed quickly. The system has been improved that 90% TFWT was removed by freeze-drying and 10% TFWT was removed by heating drying. Then, the freeze-dried samples were divided for reducing the sample volume of one batch. Since the time of freeze-drying depends on the size of the sample, it could be shortened to about 3 days by dividing the sample into four. It successfully leads to reduction of measurement duration, from 14 days to 3 days. And in order to prevent isotope exchange, freeze- drying and heating-drying were performed inside the evacuated cell. In the OBT separation process, the pretreatment time was shortened by reducing the amount of combustion, and the value of detection limit was evaluated. When measured by the above-mentioned method, the tritium concentration of fish samples in Fukushima was below detection limit (several Bq/kg) and committed effective dose was very small (under 10 mSv).