A. Maekawa

H. Kakiuchi

K. Miyamoto

M. Ando

M." Tanaka

N. "Momoshima

R. Ichikawa

S. Sugihara

S. Yokoyama

T. Fukutani

T. Takahashi

T. Tamari

National Institute for Fusion Science (NIFS-Repository)

  i) Introduction   Many studies were devoted to elucidate Plasma Surface Interaction (PSI) and hydrogen isotope transportation and purification understanding for tritium transfer dynamics in fuel cycle for fusion nuclear system. However, it is quite difficult to apply these data derived under well-controlled condition to actual and larger scale plasma devices or fusion devices due to lots of uncertainties. Therefore, the key parameters should be elucidated for the extrapolation of laboratory data (well-controlled condition data) to actual reactor design and it is necessary to treat deuterium and tritium cycle control and tritium recycle to introduce as one system. In our previous work,1) Plasma experimental device, namely EXPRESS (Effluence and Exchange Probe for Recycling Estimation at NIFS and Shizuoka University) was established to study plasma-surface-interaction (PSI) and tritium recovery & storage behavior. In this study, the fuel circulation system has been established and exhaust hydrogen gas obtained from PSI part was recovered, purified, stored and finally, re-introduced into PSI part. The key parameters correlated with hydrogen behavior in these parts such as hydrogen concentrations or throughput speeds were evaluated.   ii) Establishment of purification system   The schematic view of experimental apparatus was shown in Fig. 1. The deuterium (D2) gas with the flow rate of 2 sccm was introduced into PSI part and the plasma experiment was done. The exhaust gas was stored in the preserver tank. Thereafter, it was primarily purified using the zirconium cobalt (ZrCo) bed at 973 K. After the primary purification, the secondary purification process was performed using permeation technique through palladium membrane manufactured by Nihon pionics to recover higher purified hydrogen isotope gas. The obtained gas was stored in ZrCo bed. The gas was re-introduced to PSI part to continue the plasma experiment. The gas composition after the primary and secondary purifications were measured by a quadrupole mass spectrometer (QMS) and the D2 concentration and throughput speed were calculated. Fig. 2 shows the typical experimental results after the primary purification. The concentration of hydrogen isotope was also shown in the top of Fig. 2. The D2 concentration in the recover gas was 98.37% and the major impurity of nitrogen was detected before purification. As the purification was proceeded, the D concentration in exhaust gas was reached to be 99.98%, which was much higher than the requirement (99.50%) of secondary purification by Pd membrane with the flow rate of 4.4 sccm. By the secondary purification, 99.98% of hydrogen isotope was derived. It is thought that 0.02% of impurities was contaminated from the wall of tubing. At the palladium membrane, hydrogen gas can be purified with the maximum of 1500 sccm and ZrCo can absorb hydrogen gas with maximum of 250 sccm.  In the initial test, the isotope ratio of supply gas from ZrCo bed was consisted of 50% of D2 and 50% of the others, HD and H2. However, in the second test, highly purified D2 gas can be supplied to PSI part. In addition, recovery ratio of D2 gas by ZrCo bed was 0.5% in the initial test, but it was improved until 50% in the second test. It was found that primary purification process would be the determining step in the EXPRESS system.    1) Miura. R. .et al.:JPFR-S 10 (2013) 85.  Fig.1 Schematic view of EXPRESS                       Fig.2 The gas concentration after primary  purification  Tritium in plant is found as OBT (Organically Bound Tritium) and FWT (Free Water Tritium). The OBT which are produced by photosynthesis plays a key role in radiationexposure to human through food chain. The OBT isproduced by tritium supplied from atmospheric water vapor and from soil water because leaf water contamination occursby both waters. If atmospheric water vapor contains tritium, it would enter intercellular space of leaf through stoma and permeate into plant cells through cell membrane where photosynthesis is undergoing. The tritium moves depending on tritium concentration gradient between intercellular space and cell though water flux is always from leaf to atmosphere.If soil water contains tritium, it moves to leaf as conductingwater and is released to the intercellular space followed by atmosphere. Chloroplast in plant cell usually occupies a position close to cell membrane which makes easy and effective access to CO2 permeating from intercellular space to cell through cell membrane. The FWT concentration is evaluated from analysis of whole leaf water; therefore waterusing by chloroplast for photosynthesis would have differenttritium concentration from FWT because tritium composition of cell water close to cell membrane always vary depending on hydrogen isotopic concentration gradient.We investigated OBT formation at two different tritium contamination conditions and compared OBT/FWT. We used D2O for cultivation of plant. The soil contamination condition was modified by hydroculture of peppermint(Mentha piperita) in a conical beaker using D2O containing culture solution. Evaporation and exchange of the culture solution with ambient water vapor was avoided by sealing the mouse of the conical beaker and the plants were grown in open air. The contamination by atmospheric water vapor was modified using a culture system shown in Fig. 1. Dehydrated air with silica gel was passed in D2O solution and introduced to the chamber in which hydroculture of peppermint in a conical beaker without D2O containing culture solution was placed. The culture was carried out for 28 days. Leaf water was recovered by heating the leaf with amicrowave oven and FWD concentrations in leaf water and in the culture solution and atmospheric water vapor were determined by gas chromatography. The OBT concentrations were determined by mass spectrometry usingwater obtained by combustion of dried leaf samples. The number of leaf became about twice at the end of cultivation. The D2O concentration of the D2O containing culture solution was stable during the cultivation, 28780 ±90 ppm, while on the D2O vapor exposure the D2Orecovered at the cold trap had larger variation 12270 ±4360 ppm due to difficulty in controlling humidity in the chamber. We used two pumps and regulated the flow rate of introduction of D2O vapor to the chamber to prevent condensation of water vapor on the leaf surface at night. The OBD/FWD of leaf and stem observed is shown in Fig. 2. The OBD/FWD exceeded one for the D2Ovapor exposure while less than one for the cultivation by D2O solution. The OBD formed by photosynthesis was obviously different from the FWD that is the average of deuterium concentration of leaf water. The tendency of OBD/FWD value is quite reasonable and explains the contamination situation; in case of atmospheric contamination D2O distribution in chloroplast would bemuch higher than leaf water due to closer position of chloroplast to intercellular space and in case of soil water contamination D2O in chloroplast would be diluted with water vapor without D2O in intercellular space at lager extent than leaf water. The OBD/FWD in the stem supports the above explanation because the stem showed lower value than that of leaf in the D2O vapor exposure and higher in the D2O culture experiment. The presentresults indicate that contamination situation is importantin OBT formation in plants. Fig. 2 OBD/FWD ratio of leaf and stem exposed to D2O water vapor or culture in D2O solutionSilica gelPumpD2O Chamber Cold trapFlowmeterHygrometerFig. 1 Culture system for D2O vapor exposure253§18. Investigation on Environmental Behavior of Organically Bound TritiumMomoshima, N., Sugihara, S., Ichikawa, R.,  Maekawa, A. (Kyushu Univ.), Takahashi, T.,  Fukutani, T. (Kyoto Univ.), Tamari, T. (Kyushu Environ. Evaluation Assoc.), Miyamoto, K. (NIRS), Yokoyama, S. (Fujita Health Univ.), Ando, M. (JAEA), Kakiuchi, H. (IES), Tanaka, M.

Investigation on Environmental Behavior of Organically Bound Tritium

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Investigation on Environmental Behavior of Organically Bound Tritium

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