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A passive catalytic reactor without heating is required to enhance the safety of a fusion facility. A precious metal catalyst without heating is not suitable to oxidize tritium under conditions of low hydrogen concentration and room temperature. In addition, under a moisture condition, tritium oxidation of a precious metal catalyst drops drastically since moisture adsorbs active sites on the surface of the catalyst. Hence, as a method of tritium oxidation under a moisture condition at room temperature, we have focused on bacterial oxidation of tritium by hydrogen-oxidizing bacteria in natural soil to realize a passive reactor. In this study, we investigated the effect of hydrogen concentration on tritium oxidation by hydrogen-oxidizing bacteria in natural soils to understand the characteristic of tritium oxidation by hydrogen-oxidizing bacteria from the viewpoint of engineering. In our experiment, efficiency of tritium oxidation by a natural soil was obtained at room temperature in the range of hydrogen concentration from 0.5 to 10000 parts per million (ppm) under a moisture condition. The efficiency of tritium oxidation was the highest at a hydrogen concentration of 0.5 ppm, which equals the value of the hydrogen concentration in air. Our results show that hydrogen-oxidizing bacteria could efficiently oxidize tritium with a low concentration of hydrogen, at room temperature, with high moisture. This showed a tendency opposite to a metal catalyst. A bioreactor using hydrogen-oxidizing bacteria complemented a conventional catalytic reactor using a precious metal catalyst since hydrogen-oxidizing bacteria could oxidize tritium efficiently with a low concentration of hydrogen, at room temperature, with high moisture.

Detritiation system of a nuclear fusion facility should maintain its detritiation performance even in an event of accident of the facility such as fire. The major technical background on design of recombiner for tritium oxi-dation are detailed kinetics data including the impact of possible gaseous impurities on kinetics, characteristics on pressure drop, impact of heat of reaction and effect of flow rate on kinetics. The volume of catalyst is evaluated by the overall reaction rate constant, required conversion efficiency and gas flow rate for processing. Our demonstration showed that the overall reaction rate constant has the effect of hydrogen concentration. Among possible impurity gases, water vapor, ammonia and halogenated acids affect tritium oxidation. Experimental demonstration revealed that the scale of recombiner have little effect on the overall reaction rate constant. The height of the catalyst layer is determined first from the allowable pressure drop considering the upstream line of the recombiner not to be pressurized. Then, the inner diameter of the recombiner is determined from the volume of catalyst. The concept of adiabatic recombiner with a vacuum jacket can be applied to avoid tritium permeation. The rate of tritiated methane produced by side reactions on the catalyst between methane and tritium was negligibly small.

核融合施設では、各種化学形で漏洩しうるトリチウムを施設内に閉じ込める設備を設け、万一の事故の発生に対してもトリチウム除去設備を用いて影響を緩和させる。トリチウム閉じ込めとトリチウムモニタリング、核融合施設の異常事象を想定したトリチウム除去設備の設計と実証研究例を紹介するとともに、固相からのトリチウム回収のあり方につき解説する。

A detritiation system (DS) is required to remove tritium from the atmosphere of a nuclear contain-ment in any extraordinary situations. Realization of a DS that does not require heating of a catalyst reactor for tritium oxidation and frequent switching operation of adsorption columns for tritiated vapor collection will greatly contribute to the improvement of fusion safety. Concerning the catalyst reactor, it has been demonstrated that tritium can be oxidized at room temperature without any heating by the developed hydrophobic catalyst. To achieve a high tritium conversion efficiency for detritiation, it has already been revealed that suppression of production of tritiated hydrocarbons by hydrogenation reactions as side reactions of tritium oxidation in a catalyst reactor is the key issue to be solved. We have to pay special attention to ethylene among hydrocarbons because ethylene is easily tritiated by reaction of hydrogenation. In this study, complete combustion of ethylene at room temperature in the catalyst reactor is proposed as a measure to suppress the formation of tritiated hydrocarbons. Catalytic combustion characteristics of hydrocarbons were obtained, and the change in the ignition temperature by a change in each design parameter of the catalyst was demonstrated. Concerning noble metal species, platinum is superior to palladium due to less susceptibility to water vapor. The smaller the particle size of noble metal is, the higher the activity is, but because it is more susceptible to water vapor, the particle size of noble metal can be optimized. It was suggested that there is an optimum value for the pore size of the catalytic support.

It is necessary to establish the closed deuterium-tritium fuel cycle in the JA DEMO facility for the purpose of fuel recycling. Tokamak exhaust processing system (TEP) is the key system where hydrogen isotopes and other impurity gases are separated, then impurity gases are decomposed into hydrogen isotopes and chemical compositions without hydrogen isotopes in their chemical forms. The main impurity gas to be processed in TEP is a tritiated hydrocarbon. There are some techniques to process tritiated hydrocarbons. Among existing techniques, chemical processing with precious metal catalyst is a practical technique and precious metal catalyst for TEP needs more consideration to improvement. Among tritiated hydrocarbons, tritiated methane is hard to decompose and temperature of catalyst for decomposition of tritiated methane is 500oC or above with a commercial precious metal catalyst. QST is developing precious metal catalyst for TEP to decompose tritiated methane effectively at a lower temperature. We found that the activity of catalyst for decomposition of methane depends on some parameters such as a kind of precious metal, particle size of precious metal on catalyst surface, pore size of catalyst carrier and so on. Our experimental result revealed that methane can be decomposed effectively by selecting the particle size of precious metal and pore size of catalyst carrier suitable for methane decomposition. In addition, the application of hydrophobization technique to a catalyst for TEP improved the efficiency of methane decomposition much more at a lower temperature., 3rd Asia Pacific Symposium on Tritium Science (APSOT-3)

In the fuel cycle of ITER and future fusion reactors, which consists of processes which are fueling, burning (by fusion reaction), vacuum pumping, purification (with recovery from tritiated impurities), isotope separation and storage, tritium accountancy control will be required from the viewpoint of operation and safety. Under ITER-EDA, as one of activities to establish the technological basis for tritium tracking and process system control, a remote and multi-location analysis system of hydrogen isotopes and tritiated molecular impurities in process gases by application of laser Raman spectroscopy was developed and validated at Tritium Process Laboratory (TPL) of JAEA [1]. Then, we had validated followability to process gas composition change and a detection limit of T2 gas of less than 1 kPa with an optimized system. More than 20 years after that development, great progress of laser and detection systems (especially, semiconductor lasers and Peltier-cooled CCD detectors, which have become very conventional and of high performance) has been achieved, which will enable us to make the system much simpler and conventional. A prototype system has been made at TPL, which consists of a semiconductor laser (488 nm, 200 mW, size: 31 mm×32.5 mm×63.5 mm), spectroscope (FL: 460 mm, 1200/mm grating) with a Peltier-cooled CCD detector, and an optical system. Results of some preliminary tests being conducted to confirm sensitivity (detection limits), stability, accuracy with static and dynamic test systems are presented., 3rd Asia Pacific Symposium on Tritium Science (APSOT-3)

Detritiation system (DS) is the key system to ensure safety of a fusion reactor. DS must be designed to make sure of detritiation when an extraordinary event such as fire happens. DS comprises catalytic reactors to oxidize tritium and a system for removal of tritiated vapor. One of chemical components of tritium to be oxidized by the catalytic reactors is tritiated methane. Tritiated methane is oxidized by a high temperature reactor at 773K generally. Assuming that an accidental release of tritium and production of hydrocarbons by combustion of cables in case of fire occurs simultaneously, tritiated methane should be oxidized at lower temperature from the viewpoint of mitigation for temperature runaway of a catalytic reactor by heat of reaction of hydrocarbons combustion and tritium permeation through a catalytic reactor of DS. In addition, there is concern generation of tritiated methane by the reaction between tritium and methane in a catalytic reactor at low temperature. In the present study, we investigated experimentally a method of processing tritiated methane by a catalytic reactor at low temperature. Our experimental investigation indicated follows: 1) a generation of tritiated methane was sufficiently small; 2) tritiated methane could be oxidized below 773K by optimization of catalyst., 31st Symposium on Fusion Technology (SOFT 2020)

重水素-トリチウム燃料で動作する核融合炉は施設内に燃料循環システムを設ける。原型炉の燃料循環システムは、ブランケットで増殖されたトリチウムの燃料純度までの精製機能と高い信頼性を兼ね備え、前例のない大流量の重水素-トリチウム流を処理しなければならない。燃料循環システムの各機器の開発状況と原型炉に向けた燃料循環システムのあり方につき解説する。, NIFS一般共同研究 研究会「原型炉におけるトリチウムの課題」

Air detritiation system of a fusion facility consists of catalyst reactors for tritium oxidation and a system to remove tritiated vapor from air. In an event of fire in the facility, the impact of gaseous impurities produced by polymeric materials on catalytic oxidation of tritium is one of the points to be evaluated. A point is the impact of reaction heat since gaseous impurities such as hydrocarbons are probable to combust in the catalyst reactors and heat of reactions is large. The distributions of temperature and concentration in a catalytic reactor is numerically evaluated using the thermophysical and chemical properties. The numerical analysis showed that the temperature in a catalytic reactor rose to 1173K by combustion of ethylene and hydrogen when air containing 1% hydrogen and 1% ethylene was fed in a reactor heated initially at 473K. Since the increase in temperature is large, additional technical consideration will be surfaced for design such as selection of material for a reactor, increase in tritium permeation and impact of excessively heated air on equipment located in the downstream. The heat removal mechanism needs to be designed for catalytic reactors., 14th International Symposium on Fusion Nuclear Technology (ISFNT-14)

ITERのトリチウム除去系では、施設で想定される火災等の異常事象発生時においても確実にトリチウムを取り除くことが求められる。本発表では、火災時に発生する各種不純物ガスの中でも触媒毒として知られるハロゲンがトリチウム除去系の触媒酸化塔の水素酸化効率に及ぼす影響を実験的に評価した結果について報告する。, 日本原子力学会2019年秋の大会参加・成果発表

The atmosphere detritiation system (DS) function is to remove tritium from the atmosphere to mitigate the impact on the environment in the case of accidental tritium release. The DS uses the technique of catalytic oxidation of tritium and collection of tritiated water vapor. The DS of ITER uses scrubber column technology for collection of tritiated water vapor, not water adsorption method conventionally used for DS, considering a failure occurrence probability of switching valves during long term operation. This technology is based on isotopic exchange between water vapor and liquid water to be arranged in a packed type column. This technology has never been applied to the DS of existing tritium facilities. Hence, qualification tests have been performed for detritiation to validate scrubber column technology for use at ITER with a pilot-scale scrubber column of 1/5 size of an ITER scale. The test column was filled with packing material and internal components of the same type as is used at ITER. In this qualification test, key objectives of the scrubber column design are to 1) collect tritium to meet DS design requirements, 2) achieve high reliability/availability and 3) avoid production of excessive tritiated water to be processed by WDS. The qualification test shows that detritiation factor (DF) is strongly influenced by a liquid flow rate with increasing the liquid flow rate resulting in higher DF. DF of scrubber column highly depends on lambda value which is defined as a flow rate of gas/liquid. A scrubber column of 2.88 m height with sufficient wettability of packing and efficient water dispersion by water distributor achieved high DF in the condition that lambda is one. Test results indicate that scrubber column technology achieves both high availability for detritiation and reduction of tritiated water production., 12th International Conference on Tritium Science and Technology (Tritium 2019)

We have performed the tritium release experiment on the fusion reactor blanket at JAEA/FNS since 2009, and then clarified the ratio of tritium release and the recovered tritium chemical form. In order to acquire the detailed tritium recovery performances, we have started a new blanket tritium recovery experiment with ionization chamber (IC) at JAEA/FNS. For the appropriate tritium measurement with IC, we improved the experimental container and carried out with an intense DT neutron source at JAEA/FNS. From our new experiment, the tritium recovery radioactivity from the LSC measurement corresponds with the calculation within 6%. However, it was pointed out that further improvement in the quantitative tritium measurement by IC method was needed.

Detritiation system is required to remove tritium from the atmosphere of a nuclear containment in any extraordinary situations. Realization of detritiation system that does not require heating of catalyst reactor for tritium oxidation and frequent switching operation of adsorption columns for tritiated vapor collection will greatly contribute to the improvement of fusion safety. Concerning on the catalyst reactor, it has been demonstrated that tritium can be oxidized at room temperature without any heating by the developed hydrophobic catalyst. To achieve a high tritium conversion efficiency for detritiation, it has already revealed that suppression of production of tritiated hydrocarbons by hydrogenation reactions as side reactions of tritium oxidation in a catalyst reactor is the key issue to be solved. We have to pay special attention to ethylene among hydrocarbons because ethylene is easily tritiated by reaction of hydrogenation. In this study, complete combustion of ethylene at room temperature in the catalyst reactor is proposed as a measure to suppress the formation of tritiated hydrocarbons. Catalytic combustion characteristics of hydrocarbons were grasped and the change in the ignition temperature by change in each design parameter of catalyst was demonstrated. Concerning on noble metal species, platinum is superior to palladium due to less susceptibility to water vapor. The smaller the particle size of noble metal, the higher the activity, but because it is more susceptible to water vapor, the particle size of noble metal can be optimized. It was suggested that there is an optimum value for pore size of catalytic support., 23rd Topical Meeting on the Technology of Fusion Energy