Toshiharu Takeishi

Ionization chamber system to eliminate the memory effect of tritium

Abstract The memory effect due to the adsorption of tritium onto the electrodes of an ionization chamber sometimes reduces the accuracy of the results. Ionization chambers are generally used to monitor the tritium level in a gas stream because of their reliability, flexibility and wide range of measurement. In this paper we shown that the memory effect is mainly brought about by the transfer of tritium from the gas stream to surface water on the electrode wall by adsorption or isotope exchange reactions. A way to simulate the extent of the memory effect is proposed. An ionization chamber system which can eliminate the memory effect by applying an isotope exchange reaction is also proposed.

Behavior of tritium in a Pt-MS 5A catalyst bed

Abstract As hydrophilic porous materials usually have a certain amount of structural water, it is naturally inferred that a considerable amount of tritium can be uptaken to the precious metal catalyst with hydrophilic porous substrate through the catalytic isotope exchange reaction between gaseous tritium and structural water of the substrate and the isotope exchange reaction between tritiated water in the gas stream and structural water besides adsorption of tritiated water. Tritium uptaking performances of a Pt-molecular sieve 5A (Pt-MS 5A) catalyst are experimentally discussed in this study. Use of the precious metal catalyst bed itself as the main reactor also to uptake tritium with a removing adsorption bed is proposed in this paper.

Effect of water vapor on tritium release from ceramic breeder material

In most current designs of D-T fusion reactor breeding blankets employing Li-based ceramic as breeder materials, the use of a helium sweep gas containing 0.1% of hydrogen is contemplated to extract tritium via isotopic exchange reactions. However, at lower temperatures, the release process of tritium from the breeders is known to be rather slow. For this reason, there is still a need to develop techniques that promote the release of bred tritium. In order to improve the recovery of tritium from blankets over a wide range of temperature, the effect of the difference in sweep gas composition was investigated. Out of pile tritium release experiments were conducted using the ceramic breeders irradiated in a research reactor. The experimental results reveal the benefit of the sweep gas with water vapor, which is effective to increase the tritium release rate from ceramic breeder materials especially at comparatively lower temperatures. The experimental result was analyzed using several numerical models, and it was found that the consideration of trapping and detrapping reactions is necessary to reproduce the experimental tritium release curve.

Tritium release from catalytic breeder materials

Abstract In most current designs of D–T fusion reactor breeding blankets employing Li-based ceramic as breeder materials, the use of a helium sweep gas containing 0.1% of hydrogen is contemplated to extract tritium via isotopic exchange reactions. However, at lower temperatures, the release process of tritium from the breeders is known to be rather slow. For this reason, there is still a need to develop techniques that promote the release of bred tritium. In order to improve recovery of tritium from blankets over a wide range of temperature, platinum and palladium were deposited on the solid breeder materials such as Li4SiO4 and Li2TiO3 by the incipient wet impregnation method. Out of pile tritium release experiments were conducted using the ceramic breeders irradiated in a research reactor. The experimental results of this work reveal the benefit of the addition of catalytic additive metals, which is very effective to increase the tritium release rate from ceramic breeder materials especially at comparatively lower temperatures.

Tritium mass balance in the piping system of a fusion reactor

This paper discusses the behavior of tritium on the surface of various piping materials considering the various mass transfer steps. It is observed in this study that the isotope exchange reaction between gaseous hydrogen in gas stream and surface water and transfer of hydrogen isotopes and water through surface layer formed on materials or pores are most effective when an oxide film layer is formed on a material surface such as stainless steel. The amount of tritium sorbed on the stainless steel is correlated and compared with that observed for copper or quartz. The memory effect observed for an ionization chamber having stainless steel electrodes is also compared with that having copper electrodes.

Oxidation of tritium in packed bed of noble metal catalyst for detritiation from system gases

Abstract Catalytic oxidation rates of tritium in the bed of the noble metal catalysts are obtained and compared with the oxidation rates observed for the packed bed of spongy copper oxide or hopcalites. Use of Pt- or Pd-alumina catalysts is recommended in this study because they give effective oxidation rates of tritium in the ambient temperature range. The adsorption performance of tritiated water in the catalyst bed is also discussed.

Sorption Behavior of Tritium to Isotropic Graphite

ABSTRACTSorption behavior of tritium to isotropic graphite is discussed including solubility, diffusivity in the bulk of isotropic graphite and sorption capacity of hydrogen isotopes on the surface of isotropic graphite. The solubility and the diffusion coefficient of hydrogen isotopes in the graphite are obtained using the breakthrough method in the temperature range of 673–1273K and the activation energy of diffusivity is about 100kJ/mol. The sorption capacity of tritium on the graphite surface is also obtained by the breakthrough method using tritiated water in nitrogen gas flow. Tritium is captured on the graphite surface by isotope exchange reaction between tritium in gas flow and hydrogen in hydroxyl group on the graphite surface.These hydrogen atoms in hydroxyl group can easily exchange with other hydrogen isotopes in gas flow though they cannot be easily released from the graphite by drying or evacuating.

CATALYTIC OXIDATION OF TRITIUM IN WET GAS.

Use of precious metal catalyst is recommended in the tritium recovery system because it can oxidize tritium at ambient temperature. The ability to operate at the ambient temperature without preheating and past cooling is a large advantage in the ease of system operation. It is observed in this study, however, that the catalytic oxidation characteristics of the precious metal catalysts are largely affected by the water vapor to such extent that almost no oxidation rate of tritium is expected in the wet gas. Effect of the water vapor on the oxidation rate is quantitatively discussed based on data obtained in this study and an emergency cleanup system from the room air with pre-adsorption bed is proposed.

Enhancement of tritium release from ceramic breeders with impregnated catalytic additives

Abstract In most current designs of D-T fusion reactor breeding blankets employing Li-based ceramic as breeder materials, the use of a helium sweep gas containing 0.1% of hydrogen is contemplated to extract tritium via isotopic exchange reactions. However, at lower temperatures, the release process of tritium from the breeders is known to be rather slow. For this reason, there is still a need to develop techniques that promote the release of bred tritium. In order to improve recovery of tritium from blankets over a wide range of temperature, the effect of catalytically active metal additives on the heterogeneous isotope exchange reactions at the breeder/sweep gas interface was examined, and main results are summarized in this paper. Platinum and palladium were deposited on solid breeder material pebbles (Li4SiO4) by the incipient wet impregnation method. Experiments were performed with hydrogen and predeuterated water using packed bed reactors and gas chromatographic analysis. Out of pile tritium release experiments were also conducted using ceramic breeders irradiated in a research reactor. The experimental results of this work reveal the benefit of the addition of catalytic additive metals, which is very effective to increase the tritium release rate from ceramic breeder materials especially at comparatively lower temperatures.

Concentration profiles of tritium penetrated into concrete

Abstract Concentration profiles of tritium in cement paste, mortar and concrete were measured after exposure to tritiated water vapor for a given time. Tritium penetrated a distance of about 5 cm from the exposed surface during an exposure of 6 months. The model of tritium behavior in concrete materials reported by the present authors was developed in this study with the consideration of the effects of sand and aggregate on both the diffusion coefficient of tritiated water vapor and the isotope exchange capacity. Predictive calculations based on the tritium transport model were also carried out in some situations of tritium leakage. The results of the calculations show that a large amount of tritium will be trapped in the concrete walls, and the trapped tritium will be gradually released back to the tritium handling room over the time of months to years even after the decontamination of the room is completed.