Kenzo Munakata

Adsorption equilibria of krypton, xenon, nitrogen and their mixtures on molecular Sieve 5A and activated charcoal

The adsorption equilibria of Kr, Xe and N2, which are constituents of the off-gas from nuclear reprocessing processes, on representative adsorbents (Molecular Sieve 5A (MS5A) and activated charcoal) were studied. Adsorption experiments were conducted in the temperature range of 77 to 323 K using a packed bed column. The adsorption isotherms for the activated charcoal adsorbent were successfully correlated by the vacancy-solution model. The adsorption isotherms for the MS5A adsorbent were properly correlated by the Langmuir model and the vacancy solution model. The adsorption experiments for the binary component systems (Kr—Xe, Kr—N2 systems) were also performed, and the results suggest that the coexistence of Xe greatly inhibits the adsorption of Kr. The coexistence of large amounts of N2 was also found to inhibit the adsorption of Kr. The experimental results for the adsorption equilibrium of binary component systems on the activated charcoal adsorbent were well reproduced by the vacancy solution model w...

Adsorption of Noble Gases on Silver-mordenite

Reprocessing of spent nuclear fuel (PUREX process) produces nuclear off-gas that contains various radioactive components such as 85Kr, 129I, 3H, 14C and so forth. Except for 129I and particulates, these gases are being released to the environment. Krypton and xenon formed during nuclear fission are radioactive. Among these radioactive noble gases, the two longest-lived isotopes are 133Xe (half-life of 5.3 d) and85Kr (half-life of 10.7 yr). Krypton-85 decays with beta (99.6%) and gamma (0.4%) radiation. As a chemically inert gas with a long half-life,85Kr will continue to accumulate in the environment. Although the noble gases removal process is very complicated, future regulatory standards will probably require the radioactive krypton be removed from the off-gas of fuel reprocessing facilities before discharge to the atmosphere. Public acceptance must be considered since it could play a main role upon the approval of new facilities construction. Thus, effective technologies need to be developed before radioactive krypton regulations come into force. The volume of noble gases in PUREX process is appreciable and 80% or more of it is xenon. The half-life of133Xe is sufficiently short. Thus, provided that the irradiated fuel is stored for a period of several months prior to reprocessing, the xenon activity would be negligible. Thus this xenon could be a significant commercial source. Various processes have been proposed and investigated for the recovery of 85Kr from the off-gas. These include the cryogenic distillation process, 1,2) the membrane separation process3) and the adsorption process. 4) The cryogenic distillation is said to be promising, however related problems include higher operating cost and the potential for fire hazard caused by ozone accumulation. The problem with membrane separation is the lower throughput. The adsorption process is a simple and a reliable method and its operating cost could be made relatively low. 5,6) Test operations of the adsorption process were carried out at several institutes. However, further development of this technique was suspended due to the explosive reaction between NO X and the activated charcoal. 7)

Material conversion from paper sludge ash in NaOH solution to synthesize adsorbent for removal of Pb2+, NH4+ and PO43− from aqueous solution

Material conversion from paper sludge ash (PSA) in NaOH solution was attempted to synthesize the adsorbent for removal of inorganic pollutants, such as Pb2+, NH4+ and PO4(3-) from aqueous solution. PSA of 0.5 g was added into 10 mL of 3 mol/L NaOH solution, and then heated at 80, 120, and 160 degrees C for 6-48 hr to obtain the product. PSA mainly composed of two crystalline phases, gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8), and amorphous phase. Hydroxysodalite (Na6Al6Si6O24 8H2O) was formed at 80 degrees C, and anorthite dissolved, whereas gehlenite remained unaffected. Katoite (Ca3Al2SiO4(OH)8) was formed over 120 degrees C, and hydroxycancrinite (Na8(OH)2Al6Si6O24 2H2O) was formed at 160 degrees C, due to the dissolution of both gehlenite and anorthite. Specific surface areas of the products were almost same and were higher than that of raw ash. Cation exchange capacities (CECs) of the products were also higher than that of raw ash, and CEC obtained at lower temperature was higher. Removal abilities of products for Pb2+, NH4+, and PO4(3-) were higher than that of raw ash. With increasing reaction temperature, the removal efficiencies of Pb2+ and NH4+ decreased due to the decrease of CEC of the product, while removal efficiency for PO4(3-) was almost same. The concentrations of Si and Al in the solution and the crystalline phases in the solid during the reaction explain the formation of the product phases at each temperature.

Adsorption of Noble Gases on H-Mordenite

Release of nuclear off-gas that contains various radioactive components to the environment could jeopardize public health and safety. Therefore a screening test was carried out to find suitable adsorbents for the removal of the radioactive noble gases from the reprocessing off-gas. The adsorption capacities of various adsorbents for Kr and Xe were investigated. Carbon based adsorbents such as Ambersorb 572, Merck, Ryujou-Shirasagi G2x4/6-3 and Molsievon X2M4/6 were found to have large adsorption capacities. The H-mordenite, a non-carbon based adsorbent prepared from natural mordenite by ion exchange with a HCl solution, demonstrated a large adsorption capacity for Kr almost comparable to that of activated charcoal. Since non-carbon based adsorbents have a very important feature, that is they are not susceptible to fire hazards, the adsorption equilibria of Kr and Xe on the H-mordenite adsorbent were more thoroughly studied. The adsorption experiments on the H-mordenite were conducted in the temperature range of 77 K to 353 K using a packed bed column. The adsorption isotherms obtained for the H-mordenite were successfully correlated with the vacancy solution model over wide partial pressure and temperature ranges.

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.

Development of advanced tritium breeders and neutron multipliers for DEMO solid breeder blankets

In efforts to develop advanced tritium breeders, the effects of additives to lithium titanate (Li2TiO3) have been investigated, and good prospects have been obtained by using oxide additives such as TiO2, CaO and Li2O. As for the neutron multiplier, the development of a real-size electrode fabrication technique and the characterization of beryllium-based intermetallic compounds such as Be?Ti and Be?V have been performed. Properties of Be?Ti alloys have been found to be better than those of beryllium metal. In particular, steam interaction of a Be?Ti alloy was about 1/1000 as small as that of beryllium metal. These activities have led to bright prospects for the realization of the water-cooled DEMO breeder blanket by application of these advanced materials.

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.

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.

Development of High Performance Catalyst for Oxidation of Tritiated Hydrogen and Methane Gases

Recovery of tritium released into a working area in a nuclear fusion plant is a key issue of safety. The catalytic oxidation of isotopic hydrogen including tritium is a conventional method for removing tritium from the air of the room. If a tritium release accident occurs in the fusion plant, large volumes of air should be processed by the air cleanup system. The system should be designed to be able to process the gas with high volumetric velocity. However, the high throughput of air causes pressure drop in the catalyst bed, which results in high load to the pumping system. In this study, and their applicability of honeycomb catalysts to the tritium recovery system was examined. The honeycomb catalyst has an advantage in terms of pressure drop, which is far less than that in conventional particle-packed catalyst beds. The experiments on honeycomb catalysts such as cordierite and Al-Cr-Fe metal alloy indicate their preferable oxidizing performance. Particularly, the metal honeycomb has an advantage for hydrogen gas oxidization at room temperature because it is expected to be less affected memory effect by tritium contamination. Thus, these honeycomb catalysts are applicable to the tritiated gases recovery system with high performance.