Recovery of Tritium in Room Air by Precious Metal Catalyst with Hydrophilic Substrate

The catalytic oxidation and adsorption method is considered to be a potential and reliable measure to recover tritium released into room air in fusion power plants. The activity of precious metal catalysts that are expected to be useful in recovery of tritium released into the room air is affected by moisture in the air, and tritium in the gas phase can be captured into the catalyst substrate not only through adsorption but also through isotopic exchange reaction. The simulation study on tritium behavior in the catalyst bed was carried out quantitatively on the basis of experimental results. It is confirmed by the simulation study that the installation of the preadsorption bed decreases water vapor before the gas is passed through the precious metal catalyst bed; this is an effective countermeasure against the deterioration of the catalytic oxidizing performance caused by moisture. It is also shown that large amounts of tritium can be captured by the catalyst itself when the preadsorption bed is introduced.     

Comparison of Precious Metal Catalysts with Hydrophilic Porous Substrate for Tritium Cleanup System

Abstract

Existence of the structure water in the hydrophilic porous substrate can give various effects on tritium behavior in a tritium recovery system consisted of precious metal catalyst bed and adsorption bed because the catalytic isotope exchange reaction or the isotope exchange reaction proceeds besides oxidation reaction or adsorption reaction.

Properties of Pt catalysts loaded on various hydrophilic porous substrate are compared in this study. It is known from experiment that Pt-Alumina catalyst has the largest exchange capacity and that Pt-MS 5A catalyst has the largest total capacity because it has the effective adsorption capacity. For silica gel substrate, certain transport reaction of tritium in the substrate whose rate is rather slower than the rate of isotope exchange reaction is observed and it gives peculiar undesirable quality for silica gel to use in a tritium recovery system.     

Recovery of Tritium in Inert Gas by Precious Metal Catalyst Supported by Hydrophilic Substrate

In this study, it is proposed to use exchange reaction between hydrogen isotopes in gas phase and those in hydrophilic substrate of the precious metal catalyst for the purpose of recovering tritium as molecular form from the inert gas such as the atomosphere in the secondary containment, where no oxygen is expected. The values of the equilibrium constant and reaction rate constant for this reaction are obtained for tritium-protium system and deuterium-protium system experimentally. It is also certified that the tritium concentration at the outlet of Pt-alumina catalyst bed is evaluated in good agreement with experimental values. Behaviors of the tritium cleanup system for the secondary containment applying this reaction are compared for various operational conditions, and it is concluded that isotope exchange reaction using hydrophilic catalyst bed is applicable for the secondary contaiment. The bed of the precious metal catalyst can also be active as oxidation catalyst even when air in the tertiary containment is introduced by some accidents.     

Effect of hydrocarbons on the efficiency of catalytic reactor of detritiation system in an event of fire

Detritiation system of a nuclear fusion plant is mandatory to be designed and qualified taking carefully into consideration all the possible extraordinary situations in addition to that in a normal condition. We focused on the change in the efficiency of tritium oxidation of a catalytic reactor in an event of fire where the air accompanied with hydrocarbons, water vapor, and tritium is fed into a catalytic reactor at the same time. Our test results on the effect of these gases on the efficiency of tritium oxidation of the catalytic reactor indicated; (1) tritiated hydrocarbon produces significantly by reaction between tritium and hydrocarbons in a catalytic reactor; (2) there is little possibility of degradation in the detritiation performance because the tritiated hydrocarbons produced in the catalyst reactor are combusted; (3) there is no possibility of uncontrollable rise in the temperature of the catalytic reactor by heat of reactions; and (4) saturated water vapor could temporarily poison the catalyst and degrades the detritiation performance. Our investigation indicated a saturated water vapor condition without hydrocarbons would be the dominant scenario to determine the amount of catalyst for the design of catalytic reactor of the detritiation system.     

Isotope exchange reaction between tritiated water and hydrogen on SiC

SiC has been considered as a primary candidate material for a first wall component in future fusion reactor because it has been claimed that SiC has excellent high-temperature properties, good chemical stability and low activation. However, the behavior of tritium on SiC has not been discussed yet. In this study, tritium trapping capacity on the surface of SiC was experimentally obtained at the temperature range of 25-800 °C in consideration of tritium trapping to the experimental system. The capacity, which was independent of the water vapor pressure in the gas phase and the temperature, was determined as about 10⁶ Bq/cm². The isotope exchange reaction rate between tritiated water in a gas phase and hydrogen on the surface was quantified at the temperature of 25, 500 and 700 °C in consideration of the behavior of tritium trapping at change of experimental condition by the numerical curve fitting method applying the serial reactor model. The reaction rate was observed to be constant as 3.48 × 10⁻⁵ m/s. Additionally tritium release behavior from the surface of SiC in water vapor atmosphere was predicted and compared with that for graphite and stainless steel.     

On the likelihood that underwater metal ignition is a vapor phase phenomenon

The hypothesis that the ignition of metals (particularly aluminum) under water occurs in the vapor phase is examined theoretically. An available two-phase stagnation flow film-boiling model is used to calculate the oxidizer (steam) flux towards the vaporizing metal surface. Combining this model with the fact that there is an upper limit to the magnitude of the metal vaporization rate at which the reaction regime must change from vapor to surface oxidation, as first observed by Turkdogan, Grieveson and Darken, leads to prediction of critical metal temperature below which vapor phase oxidation is impossible. The predicted critical temperatures are compared with observed ignitions of metal spheres in free-fall in water during violent metal/water interactions (steam explosions). It appears from the theory that observed underwater metal ignitions did not occur in the vapor phase.     

Desorption Rate of Adsorbed Tritiated Water from Molecular Sieve 5A by Exchange with Environmental Water Vapor

The desorption rate of tritiated water from molecular sieve adsorbed HTO, by exchange with the environmental water vapor, was measured. The molecular sieve, packed in a column, was initially changed with tritiated water and then humidified Ar gas was made to flow through it and the tritium concentration of effluent gas was measured. The desorption rate of tritiated water increased linearly with the water vapor pressure in the gas at constant flow rate. In the case where both the flow rate and the vapor pressure were kept constant, the amount of tritium left adsorbed on the molecular sieve decreased exponentially with time. It should be noted that the desorption rate was rather rapid even at room temperature and nearly all the tritiated water adsorbed on the molecular sieve was recovered by the flowing humidified gas at room temperature within several hours.     

Effect of sweep gas species on tritium release behavior from lithium titanate packed bed during 14 MeV neutron irradiation

In a fusion reactor, the prediction of tritium release behavior from breeder blanket is important to design the tritium recovery system, but the amount of tritium generated is necessary information to do that. Hence, tritium generation and recovery studies on lithium ceramics packed bed have been started by using fusion neutron source (FNS) in Japan Atomic Energy Agency (JAEA). Lithium titanate (Li₂TiO₃) was selected as tritium breeding material, and its packed bed was enclosed by the beryllium blocks, and was kept at certain temperature during fusion neutron irradiation. During irradiation, the packed bed was purged with the sweep gas continuously, and tritium released was trapped in each gas absorber selectively by chemical form. In this work, the effect of sweep gas species on tritium release behavior was investigated. In the case of sweep by helium with 1% of hydrogen, tritium in water form was released sensitively corresponding to the irradiation. This is due to existence of the water vapor in the sweep gas. On the other hand, in the case of sweep by helium without water vapor, tritium in gaseous form was released first, and release of tritium in water form was delayed from gaseous tritium and was gradually increased.     

Li transfer from a Li2O blanket

Lithium oxide is one of the most potential breeding materials among the various candidates because of its high melting point and high lithium atomic density and preferable capability to give a large tritium breeding ratio. Leak of a fair amount of water to the blanket purge gas from the cooling water system is anticipated from the experience at the cooling system of fission reactors. Then, the behavior of lithium hydroxide formed in the lithium oxide blanket must be understood because lithium oxide is highly reactive with water vapor. In this work, the relation of the partial pressure of LiOH(g) with temperature or pressure of water vapor is studied experimentally, and the effect of Li transfer from the blanket on tritium recovery is also discussed.     

Adsorption and Desorption Behavior of Tritiated Water on the Piping Materials

The behavior of tritium on the surface of various piping materials must be investigated for establishment of the safety confinement technology of tritium or for development of the effective fuel handling technology in a D-T fusion reactor, because tritiated water or gaseous tritium is captured on the piping surface through adsorption or isotope exchange reaction. The present authors carried out the water adsorption and desorption experiments on 304 stainless steel, copper, and aluminum in the temperature range from 5 to 100°C and in the partial pressure range of water vapor between 11.8 and 198Pa using a breakthrough method and quantified the amount of water adsorbed and the overall mass transfer coefficients in adsorption and desorption of water. It was observed in this study that aluminum adsorbed more water than stainless steel or copper. It was also observed that the adsorption and desorption rates of water for three materials showed almost the same values. The breakthrough behavior of tritiated water in a 100 m pipe of stainless steel was also evaluated applying the results of this work. It is concluded that water adsorption and desorption reactions influence the behavior of tritiated water in the piping system under the condition where the partial pressure of tritiated water vapor is lower than several pascals.     

Effect of water vapor on tritium permeation behavior in the blanket system

It is found that most hydrogen supplied to the purge gas changed to water vapor due to the water formation reaction in the early stage of the blanket operation and that physical or chemical adsorbed water is released in the high concentration into the blanket purge gas when the blanket temperature becomes higher than several hundreds of degrees K if the pre-treatment is not applied to the solid breeder materials. Effect of coexistence of water vapor in the purge gas on permeation behavior of hydrogen through F82H ferritic steel in the breeding part and palladium–silver (Pd–Ag) in the recovery part is discussed because use of them is generally considered for recovery of bred tritium from the solid blanket. Almost no decrease in permeation rate of F82H is observed in this study when water vapor exists in the blanket purge gas. The permeability of hydrogen isotopes through the Pd–Ag pipe gradually decreases when water vapor exists in the blanket purge gas. Properties required in estimation of the hydrogen permeated to the purge gas are experimentally obtained in this study.     

Tritium release behavior from the graphite tiles used at the dome unit of the W-shaped divertor region in JT-60U

Release behavior of tritium from the graphite tiles used at dome top and inner dome wing in JT-60U was investigated by the thermal desorption method in dry argon, argon with oxygen and water vapor, or argon with hydrogen. It was found that approximately 20-40% of total tritium is left in graphite even after heating to the high temperature above 1000 °C in dry argon. The residual tritium could be removed by exposing the graphite tile to oxygen with water vapor or hydrogen at the high temperature above 1000 °C. The tritium retention of the dome top tile was quantified as 84-30 kBq/cm². The inner dome wing tile had a steep tritium distribution from 8 to 0.1 kBq/cm². It is observed that a measurable amount of tritium existed in the deep site of the graphite tile.     

Overview of R&D activities on tritium processing and handling technology in JAEA

In JAEA, the tritium processing and handling technologies have been studied at TPL (Tritium Process Laboratory). The main R&D activities are: the tritium processing technology for the blanket recovery systems; the basic tritium behavior in confinement materials; and detritiation and decontamination. The R&D activities on tritium processing and handling technologies for a demonstration reactor (DEMO) are also planned to be carried out in the broader approach (BA) program by JAEA with Japanese universities. The ceramic proton conductor has been studied as a possible tritium processing method for the blanket system. The BIXS method has also been studied as a monitoring of tritium in the blanket system. The hydrogen transfer behavior from water to metal has been studied as a function of temperature. As for the behavior of high concentration tritium water, it was observed that the formation of the oxidized layer was prevented by the presence of tritium in water (0.23 GBq/cc). A new hydrophobic catalyst has been developed for the conversion of tritium to water. The catalyst could convert tritium to water at room temperature. A new Nafion membrane has also been developed by gamma ray irradiation to get the strong durability for tritium.     

Detritiation of Glovebox Atmosphere by Using Compact Tritium Removal Equipment

A compact tritium removal equipment (TRE), assembled in a console with casters, has been developed for detritiation of air in a glovebox used for handling of several curies of tritium. The TRE was designed to remove gaseous tritium in the form of T₂, HT and CH₃T through oxidation with precious metal/alumina catalysts followed by adsorption on zeolite pellets.

From the detritiation experiments with hydrogen tritide (HT, 2–20 mCi), the TRE was confirmed to have sufficient performance for the practical use. The tritium concentration in the test gas (total volume –32l; 1%H₂, 5%O₂, 94%N₂) decreased from 0.64 to 6.4 ×10-⁷ Ci.m³ within 155 min when the TRE was operated under the recirculation mode with the flow rate of 200 l-h¹ at the catalyst temperature of 200°C. In addition, the HT-to-HTO fractional conversion was determined at various catalyst temperatures (25–200°C) and flow rates (100–360 lh-¹).     

氚在RAFM钢表面的吸附研究

采用气相吸附法研究了室温下RAFM钢表面对氚的吸附与释放行为,并使用316L钢、1Cr18Ni9Ti钢进行了对照实验。结果表明,RAFM钢表面的氚吸附与释放性质与316L钢、1Cr18Ni9Ti钢的非常相似,相同表面状态的样品,在相同实验条件下的吸附氚量相差不超过50%。可推测,未经深度除水处理的RAFM钢暴露于氚后,表面会形成富氚层,浓度远高于基体溶解氚,厚度不大于10 μm。表面氚的形态以化学吸附和物理吸附的氚化水为主,约占90%以上。室温下RAFM钢表面吸附的氚在干燥气氛中的释放非常缓慢,但遇水会因氚-水间的同位素交换而加速释放。     

金属Ce在高温蒸汽下的氧化动力学初步研究

Ce常用于模拟研究U、Pu等锕系元素的腐蚀行为和转化行为,其在高温水蒸气条件下的氧化动力学目前还未见报道。本研究采用热重分析仪对金属Ce的高温水蒸气氧化动力学开展了研究,得到了动力学曲线,并对反应过程进行了探讨,金属Ce的水蒸气氧化动力学过程呈现明显的三个阶段,反应后主要产物为CeO₂和氢气。     

铀及其合金的水氧腐蚀机理分析

铀及其合金是重要的核材料,其水氧腐蚀研究受到广泛关注。本文系统总结了铀及其合金在真空和大气环境下的氧化反应过程、腐蚀速率和产物,并对不同研究者提出的反应机理进行讨论分析。其中铀-氧反应过程为氧气在表面的吸附、解离,O2-在晶格中进行扩散,O2-在金属和氧化物界面发生反应,其中扩散是反应速率的控制步骤。铀-水体系反应机理存在争议:进行扩散反应的离子是O2-还是OH-还没有一致的认识。铀-氧-水体系的反应是氧气和水气共同作用的结果,反应速率介于前两种体系之间。     

Influence on moisture and hydrocarbons on conversion rate of tritium in catalytic reactors of fusion-DEMO detritiation system

Thoughtful consideration of abnormal events such as fire is required to design and qualify a detritiation system (DS) of a nuclear fusion facility. Since conversion of tritium to tritiated vapor over catalyst is the key process of the DS, it is indispensable to evaluate the effect of excess moisture and hydrocarbons produced by combustion of cables on tritium conversion rate considering fire events. We conducted demonstration tests on tritium conversion under the following representative conditions: (I) leakage of tritium, (II) leakage of tritium plus moisture, and (III) leakage of tritium plus hydrocarbons. Detritiation behavior in the simulated room was assessed, and the amount of catalyst to fulfill the requirement on tritium conversion rate was evaluated. The dominant parameters for detritiation are the concentration of hydrogen in air and catalyst temperature. The tritium in the simulated room was decreased for condition (I) following ventilation theory. An initial reduction in conversion rate was measured for condition (II). To recover the reduction smoothly, it is suggested to optimize the power of preheater. An increase in catalyst temperature by heat of reaction of hydrocarbon combustion was evaluated for condition (III). The heat balance of catalytic reactor is a point to be carefully investigated to avoid runaway of catalyst temperature.     

空气除氚系统中氚氧化催化剂的研究进展

在大型氚设施中空气除氚系统必不可少,通过气-水转换除去气态氚是目前应用最广泛也是最有效的工艺,过程中氧化催化剂至关重要。总结了气态氚的催化氧化研究进展、催化剂的催化性能及影响催化性能的主要因素。贵金属Pt和Pd在室温下对氚的转化效率接近100%,因而被广泛用于氚的催化氧化。通过负载分散载体、添加催化助剂、使用规整结构催化剂、设计新型的催化反应器能够进一步提高催化剂性能。以蜂窝状催化剂为研究热点的规整结构催化剂以其高比表面积和低压力降而显示出良好的催化性能,将它用于氚的催化氧化,是该领域的一个研究方向。氢、氘、氚在氧化过程中的同位素效应会影响除氚效率,需进行深入研究。     

Hydrogen Extraction Characteristics of Proton-conducting Ceramics under a Wet Air Atmosphere for a Tritium Stack Monitor

For the purpose of designing a tritium monitoring system combined with proton-conducting ceramics as a membrane separator, the hydrogen pump characteristics of CaZr_0.9In_0.1O_3–α proton-conducting ceramics were evaluated. In the experiments, argon gas containing 20.7% oxygen and 1.2% water vapor was fed to the anode at a rate of 47–137 ml/min at 600–800°C and an applied voltage until 3.5 V. The resulting hydrogen evolution rate reached maximally 0.67ml/min and the hydrogen recovery rate was 60%. However, the proton transport number decreased to 0.52 because the electron-hole current increased along with protonic current according to the defect equilibrium reaction occurring under a wet atmosphere containing oxygen. During operation, the hydrogen evolution rate fluctuates over time by at least 0.1 ml/min, which is approximately 20% of the hydrogen evolution rate. Additionally, the hydrogen evolution rate increased with an increase in the partial pressure of water vapor at the anode. It is important to design the tritium monitoring system taking into consideration the fluctuation in hydrogen evolution rate.