Enhancement of pumping performance of electrochemical hydrogen pump by modified electrode

Electrochemical hydrogen pump with ceramic proton conductor membrane has been proposed to apply for a blanket tritium recovery system (BTR) of a fusion reactor. The advantage of this system is that it can apply to the system of low hydrogen pressure, because the driving force of hydrogen permeation is the electric potential difference. Perovskite-type ceramic such as SrCe_0.95Yb_0.05O₃₋ (SCO) is one of the candidates of membrane. To apply hydrogen pump to BTR, the enhancement of the hydrogen transportation capability is necessary. Modification of electrode is one of the methods to enhance the hydrogen transportation capability. In this work, the electrodes of platinum (Pt) and palladium (Pd) were attached to the SCO sample by the sputtering method (sputtering electrode), and its electric conductivity and proton conductivity were measured. Then, they were compared with that of the usual Pt paste electrode. Hydrogen transportation capability was enhanced when the sputtering electrode was applied. Especially, in case of the Pd sputtering electrode, the current density which was about 4 or 5 times larger than the usual Pt paste electrode was observed at 0.1% of H₂ concentration.     

Conceptual design of a high-voltage compact bushing for application to future N-NBI systems of fusion reactors

The energy of future neutral beam injector (NBI) heating systems of fusion power plants ranges from 1 to 2 MeV. They are based on powerful (several tens of MW) hydrogen negative ion electrostatic accelerators where electrodes are polarized by DC high-voltage. The beam line under vacuum is supplied by HV power supplies via a transmission line pressured under SF₆ and a high voltage feedthrough called bushing. The paper presents results obtained over experimental campaigns dedicated to high voltage vacuum insulation for future NBI systems (ITER). It addresses the problematic of the electron field emission and the high voltage breakdown limit under vacuum between large electrode surfaces. The paper highlights the dependence of the electron emission (dark current) with the voltage and the background tank pressure: at low pressure (～1E?3 Pa in hydrogen), an important dark current of I ～ 100 mA has been measured at 500 kV, while at higher pressure (～0.3 Pa in helium), the dark current has been nearly suppressed (less than 3 mA of dark current at 970 kV). The paper shows that a field induced gas adsorption process could occur on the emitting surfaces (cathode), and this process tends to lower the electron field emission current by increasing the work function of the electrode surface. The Fowler–Nordheim law applied to the measured dark current indicates about 70% of work function increase at 0.3 Pa in helium. Finally, a new high-voltage bushing concept relevant to the future NBI systems is presented; it is based on these experimental findings in high voltage vacuum insulation; the main feature of the new bushing concept is to take benefit of the field induced adsorption effect, i.e., the suppression of the dark current with helium gas, in the inner part of the bushing where the electric field intensity is highest.     

Mass transfer process of hydrogen via ceramic proton conductor membrane of electrochemical hydrogen pump

The blanket tritium recovery system using the electrochemical hydrogen pump with proton conductor membrane has been proposed. The feature of the electrochemical hydrogen pump is that the driving force of hydrogen transportation is a potential difference. Therefore, it might be effective to apply the hydrogen pump to the blanket sweep gas (the low hydrogen and water vapor pressure). Perovskite-type ceramic such as SrCe_0.95Yb_0.05O_3−α, is one of the candidate proton conductor for hydrogen pump and its ionic hydrogen transportation properties have been investigated. In this work, the basic mass transfer equation for hydrogen, in which the apparent proton conductivity is used as the over-all mass transfer coefficient, is proposed. And then, the apparent proton conductivities were estimated from experimental data using these equations, and mass transfer of hydrogen via proton conductor membrane was discussed by using the apparent conductivity.     

In situ ERDA measurements of hydrogen isotope concentrations in palladium at atmospheric pressure

In situ elastic recoil detection analysis (ERDA) measurements in gases at atmospheric pressure have been carried out using 15 MeV ⁴He ion beams. The beams are extracted through a molybdenum foil having a thickness of 5 μm. The maximum depth of analysis is about 4 μm for the palladium hydride and palladium deuteride (PdH_x and PdD_x, x = 0.7-0.8) samples. The temperature of the samples rises stepwise from room temperature to 180 °C. ERDA spectra are obtained every 2 min. Hydrogen and deuterium in the samples are discharged in the temperature range of 120-140 °C in a vacuum. Decrease in the hydrogen concentration in the PdH_x sample heated in a vacuum follows a first order in the value of x and an apparent activation energy of discharge of hydrogen is 1.05 eV. On the other hand, the hydrogen and deuterium concentrations decrease at about 80 °C in air. No isotope effects are observed in both a vacuum and air. The temperature at which the hydrogen concentration decreases in helium gas is almost the same as that in a vacuum. It indicates that hydrogen and deuterium atoms are discharged by chemical reactions with air and that there are no effects of cooling of the thermocouple by convection of air.     

Depth profiling of hydrogen under an atmospheric pressure

Nuclear reaction analysis of hydrogen with a use of the ¹H(¹⁵N,αγ)¹²C reaction was performed under a atmospheric condition. A 100 nm-thick silicon nitride membrane coated with gold of 10 nm was used for the extraction of the ¹⁵N beam into the sample chamber filled with gas molecules. Hydrogen contained in a Y film with a thickness of 80 nm was detected in N₂ of 10⁵ Pa. This nuclear reaction analysis (NRA) setup was also applied to H₂ gas, and the yield curve revealed a plateau feature. The plateau level was, furthermore, found to be constant independent of the H₂ pressure. We show that this plateau intensity can be used to obtain the detection efficiency of a NRA setup.     

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.     

Reaction of atomic hydrogen with chemisorbed species: C2H4 on W

Atomic hydrogen produced in controlled thermonuclear reactors is capable of very rapid reaction with both the material of the first wall and with adsorbed species. We have been investigating the reactions of thermal atomic hydrogen with ethylene adsorbed on a tungsten film. When adsorbed at room temperature ethylene reacts to yield gaseous hydrogen and a carbonaceous residue which reacts with incident atomic hydrogen to yield methane along with minor amounts of ethane. This product distribution, however, depends on the chemical nature of the adsorbed species. Adsorption at 178K, a temperature at which the ethylene molecule is stable, inverts this distribution with ethane now being the major product. Analysis of the system dynamics shows that the rate of production of each product is proportional to the product of the steady state increase in the product pressure and the pumping time constant of the vacuum system. For the case of methane production following room temperature adsorption of ethylene, the reaction probability for the production of methane for the reaction C + 4H → CH₄is ~0.001 and arguments are given for a larger value.     

Radiolytic hydrogen evolution in a closed vessel

Radiolytic hydrogen gas evolution under the liquid-gas two-phase condition has been studied using a closed vessel and γ -rays from a Co-60 source to develop an evaluation method for the H₂ evolution amount during transfer and storage of radioactive materials. An experiment was conducted using a closed vessel in which air and aerated pure water were present at room temperature. Several vessels were irradiated once with ?-rays at 5.2 × 10³ Gyh^–1. It was found that apparent G-values of H₂ production, calculated with a pressure increase of the closed vessel, became smaller for the cases of higher ratio of gas phase volume to liquid phase volume due to the recombination reaction of radiolytic H₂ with O₂ and H₂O₂. Also, equilibrium H₂ partial pressure became 10 times higher than the expected value using Henry’s law. These behaviours were explained by the developed model, which includes the liquid-gas distribution ratio of radiolytic H₂, the equation of state for H₂ in the gas phase, and the effective volume of liquid phase relevant to the liquid-gas distribution under the irradiation conditions. The effective volume of liquid phase was determined by considering the extent of the recombination reaction of radiolytic H₂ during mass transfer from the liquid phase to the gas phase.     

The inhibiting effects of hydrogen on the corrosion of uranium dioxide under nuclear waste disposal conditions

A number of electrochemical experiments were employed to investigate the effects of hydrogen on the corrosion of UO₂ under nuclear waste disposal conditions. A combination of corrosion potential (E_CORR) measurements and cyclic voltammetry have indicated that dissolved hydrogen can polarize the UO₂ surface to reducing potentials; i.e., to E_CORR values more negative then those observed under anoxic (argon-purged) conditions. A comparison of the behaviours of SIMFUEL specimens with and without incorporated noble metal ε-particles indicates that these particles may act as catalytic electrodes for H₂ oxidation, H₂ ↔ 2e⁻ + 2H⁺. It is the galvanic coupling of these particles to the UO₂ matrix which suppresses the fuel corrosion potential.     

Effects of surface impurity layers on the hydrogen permeability of vanadium

Surface impurity layers were found to reduce the hydrogen permeability of vanadium by a factor of several hundred over the temperature range from 200 to 850°C. These impurity layers formed during hydrogen permeation studies that were carried out over periods of ~1500 h under conditions where the ultimate vacuum ranged from ~10^?5 to 10^?8 torr. Studies using Auger, ion microprobe, and nuclear microprobe methods showed that the major impurity elements on the surface of contaminated vanadium samples were oxygen and carbon. The depth of penetration of oxide into the vanadium due to diffusion was found to be in excess of 10 μm. Hydrogen perméation rates determined for surface-oxidized vanadium samples appeared to have a half-power dependence on upstream hydrogen pressure. The implications of these results with respect to the use of vanadium as a structural or clad material in fission and fusion reactors are discussed.     

Study on Glow Discharge Plasma Used in Polyester Surface Modification

To achieve an atmospheric pressure glow discharge(APGD)in air and modify the surface of polyester thread using plasma,the electric field distribution and discharge characteristics under different conditions were studied.We found that the region with a strong electric field,which was formed in a tiny gap between two electrodes constituting a line-line contact electrode structure,provided the initial electron for the entire discharge process.Thus,the discharge voltage was reduced.The dielectric barrier of the line-line contact electrodes can inhibit the generation of secondary electrons.Thus,the transient current pulse discharge was reduced significantly,and an APGD in air was achieved.We designed double layer line-line contact electrodes,which can generate the APGD on the surface of a material under treatment directly.A noticeable change in the surface morphology of polyester fiber was visualized with the aid of a scanning electron microscope(SEM).Two electrode structures–the multi-row line-line and double-helix line-line contact electrodes–were designed.A large area of the APGD plasma with flat and curved surfaces can be formed in air using these contact electrodes.This can improve the efficiency of surface treatment and is significant for the application of the APGD plasma in industries.     

Preparation of Cr2O3 film by MOCVD as hydrogen permeation barrier

Cr₂O₃ film on structural material as hydrogen permeation barrier can be applied in many areas such as hydrogen storage devices, vacuum solar receivers and fusion reactors. In this study, the Cr₂O₃ film was prepared by MOCVD on 316L stainless steel using chromium(III) acetylacetonate as precursor. The film was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The hydrogen permeation inhibition performance of films was investigated by deuterium permeation experiment. The 366 nm thick Cr₂O₃ film on 316L could reduce the deuterium permeability by 24–117 times at 823–973 K, revealing efficient inhibition to hydrogen permeation. The Cr₂O₃ film is dense, crack-free and has a corundum structure which possesses a more stable structure than a metastable phase or an amorphous phase. Moreover, the crystalline Cr₂O₃ could be easily obtained by MOCVD at a low temperature, e.g. 773 K.     

On Effective Thermal Conductivity and Structural Change of UO2 Powder Compacted Fuel in Thermal Simulation Experiment

The studies described in this paper relate to the effective thermal conductivity K^eff of powder compacted UO₂ fuel material under simulated thermal conditions of a reactor, and to structural changes brought about by large thermal gradient at elevated temperatures. From the results obtained, it is concluded that: (1) Structural change in the material occurring under such thermal conditions greatly affects K_eff , which is thereby increased as a result of columnar grain growth; (2) Columnar grains developed the path of heat flow are single crystals growing in the direction (111); (3) K_eff is higher when measured during cooling than during heating, and the relation between K_eff and temperature is similar to the case of a UO₂ single crystal; (4) K_eff measured in 90% He- 10% H₂ at atmospheric pressure is much higher than in vacuum, from which it can lie inferred that the gas contained in the fuel pin has marked influence on K_eff .     

Stable absolute calibration standards for hydrogen profile analysis using nuclear resonance techniques

Samples of pyrolytic carbon, CH_x (x ≈ 0.03) from pyrolysis of methane, and amorphous silicon, SiH_x (x ≈ 0.2) from plasma deposition of silane, are shown to be stable during ion bombardment and vacuum storage. They are therefore well suited as calibration standards for hydrogen profile analysis using nuclear resonance reactions such as ¹H(¹⁵N, αγ)¹²C. Due to its high hydrogen content and the possibility of absolute calibration by vacuum extraction, tantalum hydride, TaH_x (x ≈ 0.5) has been used for the absolute calibration of these standards.     

Theoretical Study on the Characteristics of Atmospheric Radio Frequency Discharges by Altering Electrode Gap

In this paper, we present a theoretical study on the discharge characteristics of radio-frequency discharges at atmospheric pressure driven by a higher frequency of 40.68 MHz while the electrode gap is altered. Based on the analytical equations and simulation data from a one-dimensional fluid model, an optimal gap between electrodes, at which the largest electron density is obtained, can be observed under a constant power condition; however, as the electrode gap increases the time-averaged electron temperature decreases, and the underpinning physics is also discussed based on the simulation results. This study indicates that at a constant power by choosing an appropriate electrode spacing, the rf discharge can be effectively optimized at atmospheric pressure.     

Characteristics of Electrode-Water-Electrode Discharge and its Application to Water Treatment

Atmospheric air discharge above the surface of water is an effective method for water treatment.The leakage current and Joule heating of water are reduced by the air gap,which raises the energy efficiency of the water treatment.However,the application of this kind of discharge is limited by a pair of conflicting factors：the chemical efficiency grows as the discharge gap distance decreases,while the spark breakdown voltage decreases as the gap distance decreases.To raise the spark breakdown voltage and the chemical efficiency of atmospheric pressure water surface discharge,both the high-voltage electrode and the ground electrode are suspended above the water surface to form an electrode-water-electrode discharge system.For this system,there are two potential discharge directions：from one electrode to another directly,and from the electrodes to the water surface.The first step in utilizing the electrode-water-electrode discharge is to find out the discharge direction transition criterion.In this paper,the discharge direction transition criterions of spark discharge and streamer discharge are presented.By comparing the discharge characteristics and the chemical efficiencies,the discharge propagating from the electrodes to the water surface is proved to be more suitable for water treatment than that propagating directly between the electrodes.     

Low voltage hydrogen plasma interaction with palladium surface

Hydrogen penetration, distribution and content in palladium under low voltage glow discharge plasma, depending on plasma component structure (H₂, He, Ar and their mixtures), irradiation succession and possible surface structure changes have been investigated, using mass-spectrometry, SIMS, proton-proton scattering and electron microscopy techniques. Dependences of hydrogen permeability through palladium membranes upon the plasma component structure and the preliminary irradiation dose in the inert gas atmosphere have been revealed.     

Development of a new X-ray source using backscattered X-ray with the use of a cold cathode

The development of an intense X-ray source using backscattered X-ray produced using an advanced electrode configuration is described. The electrodes were composed of field emitters deposited on a wire mounted on a perforated plate as the cathode and a copper plate as the anode. Electrons from these emitters collided with the copper plate and X-ray was generated at collision points. The backscattered X-ray in the direction normal to the electron trajectory through a hole in the anode escaped from the vacuum chamber through a beryllium window. Continuous and characteristic X-rays were detected at an applied voltage lower than that of a conventional X-ray source from 3.0 to 9.4 kV, respectively. Moreover, the X-ray dosage measured with a survey meter reached 0.95 mSv/h at 5.0 kV of applied voltage. The transmission images of three types of material used as an X-ray source for the X-ray imaging system indicate three advantages; low power consumption, focal point controllable by adjusting applied voltage, and photographable motion picture of X-ray transmission.     

Separation Factor of Hydrogen and Deuterium by Suspension Electrolysis Using Mercury Cathode

The electrolytic separation factor between hydrogen and deuterium was examined using mercury or else platinum cathode immersed in IF NH₄Cl in 10v/oD₂O water containing cobalt sulfide powder in suspension. Several other kinds of powdered materials in suspension were also studied. In the case of mercury pool electrode, the materials added in suspension were effective in enhancing the hydrogen/deuterium separation factor, but powder suspension was ineffective on platinum plate electrode. The powdered material added in suspension served as catalyst on the hydrogen evolution reaction at the mercury cathode. The influence of the applied potential on the separation factor was studied over the temperature range of 15°~80°C. The results provided an indication of the rate-determining steps governing the electrolytic hydrogen evolution.

The experimental values obtained for the separation factor and activation energy gave an insight into the mechanism of the rate-determining step of the hydrogen evolution at the working electrode.