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.     

Extraction of hydrogen into vacuum by electrochemical hydrogen pump for hydrogen isotope recovery

For hydrogen isotope recovery, an electrochemical hydrogen pump using CaZr_0.9In_0.1O_3?α under vacuum in the cathode compartment has been proposed. Two types of electrodes—pasted and electroless plated electrodes—were used to evaluate the mass transfer process. When the hydrogen pump was operated using direct current, the voltage between electrodes under vacuum was less than that under atmospheric pressure, regardless of electrode type. This result indicates that the hydrogen pump performance is improved under vacuum. In the case of the pasted electrode, however, the voltage decrease was larger than that in the case of the plated electrode. Electrochemical impedance spectroscopy revealed that the diffusivity in the pasted electrode was smaller under atmospheric pressure than under vacuum. From comparing the microstructures of the two electrode types, the pasted electrode has larger pores and would be more suitable for the hydrogen pump under vacuum.     

Evaluation of the response time of H-concentration probes for tritium sensors in lead–lithium eutectic alloy

Dynamic tritium concentration measurement in lead–lithium eutectic is of major interest for a reliable tritium testing program in ITER TBM and for an experimental proof of tritium self-sufficiency in liquid metal breeding systems. Potentiometric hydrogen sensors using different solid-state electrolytes for molten lead–lithium eutectic have been reported and tested by the Electrochemical Methods Lab at Institut Quimic de Sarria (IQS).In the present work the following ceramic elements have been synthesized and characterized by X-ray diffraction (XRD) in order to be tested as a Proton Exchange Membranes (PEM) H-probes: BaCeO₃, BaCe_0.6Zr_0.3Y_0.1O_3−δ and Sr(Ce_0.9–Zr_0.1)_0.95Yb_0.05O_3−δ. Potentiometric measurements of the synthesized ceramic elements have been performed shifting from a fixed hydrogen partial pressure at the working electrode to high purity argon. In this experimental campaign a fixed and known hydrogen pressure has been used in the reference electrode. The goal of these experiments is to evaluate the sensor response time when the hydrogen concentration in the environment is rapidly changed. All experiments have been done at 500 °C and 600 °C. The sensor constructed using the proton conductor element BaCe_0.6Zr_0.3Y_0.1O_3−δ exhibited stable output potential and its value was close to the theoretical value calculated with the Nernst equation. In contrast, the sensors constructed using the proton conductor elements BaCeO₃ and Sr(Ce_0.9–Zr_0.1)_0.95Yb_0.05O_3−δ showed higher deviations between experimental and theoretical data, and long response times.     

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.     

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.     

An electrochemical hydrogen meter for measuring hydrogen in sodium using a ternary electrolyte mixture

An electrochemical sensor for measuring hydrogen concentration in liquid sodium that is based on a ternary mixture of LiCl, CaCl₂ and CaHCl as the electrolyte has been developed. DSC experiments showed the eutectic temperature of this ternary system to be ∼725 K. Impedance spectroscopic analysis of the electrolyte indicated ionic conduction through a molten phase at ∼725 K. Two electrochemical hydrogen sensors were constructed using the ternary electrolyte of composition 70 mol% LiCl:16 mol% CaHCl:14 mol% CaCl₂ and tested at 723 K in a mini sodium loop and at hydrogen levels of 60-250 ppb in sodium. The sensors show linear response in this concentration range and are capable of detecting a change of 10 ppb hydrogen in sodium over a background level of 60 ppb. Identification of this electrolyte system and its use in a sensor for measuring hydrogen in sodium are described in this paper.     

Conceptual design of sulfur-iodine hydrogen production cycle of Korea Institute of Energy Research

The sulfur-iodine (S-I) thermochemical cycle offers a promising approach to the high efficiency production of hydrogen from nuclear power. S-I process was originally proposed by General Atomic. Especially, HI decomposition flowsheets of S-I process were proposed by several research groups. General Atomics (GA) studied I₂ separation by extractive distillation using H₃PO₄. RWTH introduced the concept of reactive distillation. HIx stream coming from the Bunsen reaction is fed to the column. And HIx is distillated and decomposed at the same time to obtain hydrogen. Korea Institute of Energy Research (KIER) and Japan Atomic Energy Agency (JAEA) concentrate HIx using electrodialysis cell and concentrated HIx is fed to the column to produce HI vapor, which is decomposed to produce hydrogen. HI was separated from HIx solution by an extractive distillation using H₃PO₄. However, a large amount of electric energy was required to recycle H₃PO₄. Most of S-I processes have difficulties producing hydrogen because it has excess iodine in HI decomposition section. S-I cycle with electrodialysis cell uses membrane reactor to separate H₂ and HIx. The current state of the membrane technology is not compatible with the process needs.We studied several cases of flowsheets to overcome the problems mentioned above. We revised the flowsheet by adding the iodine separator and excluded membrane reactor. We analyzed the thermal efficiency of S-I process based on revised flowsheet.     

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.     

Analysis of diffusion and dissolution of two-component hydrogen (H + D) in lead lithium

A lead–lithium eutectic alloy (Pb–Li) is one of the most promising candidate materials for the liquid blanket of an advanced fusion reactor. We have experimentally determined mass-transfer properties by an unsteady permeation method, which data are necessary to design a system to recover tritium (T) from a Pb–Li blanket. An experiment of simultaneous H and D permeation through Li₁₇Pb₈₃ is performed to clarify interactions between atoms in the two-component permeation process. The experimental results are analyzed by a model of one-dimensional or two-dimensional permeation through Li₁₇Pb₈₃. The major permeation proceeds in the longitudinal direction of the present system, and the ratio of hydrogen leak in the radial direction is evaluated using the simulation. As a result, it was found that H and D atoms permeate independently regardless of the H/D component ratio within the present experimental conditions. The permeability and diffusivity of H are 1.4 times higher than that of D. The solubility of H is close to that of D. The isotope effect in diffusivity is in proportion to the square root of the mass ratio of D to H. When these data can be extended to the case of T, T permeability and diffusivity is predicted as 1/1.7 times lower than that of H in the temperature range from 773 K to 973 K.     

聚变制氢堆高温液态包层热工水力学新概念研究

在深入分析聚变堆包层设计要求和目前技术发展水平的基础上,根据热化学工艺制氢需要高温热的要求,提出了一个基于技术相对成熟的低活化铁素体/马氏体钢作为主要结构材料、高压氦气与液态LiPb合金作为冷却剂、具有创新性“多层流道插件”结构方案以获得高温热能的包层热工水力学概念,建立了热工水力学模型,在利用有限元数值模拟程序进行模拟计算的基础上分析了这种新概念包层的可行性。     

Measurement of hydrogen permeation due to atomic flux using permeation probe in the spherical tokamak QUEST

Particle retention and recycling in plasma fusion devices are generally associated with the diffusion of atomic hydrogen into the materials. The resulted permeation of atomic hydrogen is known as plasma driven permeation (PDP). This permeation may also be significant, even in the walls, which are not directly exposed to the plasma. Under similar conditions, the permeation flux (Γ_perm) of hydrogen through a 30 μm thick Ni membrane heated at 412-575 K has been measured in the spherical tokamak QUEST. Γ_perm is being measured during the scans of different operating parameters like RF power (P_RF), chamber pressure (P_chamber), discharge widths (τ_dis) and vertical magnetic field (B_Z). Simultaneously edge plasma density and spectral intensities of atomic (Balmer) lines and molecular (Fulcher) bands have been compared with the permeation measurements. A linear relationship has been established between the time integrated Γ_perm i.e. permeation fluence (Q_perm) and the time integrated H_α intensity i.e. H_α fluence (Q_α). Q_perm also shows a strong relationship with the edge plasma density and various spectral fluences. The obtained results are discussed for exploring the applicability of the permeation probes in measuring the atomic flux near the first walls.     

Release behavior of bred tritium from LiAlO2

The release behavior of bred tritium to the blanket purge gas is mainly controlled by such bulk phenomena as tritium forming reaction, diffusion of tritium in grain, interaction of tritium with irradiation defects, and absorption together with such surface phenomena as adsorption, isotope exchange reaction between molecular form hydrogen in purge gas and tritium on grain surface (isotope exchange reaction 1), isotope exchange reaction between water vapor and tritium on grain surface (isotope exchange reaction 2), and water formation reaction at addition of hydrogen. Following the observation of the present authors that the isotope exchange reaction 2 is much faster than the isotope exchange reaction 1, the release curve of bred tritium obtained at purge with humidified gas was used for estimation of the effective diffusivity of bred tritium in LiAlO₂. Then, the effective diffusivity of tritium in grain of LiAlO₂ is obtained as D_T = 2.5 × 10⁻⁷exp(−110 [kJ]/RT) [m²/s]. This equation gives the larger diffusivity than any other diffusivity presented so far because the mass transfer resistance at the grain surface is expected to be eliminated in the estimation procedure of this study.     

Progress on the development of H-concentration probes in eutectic lead–lithium: Synthesis and characterization of electrochemical sensor materials

Dynamic tritium concentration measurement in lithium–lead eutectic (17% Li–83% Pb) is of major interest for a reliable tritium testing program in ITER TBM and for an experimental proof of tritium self-sufficiency in liquid metal breeding systems. Potentiometric hydrogen sensors for molten lithium–lead eutectic have been designed at the Electrochemical Methods Lab at Institut Quimic de Sarria (IQS) at Barcelona and are under development and qualification. The probes are based on the use of solid state electrolytes and works as Proton Exchange Membranes (PEM).In this work, the following compounds have been synthesized in order to be tested as PEM H-probes: BaCeO₃, BaCe_0.9Y_0.1O_3?δ, SrCe_0.9Y_0.1O_3?δ and Sr(Ce_0.9–Zr_0.1)_0.95Yb_0.05O_3?δ. Potentiometric measurements of the synthesized ceramic elements have been performed at different hydrogen concentrations at 500 °C. In this campaign, a fixed and known hydrogen pressure has been used in the reference electrode. The sensors constructed using the proton conductor elements BaCeO₃, SrCe_0.9Y_0.1O_3?δ and Sr(Ce_0.9–Zr_0.1)_0.95Yb_0.05O_3?δ exhibited quite stable output potential and its value was quite close to the theoretical value calculated with the Nernst equation (deviation less than 100 mV). Unstable measurement was obtained using BaCe_0.9Y_0.1O_3?δ as a solid state electrolyte in the sensor.     

Experimental investigation on cryogenic hydrogen adsorption of molecular sieves

Tritium extraction system (TES) is one of the most important components in the helium cooled solid breeder test blanket modules (TBMs) of ITER. TES will extract various isotopic species of hydrogen by the liquid nitrogen cooled molecular sieve adsorber beds (MSB). The cryogenic hydrogen adsorption properties of several kinds of molecular sieves have been investigated at the pressure of hydrogen of 100 Pa, 200 Pa, and 0.2 MPa in order to offer the suitable molecular sieve for the MSB in TES. The saturated hydrogen adsorption capacities of the MS5A-2 and MS13X-2 have been measured at 100 Pa hydrogen pressure. To demonstrate the hydrogen extraction from continuous He–H₂ purge gases, the MS5A-2 has been tested in circulating 99.79% He–0.21% H₂ mixture with a flow rate of 16.8 L/min. The results show that the globular MS5A-2 with a diameter of 3–5 mm can adsorb/desorb hydrogen quickly. The saturated hydrogen adsorption capacity of MS5A-2 is 7.55 ml g^?1 (NTP) and MS5A-2 could effectively extract trace hydrogen from mixture gases. As a result, this type of molecular sieve can be the candidate of the one in the MSB in ITER TBM.     

Performance evaluation and analysis of ITER poloidal field conductors

The performance evaluation and analysis of PF5 conductor of the ITER Project in China have been performed using the Gandalf code (Bottura [1]). This study focuses on the T_cs and MQE of PF5 conductor with Cu–non Cu ratio of 2.3 NbTi strands from WST. The PF5 conductor samples have been measured in SULTAN at CRPP for evaluating the performance successfully. The measurement results are also presented with the evaluation results in the paper. The evaluation results related to T_cs and MQE are agreed well with the measurement results. The simulation with Gandalf code can predict the performance of PF5 conductor effectively and provide the helpful method for ITER conductor design and analysis.     

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.     

Isotope Exchange Capacity on Li4SiO4 and Comparison of Tritium Inventory in Various Solid Breeder Blankets

It has been pointed out by the present authors that it is essential to understand such mass transfer steps as diffusion of tritium in the grain of a breeder material, absorption of water vapor into bulk of the grain, adsorption of water on surface of the grain, and exchange capacity of tritium to be trapped to surface of the grain together with two types of isotope exchange reactions for evaluation of the tritium inventory in a solid breeder blanket under various conditions. The isotope exchange capacity on the Li₄SiO₄ surface is experimentally obtained in this study. Most of the properties required for evaluation of the tritium inventory for various blanket materials have been already quantified by the present authors. Then it has become possible to compare the tritium inventory in solid breeder blankets packed with either Li₂O, LiAlO₂, Li₂ZrO₃, Li₂TiO₃ or Li₄SiO₄ using the calculation model previously presented by the present authors.     

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.     

Permeability of hydrogen and deuterium of Hastelloy XR

Permeation of hydrogen isotope through a high-temperature alloy used as heat exchanger and steam reformer pipes is an important problem in the hydrogen production system connected to be a high-temperature engineering test reactor (HTTR). An experiment of hydrogen (H₂) and deuterium (D₂) permeation was performed to obtain permeability of H₂ and D₂ of Hastelloy XR, which is adopted as heat transfer pipe of an intermediate heat exchanger of the HTTR. Permeability of H₂ and D₂ of Hastelloy XR were obtained as follows. The activation energy E₀ and pre-exponential factor F₀ of the permeability of H₂ were E₀=67.2±1.2 kJ mol⁻¹ and F₀=(1.0±0.2)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5, respectively, in the pipe temperature ranging from 843 K (570 °C) to 1093 K (820 °C). E₀ and F₀ of the permeability of D₂ were respectively E₀=76.6±0.5 kJ mol⁻¹ and F₀=(2.5±0.3)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5 in the pipe temperature ranging from 943 K (670 °C) to 1093 K (820 °C).     

Adsorption Characteristics of Water Vapor on Gamma-Lithium Aluminate

Lithium aluminate (LiAl0₂) is one of the probable candidates for tritium breeding material because of its potential to give a fair tritium breeding ratio, stability at high temperature and stability in atmosphere containing water vapor. Clarification of the adsorption performances of water on LiAl0₂ is important for optimization of the way to recover bred tritium from LiAl0₂ and for estimation of the tritium inventory in the breeding blanket and for quantification of the hydrogen isotope exchange reaction. The amount of water captured on LiAl0₂ was studied in the temperature range of 373~1,100K using the breakthrough curve method, and adsorption isobar and isotherm of water on LiAl0₂ were proposed based on the data obtained. The water capture phenomena of LiAl0₂ was attributed to the dissociative chemisorption, and the apparent activation energy was determined to be 32.2 kJ/mol.K. The tritium inventory by sorption for LiAl0₂ was compared with that for Li₂0.