Permeation Reduction Factor Research Articles

Formation of Cr2O3 layers on coolant duct materials for suppression of hydrogen permeation

Oxidation control of candidate duct materials for liquid cooled blanket systems (i.e. SS316, SS316L, Inconel 600 and Hastelloy X) was studied to form a single composition Cr2O3 layer on the surfaces for (i) making a stable and robust underlayer for ceramic coating fabrication by the metal organic decomposition (MOD) method and (ii) suppression of hydrogen permeation by the Cr2O3 layer itself. In heat treatments of the bare plates at 700°C in air of atmospheric pressure and air of reduced pressure of 5Pa, oxidized Fe and/or Ni diffused to the outermost surfaces of the oxide layers and a single composition layer of Cr2O3 could not obtained on the surfaces. However, the Cr2O3 surface layers could be formed by a heat treatment of chrome plated plates at 700°C in air of atmospheric pressure. MOD coatings were successfully fabricated on the stable Cr2O3 surface layers. The preliminary evaluation of deuterium permeation through the Cr2O3 covered plates showed that the permeation reduction factors (PRFs) of the Cr2O3 layers would be 1–2 orders.

Hydrogen saturation and permeation barrier performance of yttrium oxide coatings

In fusion power plants a tritium permeation barrier is required in order to prevent the loss of the fuel inventory. Moreover, the tritium permeation barrier is necessary to avoid that the situation the radioactive tritium accumulates in the first wall, the cooling system, and other parts of the power plant. Oxide thin films, e.g. Er2O3 and Y2O3, are promising candidates as tritium permeation barrier layers. With regard to the application, this is especially true for Y2O3 due to its favorably low activation behavior compared to the other candidates. Y2O3 thin films are deposited on the reduced activation steel Eurofer97 by means of magnetron sputtering. To quantify the permeation reduction factor of the Y2O3 thin films a new gas-driven deuterium permeation measurement setup was constructed. Comparing the permeation flux through a bare substrate and a coated Eurofer97 substrate, the permeation reduction factor can be determined. The measurement results suggest that the permeation reduction factor is in the same range as for Er2O3. Moreover, the morphological analysis and the permeation measurements indicate that the long term stability of the permeation barrier performance depends on the deuterium saturation in the Y2O3 thin film.

A deuterium permeation barrier by hot-dipping aluminizing on AISI321 steel

Reducing the tritium permeation through structural materials is one focused issue in the International Thermonuclear Experimental Reactor (ITER). To achieve the target, the hot-dipping aluminizing (HDA) method is suggested for its potentials in applications. But low permeation reduction factors (PRFs) were often appeared on the coatings by HDA for through defects or visible cracks. In this work, we presented a deuterium permeation barrier coating with high PRF prepared by HDA method and subsequent oxidation, on AISI321 steel vessel sample. The external surface of the sample was protected by oxide passivation, inert atmosphere and molten salt before dipping in Al liquid at 1023 K for 30 min. Then the sample was oxidized at 1123 K in air for 4 h and at 1023 K in 10 ml/min Argon flow for 15 h. Four layers of the aluminide coating were obtained after HDA and oxidation processes, including Al2O3, Al, FeAl, and Fe3Al. High PRF values were achieved as 2530 at 773 K, and 790 at 823 K, through the aluminide coating.

Fast hydrogen diffusion along the Σ7 grain boundary of α-Al2O3: A first-principles study

We have studied the energetics and mobility of hydrogen in the bulk and along the rhombohedral Σ7 grain boundary (GB) of α-Al2O3 via first-principles calculations based on the projector-augmented wave method. The temperature-dependent diffusivities D(T) in the α-Al2O3 bulk and along the Σ7 GB are derived. The ratio shows that the diffusivity along the GB is 2–5 orders of magnitude greater than in the bulk at temperatures in the range of 273–973 K, revealing that the diffusion along the GB is the underlying reason for the experimentally observed permeation reduction factor being much lower than the anticipated value. Moreover, the calculations also reveal that radiation-induced O vacancies tend to aggregate to the GB plane, thereby forming a zigzag O vacancy chain. In such circumstances, however, the calculated migration energy for hydrogen diffusion along the O vacancy chain is 2.58 eV, which is much greater than that of 1.10 eV and 0.81 eV for H diffusion in the bulk and along the GB, respectively. This finding suggests that O vacancies in the GB trap hydrogen atoms and prevent their diffusion along the GB.

Deuterium permeation properties of Er2O3/Cr2O3 composite coating prepared by MOCVD on 316L stainless steel

In this work, an Er2O3/Cr2O3 composite coatings on 316L stainless steel were prepared by metalorganic chemical vapor deposition (MOCVD). Effect of Cr2O3 layer on the microstructure, mechanical properties and deuterium permeation properties of Er2O3 coating was investigated. It was found grain sizes of the coatings enlarged with increasing the thickness of Cr2O3 layer. The Er2O3/Cr2O3 (80nm) composite coating showed larger adhesion force value 9.2N than the Er2O3 coating. The Cr2O3 layer adding could significantly enhance the deuterium permeation inhibition property of the coatings. The single-layer Er2O3 coating exhibited the minimum reduction in deuterium permeability, and the permeation reduction factor (PRF) values were in the range of 95–146 at 823–973K. The maximum reduction in deuterium permeability was obtained from the Er2O3/Cr2O3 (80nm) composite coating, and the PRF values were in the range of 463–206 at 823–973K. With further increasing thickness of the Cr2O3 layer to 120nm, the hydrogen permeation inhibition performance of the composite coating lower instead. Furthermore, apparent delamination of coating was illustrated on the single-layer Er2O3 coating after the permeation measurement, and this might be the main reason for the transformation to diffusion limiting process.

Tritium permeation characterization of Al2O3/FeAl coatings as tritium permeation barriers on 321 type stainless steel containers

Accurate tritium transport properties of prospective tritium permeation barriers (TPBs) are essential to tritium systems in fusion reactors. By passing a temperature and rate-controlled sweeping gas over specimen surfaces to carry the permeated tritium to an ion chamber, the gas-driven permeation of tritium has been performed on 321 type stainless steel containers with Al2O3/FeAl barriers, to determine the T-permeation resistant performance and mechanism of the barrier. The tritium permeability of the Al2O3/FeAl coated container was reduced by 3 orders of magnitude at 500–700 °C by contrast with that of the bare one, which meets the requirement of the tritium permeation reduction factor (PRF) of TPBs for tritium operating components in the CN-HCCB TBM. The Al2O3/FeAl barrier resists the tritium permeation by the diffusion in the bulk substrate at a limited number of defect sites with an effective area and thickness, suggesting that the TPB quality is a very important factor for efficient T-permeation resistance.

Effect of Cr2O3 layer on the deuterium permeation properties of Y2O3/Cr2O3 composite coating prepared by MOCVD

In this work, Y2O3/Cr2O3 composite coatings were deposited on 316 L stainless steel by metal organic chemical vapour deposition (MOCVD). Effect of Cr2O3 layer on the microstructure, mechanical properties and deuterium permeation properties of Y2O3 coating was investigated. In order to investigate the evolution of the microstructure in working environment, Y2O3/Cr2O3 composite coating was high temperature annealed in argon or hydrogen atmosphere. After annealing at 973 K in hydrogen atmosphere, the (222) diffraction peak became weak whereas the (400) diffraction was strengthened. The adhesion force reduced from 9.7 N of the Y2O3/Cr2O3 composite coating annealed in argon atmosphere to 8.6 N of that annealed in hydrogen atmosphere. It was inferred that hydrogen atoms invasion might be responsible for this mechanical property difference. The deuterium permeation inhibition performance of the coatings was examined by deuterium permeation measurement. After annealed at 973 K in argon atmosphere, the single-layer Y2O3 coating exhibited the minimum reduction in deuterium permeability, and the permeation reduction factor (PRF) values were in the range of around 435–272 at 873–973 K. The maximum reduction in deuterium permeability was obtained from the Y2O3/Cr2O3 composite coating with Cr2O3 layer thickness of 120 nm, and the PRF values were in the range of around 612–432 at 873–973 K. With further increasing thickness of the Cr2O3 layer to 160 nm, the hydrogen permeation inhibition performance of the composite coating lowered instead. Furthermore, the Y2O3/Cr2O3 composite coating annealed in argon atmosphere exhibited better deuterium permeation inhibition performance than that annealed in hydrogen atmosphere. After adding the Cr2O3 layer, the Y2O3/Cr2O3 coating exhibited stable permeation fluxes at each driving pressure, while spikes of the permeation flux were observed for the single-layer Y2O3 coating.

Deuterium permeation properties of Y2O3/Cr2O3 composite coating prepared by MOCVD on 316L stainless steel

The effect of Cr2O3 layer on the deuterium permeation properties of Y2O3/Cr2O3 composite coating was investigated. Composite coating was prepared on 316L stainless steel by metal organic chemical vapor deposition (MOCVD). The Cr2O3 layer contributed to the completely decomposition of the original precursor with no presence of unreacted or partially reacted precursor molecules. The deuterium permeation inhibition performance of the coatings was examined by deuterium permeation measurement. The Y2O3/Cr2O3 composite coating exhibited better deuterium inhibition performance than the single-layer Y2O3 coating with the same thickness. The permeation reduction factor (PRF) value of the Y2O3/Cr2O3 composite coating was 167–477 at temperatures of 823–973 K, while the PRF value of the single-layer Y2O3 coating was 96–292 at 823–973 K. Moreover, the Y2O3/Cr2O3 composite coating exhibited a stable permeation flux, while a spike of the permeation flux was observed for the single-layer Y2O3 coating. Therefore, the Cr2O3 layer could enhance deuterium permeation inhibition properties significantly.

Study on influence factors of permeation reduction factor of Al2O3-hydrogen isotopes permeation barriers

Hydrogen isotopes permeation barriers (HIPB) play an important role in reducing hydrogen isotopes permeation and leakage in hydrogen energy and thermonuclear fusion energy. In this study, the Al2O3-HIPB were prepared on 316L stainless steel by RF magnetron sputtering. The impacts of coating thickness and annealing temperature of as-deposited coatings on deuterium permeation reduction factor (D-PRF) are studied in detail. With increasing thickness of coatings from 0.2 μm to 1.6 μm, the D-PRF did not rise significantly, which indicated that coating surface, for thin coatings, played more important role in impeding deuterium permeation than coating interior. As a result of further crystallization and grain growth, the 700 °C-annealed Al2O3 coatings had stronger ability to impede deuterium permeation than 600 °C-annealed Al2O3 coatings. This further proofs that grain boundaries may be a short-cut path for deuterium diffusion in Al2O3 coatings. The D-PRF drastically decreased when the annealing temperature reaches 800 °C, which might be ascribed to the new phases form and the coating cracks appearance after 800 °C annealing.

Thickness impacts on permeation reduction factor of Er2O3 hydrogen isotopes permeation barriers prepared by magnetron sputtering

In hydrogen energy and thermal fusion energy, hydrogen isotopes permeation barriers (HIPB) are used to reduce hydrogen isotopes permeation, which may result in hydrogen isotopes leak and construction materials brittleness. Here, we made Er2O3 HIPB by magnetron sputtering and studied the relation between deuterium-permeation reduction factor (D-PRF) and coating thickness, micro-morphology. The 0.2 μm Er2O3 coatings can reduce deuterium permeation by 1/10 ∼ 1/20 at 600 °C ∼ 300 °C. However, the reduction ability decreases gradually with the increase in coating thickness when the coatings are thicker than 0.2 μm. The reason may be ascribed to the deuterium invasion in deuterium permeation test. After deuterium invasion into coatings, the coatings suffer intrinsic stress, thermal stress and expansion stress from deuterium invasion. As a result of grain size dependence on coating thickness in PVD-derived coatings, i.e. grains become coarse with increasing coating thickness, the sum of stresses in various type increases with increasing coating thickness. The coatings crack easily when they suffer more strong tensile stress. So, the damages of coatings become serious and D-PRF reduces with increasing coating thickness.

Influence of annealing atmosphere on the deuterium permeation of Y2O3 coatings

In this work, Y2O3 coatings were deposited on 316L stainless steel substrate by metal organic chemical vapor deposition (MOCVD), followed by annealing at 700 °C in argon or hydrogen atmosphere. The effect of annealing atmosphere on the deuterium permeation inhibition properties of Y2O3 coatings was investigated. The coating was characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM). The hydrogen permeation inhibition performance of coatings was investigated by deuterium permeation measurement. When post-annealed in hydrogen atmosphere, a FeCr2O4 layer formed at the interface as a result of element diffusion, leading to obvious increase of the permeation current from the first measurement cycle to the second. The deuterium permeation reduction factor (PRF) values of the coating were in the range of 412–102 at 500°C–700 °C in the first measurement cycle. However, the PRF values decreased to 276–43 at 500°C–700 °C in the second measurement cycle. The coating post-annealed in hydrogen atmosphere exhibited lower permeation inhibition performance than that post-annealed in argon atmosphere, which might be ascribed to the cracks and holes presented in the coating.

Study of Al2O3–Er2O3 composite coatings as hydrogen isotopes permeation barriers

Composite coatings as Hydrogen Isotopes Permeation Barriers have seldom been studied. In this work, the Al2O3–Er2O3 component composite HIPB were prepared on 316L stainless steel by radio frequency reactive magnetron sputtering. The deuterium permeation reduction factor (D-PRF), structure and nano-hardness of the Al2O3–Er2O3 composite HIPB were investigated. The Er4Al2O9 (012), (023) diffraction peaks appeared. The D-PRF of 1.2 μm Al2O3–Er2O3 composite HIPB could reach 160 at 600 °C, the D-PRF was 210 at 450 °C and increased sharply to 1380 at 250 °C. The coatings kept dense during the deuterium permeation tests while the Er4Al2O9 phases changed. The nano-hardness decreased from 9.55 GPa to 7.46 GPa after the deuterium permeation tests. It was inferred that the deuterium atoms invasion and formation of oxygen vacancies should be responsible for the performance and structure changes of the Al2O3–Er2O3 composite HIPB during deuterium permeation tests.

Model improvements for tritium transport in DEMO fuel cycle

DEMO operation requires a large amount of tritium, which is directly produced inside the reactor by means of Li-based breeders. During its production, recovering and purification, tritium comes in contact with large surfaces of hot metallic walls, therefore it can permeate through the blanket cooling structure, reach the steam generator and finally the environment.The development of dedicated simulation tools able to predict tritium losses and inventories is necessary to verify the accomplishment of the accepted tritium environmental releases as well as to guarantee a correct machine operation.In this work, the FUS-TPC code is improved by including the possibility to operate in pulsed regime: results in terms of tritium inventory and losses for three pulsed scenarios are shown. Moreover, the development of a 1-D model considering the radial profile of the tritium generation is described. By referring to the inboard segment on the equatorial axis of the helium-cooled lithium–lead (HCLL) blanket, preliminary results of the 1-D model are illustrated: tritium partial pressure in Li–Pb and tritium permeation in the cooling and stiffening plates by assuming several permeation reduction factor (PRF) values. Future improvements will consider the application of the model to all segments of different blanket concepts.

Characterization of ZrO2 ceramic coatings on ZrH1.8 prepared in different electrolytes by micro-arc oxidation

ZrO2 ceramic coatings were directly prepared on the surface of ZrH1.8 in silicate and phosphate electrolytes by micro-arc oxidation (MAO) technique, respectively. The microstructure, chemical composition and phase composition of ZrO2 ceramic coatings were investigated by X-ray diffraction (XRD), energy-dispersive spectrometry (EDS) and scanning electron microscopy (SEM). The anti-permeation effect was measured by means of vacuum dehydrogenation experiment. It is found that the coating fabricated in phosphate electrolyte is more compact than that in silicate electrolyte. The coatings fabricated on the surface of ZrH1.8 are composed of M-ZrO2, T-ZrO2 and C-ZrO2. EDS analysis indicates that the coatings are mainly composed of O and Zr. Vacuum dehydrogenation experiment shows that the permeation reduction factor (PRF) of coating prepared in phosphate electrolyte is superior to that in the silicate electrolyte, and the PRF value reaches up to 11.2, which can enhance the resistance effect of hydrogen significantly.

A Uranium Bed with Ceramic Body for Tritium Storage

Tritium permeation through structural materials is a key issue in many activities linked with tritium handling both for radiological safety and accountancy reasons to say nothing of economical aspect: tritium is not the cheapest material in the world. It is widely recognized that ceramic coatings provide an attractive solution to lower tritium permeation in structural materials. Alumina based ceramic coatings have the highest permeation reduction factor for hydrogen. Nevertheless even small cracking will significantly spoil the permeation reduction factor of a protecting coating. Nowadays for hydrogenating neutron tube targets with tritium “VNIIA” uses working chambers manufactured by pressing of alumina based ceramics.These chambers have revealed extremely low hydrogen permeation upon conditions of their application. For this reason an attempt was made to apply low porous ceramics as a structural material of a bed body for tritium storage in a setup used for hydrogenating neutron tube targets at “VNIIA”. The present article introduces the design of the bed. This bed possesses essentially less hydrogen permeation factor than traditionally used beds with stainless steel body. Bed heating in order to recover hydrogen from the bed is suggested to be implemented by high frequency induction means. Inductive heating allows decreasing the time necessary for tritium release from the bed as well as power consumption. Both of these factors mean less thermal power release into glove box where a setup for tritium handling is installed and thus causes fewer problems with pressure regulations inside the glove box. Inductive heating allows raising tritium sorbent material temperature up to melting point. The latter allows achieving nearly full tritium recovery.

Tritium management and anti-permeation strategies for three different breeding blanket options foreseen for the European Power Plant Physics and Technology Demonstration reactor study

In DT fusion reactors like DEMO, the commonly accepted tritium (T) losses through the steam generator (SG) shall not exceed about 2mg/d that are more than 5 orders of magnitude lower than the T production rate of about 360g/d in the breeding blanket (BB). A very effective mitigation strategy is required balancing the size and efficiency of the processes in the breeding and cooling loops, and the availability and efficiency of anti-permeation barriers. A numerical study is presented using the T permeation code FUS-TPC that computes all T flows and inventories considering the design and operation of the BB, the SG, and the T systems. Many scenarios are numerically analyzed for three breeding blankets concepts – helium cooled pebbles bed (HCPB), helium cooled lithium lead (HCLL), and water cooled lithium lead (WCLL) – varying the T processes throughput and efficiency, and the permeation regimes through the BB and SG to be either surface-limited or diffusion-limited with possible permeation reduction factor. For each BB concept, we discuss workable operation scenarios and suggest specific anti-permeation strategies.

Tritium Migration in HCLL and WCLL Blankets: Impact of Tritium Solubility in Liquid Pb-17Li

The next generation of fusion power plants (DEMO) should rely on a breeding blanket (BB) able to efficiently convert the neutrons kinetic energy into heat, to ensure the tritium self-sufficiency and to adequately shield the toroidal field coils from neutrons and gamma rays. The eutectic lithium-lead alloy is a consolidate liquid blanket material, which simultaneously includes the breeder (Li) and the neutron multiplier (Pb). The assessment of the tritium inventory inside the blanket and its environmental release requires knowledge of the hydrogen isotopes interactions with blanket materials, in particular the hydrogen solubility in lithium-lead, which is defined by means of the hydrogen Sievert's constant. Several experiments, aiming to determine the hydrogen isotopes solubility in lithium-lead, have been performed in the past giving values of the temperature-dependent Sieverts' constant, K , distributed in a wide range (covering about two orders of magnitude on the Arrhenius plot). Starting from a literature review of the K values, this paper provides a parametric analysis for the most influencing parameters related to tritium migration in helium-cooled and water-cooled lead-lithium DEMO BBs. This analysis has been performed using the computational code FUS-TPC for several operative scenarios and considering the different K values. This paper demonstrates that the tritium Sieverts' constant in Pb-17Li has a great impact on the assessment of tritium losses, whose value can spread in more than one order of magnitude. Furthermore, the analysis suggests suitable permeation reduction factors to be adopted in the different scenarios as well as the need of addressing new accurate experiments on the solubility constant.

The Effect of MOD Process Parameters on Performance of Er2O3 Coating for Liquid Blanket Application

Previous studies by the authors showed that hydrogen permeation reduction factor (PRF) of Er2O3 coating on ferritic steels by Metal Organic Decomposition (MOD) depends on the Cr level of the substrate steels and the annealing conditions. The reason of the dependence was attributed to the composition of the oxide layer formed beneath the coating. The PRF was shown to be larger when Cr2O3 layer was formed than when Fe2O3 layer was formed. This paper reports further investigation of the effect of temperature and oxygen partial pressure of the annealing on the composition of the oxide layer. A diffusion modeling of Cr and O was performed to account for the experimental data. The results showed that the data can be well explained assuming that Cr2O3 layer is formed when supply of Cr exceeds that of O at the substrate surface.

Hydrogen permeability through beryllium films and the impact of surface oxides

Beryllium will constitute the major part of the first wall of ITER, however, several aspects of the tritium retention and recycling in fusion reactors are still open. Studying details of the hydrogen isotope interactions on Be films is in principle easier and more accurate than on the bulk Be metal since a thin (and therefore more permeable) layer of Be film could be deposited on a desired substrate by applying well controlled methods. Results of the hydrogen permeation through 8 micrometer thick Be films deposited by the thermionic vacuum arc method on Eurofer steel membranes with exposed area of 8.4cm2 are presented. The permeation reduction factor (PRF) at 400°C varied on six samples from 14 to 135 with respect to the bare Eurofer membrane. The highest PRF value enables expression of the Be film permeability coefficient P by means of a simple model which gives PBe∼2×10−15molH2/msPa0.5. Lower PRF values could be explained by microscopic imperfections which represent parallel hydrogen paths through the Be film and enhance the permeation rate. Some of them were revealed by the SEM while their presence could be confirmed also by observing permeation flux transients recorded after the hydrogen exposure. The two-step process of achieving the steady flux agrees with our numerical simulation. It was found that for unintentionally oxidized samples the extracted regular (eliminated contribution of the pinholes in Be film) permeation rate is almost identical from sample to sample and accounts to j≈1.2×10−7H2/m2 s at 1bar hydrogen driving pressure due to BeO formation. For a non-oxidized sample this value is several times higher, j≈6.5×10−7molH2/m2s. From the latter follows that PBe≈1.9×10−14molH2/msPa0.5, while PBeO∼1×10−17molH2/msPa0.5 can be estimated by assuming a 35nm thick BeO layer.

Deuterium permeation through erbium oxide coatings on RAFM steels by a dip-coating technique

A tritium permeation barrier is a promising solution for the problems of tritium loss and radiological safety in fusion blanket systems. In recent years, erbium oxide coatings have shown remarkable permeation reduction factors. One of the remaining issues for the coatings is the establishment of plant-scale fabrication. In this study, erbium oxide thin films have been fabricated by a dip-coating technique, which has the potential to coat a complex-shaped substrate, and deuterium permeation behavior in the coatings has been examined. Crack-free coatings were formed on a reduced activation ferritic/martensitic steel F82H substrate by use of a withdrawal speed of 1.0–1.4mms−1 and a heat-treatment process in hydrogen with moisture. In deuterium permeation experiments, a 0.2-μm-thick coating on both sides of the substrate showed a reduction factor of 600–700 in comparison with a F82H substrate below 873K; however, the coating degraded at above 923K because of crack formation. A double-coated sample indicated a reduction factor of up to 2000 and did not degrade at up to 923K. The driving pressure dependence of the deuterium permeation flux indicated that the permeation tended to be limited by surface reactions at low temperatures. Optimization of the number of layers has the possibility to reduce degradation at high temperatures while maintaining high permeation reduction factors.