Tritium Permeation Barrier Research Articles

In-situ oxidation of aluminized stainless-steel to form alumina as tritium permeation barrier coating

Tritium permeation barrier (TPB) coating technology is one of the most effective methods to solve the problem of permeation and leakage of tritium through the structural materials to the coolant, environment, and other functional components in tritium breeder blanket modules (TBMs). Alumina coatings prepared by in-situ oxidation of aluminized steel shows a high permeation reduced factor (PRF) and self-healing ability. However, to eliminate the Al5Fe2 phase in the aluminizing process, the stainless-steel substrate is often heat treated under high temperature conditions for a long time which leads to the degeneration of steel. In this work, a gradient alumina TPB coating was prepared on 316 L stainless steel by aluminizing process at low temperature. The influence of Al5Fe2 on the formation of Al2O3 is studied. The mechanism for the different growth behavior of Al2O3 in the polished and unpolished samples is also discussed. Compared to the polished samples, the existence of porous Al5Fe2 layer in unpolished sample changes the diffusion behavior of Al and leads to the formation of an un-continuous and rough Al2O3 layer.

First principles calculation of hydrogen adsorption, dissociation and diffusion on Cr/Ni/Mn doped α-Al2O3 (0001) surface

Cr, Ni and Mn are common impurities observed in Al2O3 tritium permeation barrier in experiments, to acquire influence of these metal impurities on the interaction of hydrogen isotopes with α-Al2O3 surface, energies and geometric structures of H2 adsorption, dissociation and diffusion on Cr/Ni/Mn doped α-Al2O3 (0001) surface have been investigated by first-principles. It's found that Cr and Ni have slight effect on H2 adsorption and dissociation on α-Al2O3 (0001) surface due to the similar adsorption and dissociation behavior and close activation energies before and after doping. On Mn doped α-Al2O3 (0001) surface, the adsorption energies for H2 are close to that on undoped surface, while the lowest activation energies for H2 dissociation and H atoms recombination are 0.32 eV and 0.24 eV, which are obviously lower than that for H2 dissociation on undoped α-Al2O3 (0001) surface. This finding reveals that Mn doping is favor to H2 dissociation and also beneficial to H atoms recombination on α-Al2O3 (0001) surface. The lowest activation energy is 1.36 eV for H atom diffusion from surface to bulk on Mn doped α-Al2O3 (0001) surface and obviously higher than that of H diffusion through RAFM steels measured through H2 gas evolution permeation experiment, indicating that Mn surface doping prevents H permeation in Al2O3 tritium permeation barrier from the view of diffusion activation energy. The results are constructive for optimization the preparation technique of TPBs.

An efficient graphene oxide reinforced aluminum phosphate/Cr2O3 double coating as an enhanced tritium permeation barrier

Fabrication of dense ceramic coating on steels is one of the most efficient methods to prevent hydrogen ingress into the structural materials. In this study, a dense graphene oxide (GO)-reinforced AlPO4 composite layer on a Cr2O3 coating was successfully fabricated by the dip coating method. The effects of GO content on the microstructure, phase compositions, mechanical and hydrogen isotope barrier properties were investigated. The results show that GO can effectively improve the mechanical properties of the double-coated specimens and the GO-reinforced AlPO4 composite layer has a positive effect on the improvement of deuterium permeation resistance. The 0.050 GO-AlPO4/Cr2O3 sample with a dense and uniform double-coating structure demonstrates the best overall performance. The microhardness, bonding strength, corrosion voltage, and corrosion current can reach 317.3 HV, 21.5 MPa, −77 mV, and 3.92 × 10−9 A·cm−2, respectively. In addition, the deuterium permeability reduction factor (PRF) in the temperature range of 450–550 °C is greatly improved. This enhancement can be attributed to the chemical inertness and barrier effect of GO and the synergy effects of GO-reinforced AlPO4 layer and Cr2O3 layer. As the test temperature rises to 600 °C, the phase transition of Cr2O3 is observed, and the phase transition process is revealed by thermodynamic method.

Design and properties of FeAl/Al2O3/TiO2 composite tritium-resistant coating prepared through pack cementation and sol–gel method

Structural steel materials exist in tritium breeding blankets and auxiliary tritium handling systems in fusion reactors. Applying a tritium permeation barrier coating on the surface of the structural material is necessary to prevent material corrosion and embrittlement. In this study, FeAl/Al2O3/TiO2 composite tritium-resistant coating on 316 L stainless steel substrate was successfully prepared by pack cementation and sol–gel method. The surface and cross-section morphologies of the composite coatings were systematically studied. The hydrogen permeation properties and adhesion between the different composite coatings were investigated. Results show that the FeAl/Al2O3/TiO2 composite tritium resistant coating is uniform, dense, and preferably connected to the substrate without defects such as cracks and pores. Compared with FeAl/Al2O3 coating, FeAl/Al2O3/TiO2 coating has better bonding strength, toughness, and ductility. Hydrogen permeation test shows that the design of the FeAl/Al2O3/TiO2 composite coating improves the hydrogen resistance of the coating at room temperature.

First-principles study on the dissolution and diffusion properties of hydrogen in α-Al2O3

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. α-Al2O3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk α-Al2O3. The calculation results show that the most energetically stable form of hydrogen in a perfect α-Al2O3 crystal under H2 gas annealing treatment is the H2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in α-Al2O3, these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in α-Al2O3 is 2Hi, with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H2 molecule. We have derived the temperature-dependent diffusivity of the H2 molecule in α-Al2O3 as D(T)=(3.65×10−7m2/s)exp(−2.27eV/KT), which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H2 molecule are so high that dissolution and diffusion of the H2 molecule in α-Al2O3 are very difficult, resulting in low hydrogen permeability.

Fe effect on hydrogen interactions with intrinsic point defects and hydrogen migration in α-Al2O3 based tritium permeation barriers

α-Al2O3 based coatings are the first candidate for tritium permeation barriers (TPBs) in fusion reactors, and the impurity effect on hydrogen resistance is one of the major concerns. In the present work, Fe effect on hydrogen interactions with intrinsic point defects and hydrogen migration in α-Al2O3 under the TPB service condition has been investigated based on first-principles calculations. It is found that Fe is unfavorable for the formation of vacancy-type defects and defect complex [VAl-H] in α-Al2O3, yet beneficial for the formation of H-related defects Hi and HO. The H trapping ability by vacancy type defects in α-Al2O3 is lowered after Fe doping. Hi is the main diffusion species in Fe-doped α-Al2O3, and Fe is favorable for the H migration in α-Al2O3, which will be unfavorable for H resistance of α-Al2O3 based TPBs. During the Hi migration in Fe-doped α-Al2O3, one step of screw motion is involved, in which the forming and breaking of the H-Fe and H-O bonds occur.

First-principles insights of hydrogen diffusion dynamics at the α-Al2O3 (0001) surface

Hydrogen diffusion dynamics in the α-Al2O3 is very important because it associates directly with the performance of α-Al2O3 as the tritium permeation barrier (TPB) material in the tritium handling systems. In this work, the kinetics of hydrogen diffusion at the α-Al2O3 (0001) surface and hydrogen permeation into the α-Al2O3 bulk have been systematically studied by using the first-principles calculations. Results show that the vacancy defect can reduce the hydrogen migration barrier, while the Fe-doping defect can enhance the hydrogen migration barriers. This result is due to the dopant Fe showing bivalent instead of trivalent, which leads to the formation of a negatively charged center due to the substitution of divalent Fe for trivalent Al. In addition, results show quantitatively that the outer three oxygen layers (9 atomic layers) play a key role in hydrogen resistance. Hydrogen permeation from the outmost surface to the subsurface is the rate-determining step, and the corresponding energy barrier is about 3.327 eV.

Different Fe(Al) transition coatings on the performance of Al2O3 coating

Three kinds of Al2O3-based tritium permeation barriers with different Fe(Al) transition layers, namely, FeAl/Al2O3, FeAl2/Al2O3 and Fe3Al/Al2O3 coatings were prepared on Si substrates using radio frequency magnetron sputtering technology. The microstructure, mechanical properties, and electron-irradiation resistance of the coatings were investigated systematically to explore the effect of Fe(Al) transition layers on the growth and performance of the Al2O3 coatings. These different coatings were fabricated successfully on Si substrates, while FeAl, FeAl2 and Fe3Al transition layers exhibited a columnar grain structure and the Al2O3 coatings were composed of amorphous and γ-Al2O3 crystalline phases. FeAl and Fe3Al transition layers enhance the formation of γ-Al2O3 crystalline phase, whereas the surface of FeAl2 transition layer is mainly amorphous Al2O3. The FeAl2/Al2O3 coating exhibits the highest adhesive force of 49.75 N among the three samples, and the adhesive forces of the FeAl/Al2O3 and Fe3Al/Al2O3 coatings are 37.75 N and 47.4 N respectively. The surface roughness values of the FeAl/Al2O3, FeAl2/Al2O3 and Fe3Al/Al2O3 coatings are 3.483 nm, 1.042 nm, 4.565 nm, while the hardness values of three samples are 4199 MPa, 6812 MPa, 4796 MPa, respectively. The surface roughness of the FeAl/Al2O3 and Fe3Al/Al2O3 coatings decreased, but the surface roughness of FeAl2/Al2O3 increased after electron irradiation tests. Moreover, the hardness of all three samples was reduced after electron irradiation. These results provide a reference for further studies of the Fe(Al)/Al2O3 coating.

Preparation of Al2O3/Y2O3 composite coating for deuterium permeation reduction

A tritium permeation barrier is required in fusion blankets for the reduction of fuel loss and radiological hazard. In this study, an Al2O3/Y2O3 composite coating was prepared on 316L stainless steel by radio-frequency magnetron sputtering in order to improve the tritium permeation resistance. The microstructure and the phase composition of the Al2O3/Y2O3 composite coating are observed by scanning electron microscopy, transmission electron microscopy and grazing incidence X-ray diffraction. Moreover, Auger electron spectroscopy was used to characterize the depth profiles of Al, Y and O elements. The results clearly indicate that the Al2O3/Y2O3 composite coating is fully dense and the total thickness is approximately 340 nm. The Al2O3/Y2O3 coating consists of an amorphous Al2O3 and the cubic Y2O3, in which Al, Y and O elements are homogeneously distributed in the vertical base direction. Furthermore, the deuterium permeation property of the Al2O3/Y2O3 composite coating was measured by the gas phase permeation method. The results show that the introduction of an interface and the existence of a tiny amount of micro-defects improve the deuterium resistance of the Al2O3/Y2O3 coating, and its deuterium permeation reduction factor is 536–750 at 873–973 K. Therefore, it is concluded that the Al2O3/Y2O3 composite coating as deuterium permeation barrier can significantly enhance the deuterium permeation resistance property.

Gamma-ray irradiation effect on deuterium retention in reduced activation ferritic/martensitic steel and ceramic coatings

Tritium permeation and retention are serious problems in D-T fusion reactors from the viewpoint of fuel efficiency and radiological safety. Functional ceramic coatings have been intensively studied for the development of tritium permeation barriers for several decades, while reports about tritium retention in the ceramic coatings are scarce. Moreover, irradiation may affect tritium retention in fusion materials, which is important to precisely evaluate tritium inventory in the reactor. In this study, the gamma-ray irradiation effect on deuterium retention in reduced activation ferritic/martensitic steel and three kinds of ceramic coatings were investigated through deuterium exposure, gamma-ray irradiation using cobalt-60 gamma-ray sources and deuterium depth profile measurements. The amount of deuterium retention in yttrium oxide, silicon carbide, and zirconium oxide coatings decreased after the irradiation in the dose rate of 2.43 Gy s−1, while no clear change in the retention was observed at the lower dose rate. From these results, the gamma-irradiation effect on deuterium retention would have a threshold dose rate. Diffusion and desorption of deuterium would be accelerated by excitation of deuterium via energy transfer from electrons generated by Compton scattering.

Deuterium permeation behavior through yttria-stabilized zirconia coating fabricated by magnetron sputtering

Abstract For the realization of a sufficient fuel cycle in a fusion reactor, tritium permeation barriers have been studied mainly using ceramics coatings to suppress tritium leakage through structural materials. In this study, the first trial of fabrication and characterization of yttria-stabilized zirconia (YSZ) coatings has been carried out. The YSZ coatings containing 4 mol% Y2O3 were deposited on reduced activation ferritic/martensitic steels by radio-frequency magnetron sputtering devices in two different deposition modes. The coating fabricated in the metallic mode showed a lower density with many cracks, and deteriorated after annealing and deuterium permeation tests at 600 °C. On the other hand, the coating fabricated in the poison mode showed relatively higher density, and did not deteriorate from the result of the deuterium permeation measurements at up to 600 °C. These results suggest that the YSZ coating deposited in the poison mode would be applied as a tritium permeation barrier.

A composite coating of GO-Al2O3 for tritium permeation barrier

To improve tritium permeation resistance and toughness of Al2O3 coating, a new graphene oxide (GO)-alumina (Al2O3) composite coating as tritium permeation barrier was designed in this paper. GO was added into Al2O3 to prepare the composite coating of GO-Al2O3 by a sol-gel method. The results showed that the deuterium permeation reduction factor (PRF) of 0.5 wt% GO-Al2O3 coating was higher than that of the pure Al2O3 coating at 500 ℃. It was related to the GO nanosheets blocking the diffusion of deuterium atoms in the Al2O3 coating and reducing the permeability. Meanwhile, the toughness of the 0.5 wt% GO-Al2O3 coating was enhanced by about 40 % than the pure Al2O3 coating. The reason can be speculated that GO nanosheets can prevent the cracks propagation in the composite coating of GO-Al2O3. This study could provide a new material of tritium permeation barrier for fusion reactors.

Tungsten nitride as tritium permeation barrier

• Three kinds of W and WN layers on steel substrates were deposited. • The samples were investigated by deuterium permeation measurements and compared. • WN and WNW layers are very suitable as tritium permeation barrier layers on steel. • The layer permeabilities of the WN and non-cracked W coatings were calculated. • The layer permeability enables a reliable comparison of permeation barriers. • The hydrogen permeation flux through components in fusion reactors can be estimated. The development and application of robust tritium permeation barrier coatings is crucial for a safe and economic fusion reactor operation. Three different tungsten and tungsten nitride layers on Eurofer97 substrates were investigated by deuterium permeation measurements and compared. The microstructure and crystal structure was characterized before and after permeation measurements. The layer permeability is independent of the layer thickness and substrate of the sample. For a reliable comparison of different tritium permeation barrier coatings, the layer permeability of each layer was calculated. With this layer permeability, the permeation flux through potential fusion device components can be estimated. As examples, the permeation flux through a 0.5 cm thick steel component can be reduced by two orders of magnitude by a 2 μ m thick WN layer and nearly four orders of magnitude by a 2 μ m non-cracked tungsten layer.

Hydrogen Permeation in Fusion Materials and the Development of Tritium Permeation Barriers

Hydrogen Permeation in Fusion Materials and the Development of Tritium Permeation Barriers

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO2/TiCx composite coating, was proposed. The preparation process of this TiO2/TiCx composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO2 coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO2 coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiCx layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.

First principles investigation of hydrogen adsorption and dissociation on Fe doped α-Al2O3 (0001) surface

Energies and geometric structures of H2 molecule adsorption and dissociation on Fe doped α-Al2O3 (0001) surface have been investigated by the first-principles calculation since Fe is a common impurity in Al2O3 tritium permeation barrier. It’s found that Fe prefers to substitute the surface Al site and the adsorption of H2 on Fe doped α-Al2O3 (0001) surface is physisorption. The adsorption energies and dissociation activation energies of H2 absorbed on Fe doped α-Al2O3 (0001) surface are slight lower than that of H2 on undoped α-Al2O3 (0001) surface. The activation energies of H atoms recombination are 0.65 eV and 0.53 eV and obviously smaller than the values of 0.92 eV and 1.03 eV for H atoms recombination on undoped α-Al2O3 (0001). Therefore, Fe surface doping has slight effect on the reactivity for H2 adsorption and diffusion on α-Al2O3 (0001) surface, but is beneficial to α-Al2O3 as a tritium barrier from the view of H atoms surface recombination.

Efficient deuterium permeation reduction coating formed by oxidizing the Fe–Cr–Al ferritic steel in reduced oxygen atmosphere at 973 K

Alumina is regarded as one of the most promising candidate tritium permeation barrier (TPB). Through thermal oxidization of Al-contained alloys Fe–Cr–Al, alumina layer with high tritium permeation reduction ability can be obtained. After bonding this kind of materials with structural materials such as reduced activation ferritic/martensitic (RAFM) steels, it can serve as TPB. In this work, efforts have been done to enhance the hydrogen isotope permeation reduction ability of the alumina layer on the Fe–Cr–Al ferritic steel by optimizing the oxidation process. The oxidation temperature of the Fe–Cr–Al ferritic steel is set to 973 K, which is lower than the final heat treatment temperature of the RAFM steel. Three different atmospheres have been employed for the oxidation process of the Fe–Cr–Al ferritic steel. Gas driven permeation (GDP) experiments have been performed to examine the deuterium permeability of the oxidized Fe–Cr–Al ferritic steel. The deuterium permeability of the Fe–Cr–Al ferritic steel oxidized in argon with 1700 ppm oxygen is 104 times lower than that of the RAFM steel at 823 K. The microstructure and chemical composition of the oxide layer of the Fe–Cr–Al ferritic steel oxidized in three different atmospheres has been clarified using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) methods. It is suggested that oxidizing the Fe–Cr–Al ferritic steel at reduced oxygen atmosphere could inhibit the growth of the iron oxide and chromium oxide in the oxide layer, thereby leading to a dense and compact alumina layer that has excellent hydrogen permeation reduction performance.

Preparation and properties of FeAl/Al2O3 composite tritium permeation barrier coating on surface of 316L stainless steel

In fusion reactors, preparing barrier coatings on the surface of steel structures is one of the most effective ways to reduce the penetration of hydrogen and its isotopes. An FeAl/Al2O3 composite tritium permeation barrier is a good choice for the current tritium permeation prevention technology because it has low enthalpy permeability, small thermal mismatch, good compatibility with lithium lead, and metallurgical bonding. In this study, FeAl/Al2O3 composite tritium permeation barrier coating was successfully prepared on the surface of 316L stainless steel by embedding method combined with in-situ thermal oxidation. By using test methods, such as TEM, SEM, EDS, and XRD, we found that the FeAl alloy transition layer was successfully formed on the surface of the aluminized matrix, thereby effectively relieving the thermal mismatch between the Al2O3 coating and the substrate. The results showed that the thickness of the aluminum-rich layer and the FeAl transition layer on the surface of the substrate were approximately 32 μm and 8 μm, respectively. In addition, α-Al2O3 layer formed on the surface of the aluminum-rich layer after oxidation was approximately 4 μm thick. According to the scratch test, the bond strength of the coating showed that the adhesion of the FeAl/Al2O3 composite coating was 52.4 N. The hydrogen permeation test results showed that the FeAl/Al2O3 composite coating remarkably improved the hydrogen barrier properties of the 316L stainless steel matrix.

Preparation and characterization of Er2O3 films on CLAM steel via sol-gel method

Er2O3 coating is one of the candidate tritium permeation barriers (TPB) to reduce tritium permeation and leakage in fusion reactors. A series of Er2O3 coatings were synthesized on China Low Activation Martensitic (CLAM) steel via sol-gel method, and systematic investigate the influence of synthesis parameters for on microstructure and properties of the Er2O3 coatings. The results showed that no cracks were observed in Er2O3 coatings as chosen 11 vol.% diethanolamine. The best heat treatment temperature could be thought 600 °C, in order to obtain a homogeneous and dense Er2O3 coating with high hardness, good crystallization and strong bonding to the substrate.

Hydrogen isotope role in the crystal orientation change of erbium oxide coatings

Erbium oxide (Er2O3) is a candidate material for tritium permeation barriers (TPB) applied to fusion blanket systems. To permeate through a TPB coating on a metal substrate, a hydrogen isotope must first adsorb on the surface, and then penetrate from the surface into the interior. In this work, Er2O3 coatings were fabricated by arc source assisted deposition and annealing at 973 K in vacuum (<10−5 Pa). The effect of hydrogen on the crystal orientation change of Er2O3 coatings was examined by X-ray diffraction and density-functional theory studies. It was found that the surface energy of a partially H-covered initially {111} Er2O3 slab increases upon annealing at 973 K, and that the associated crystal orientation change from {111} to {100} never occurs without hydrogen even at high temperature.