Component development for operation in a large-scale fusion device requires thorough testing and qualification for the intended operational conditions. In particular environments are necessary which ...are comparable to the real operation conditions, allowing at the same time for in situ/in vacuo diagnostics and flexible operation, even beyond design limits during the testing. Various electron and neutral particle devices provide the capabilities for high heat load tests, suited for material samples and components from lab-scale dimensions up to full-size parts, containing toxic materials like beryllium, and being activated by neutron irradiation. To simulate the conditions specific to a fusion plasma both at the first wall and in the divertor of fusion devices, linear plasma devices allow for a test of erosion and hydrogen isotope recycling behavior under well-defined and controlled conditions. Finally, the complex conditions in a fusion device (including the effects caused by magnetic fields) are exploited for component and material tests by exposing test mock-ups or material samples to a fusion plasma by manipulator systems. They allow for easy exchange of test pieces in a tokamak or stellarator device, without opening the vessel. Such a chain of test devices and qualification procedures is required for the development of plasma-facing components which then can be successfully operated in future fusion power devices. The various available as well as newly planned devices and test stands, together with their specific capabilities, are presented in this manuscript. Results from experimental programs on test facilities illustrate their significance for the qualification of plasma-facing materials and components. An extended set of references provides access to the current status of material and component testing capabilities in the international fusion programs.
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▶ SMSI on oxide-supported rhodium catalysts are investigated by ISS and XPS. ▶ 773
K reduction leads to encapsulation of Rh/titania model catalysts by oxide layer. ▶ Monolayer surface ...oxide leads to expected reduction in CO chemisorption. ▶ XPS following ambient pressure H
2 reduction shows no indication for Ti
3+ at surface.
Reactive processes on catalyst surfaces are studied in this work for Rh/metal oxide model systems by means of surface science techniques. Published results in the literature deal with titania, alumina, and silica as support materials and are briefly reviewed. For the present studies Rh/Al
2O
3 and Rh/TiO
2 model catalysts with about one monolayer Rh coverage are specially prepared and analyzed by ion scattering spectroscopy, X-ray photoelectron spectroscopy, and thermal desorption measurements as main techniques. In part these measurements are supported by a variety of other analytical and imaging techniques. For thermal treatment in hydrogen atmosphere at low and elevated pressures a complete Rh encapsulation by titania is observed for temperatures above 773
K. The results from ion bombardment depth profiling are corroborated by the concomitant drastic reduction of the CO adsorption capacity of these samples. The model catalysts thus exhibit the typical features of ‘strong metal–support interaction’. The effect was not found for alumina supports, for which thermal treatments mainly resulted in gradual interdiffusion of the various surface species. These results also demonstrate that characteristic catalyst behavior can be successfully studied by applying surface science methods to model catalysts.
In future fusion reactors, tungsten is a main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is an issue under the extreme fusion environment. To ...overcome this drawback, tungsten fiber-reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughening principle. In this study Wf/W composites are produced by a Field-Assisted Sintering Technology (FAST) process with different fiber–matrix interfaces. The fracture behavior was studied by 3-point bending tests on notched samples. 4-point bending tests and tensile tests are performed to measure the flexural strength and tensile strength, respectively. Wf/W with a weak interface shows a typical pseudo-ductile fracture behavior, similar to ceramic matrix composites. A strong interface is beneficial to achieve higher flexural strength and tensile strength, but in turn, weakens the pseudo-ductile behavior.
Steel components are required in the infrastructure and the facilities of the hydrogen economy. The high hydrogen pressures in the hydrogen economy lead to embrittlement and surface corrosion of the ...steels. For the functionality of the facilities it is necessary to suppress the embrittlement and the surface corrosion of the steels by protective layers, e.g. ceramic thin films. With regard to fusion power plants ceramic thin films on the structural steel materials are also required. These thin films work as a tritium permeation barrier that is necessary to prevent the loss of the radioactive fuel inventory. Oxide thin films, e.g. Al2O3, Er2O3, and Y2O3, are promising candidates as tritium permeation barrier layers. In terms of the application in the first wall, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y2O3 layers with thicknesses of 0.5 μm–1 μm are deposited on both substrate sides by RF magnetron sputter deposition. Since the microstructure of the barrier layer plays an important role for the permeation reduction, layers with three different magnetron process modes and thus three different microstructures are prepared. After annealing the cubic crystal structure of all thin films is verified by X-ray diffraction and the different microstructures are investigated by scanning electron microscopy and transmission electron microscopy. The Y2O3 stoichiometry of all thin films and a chromium oxide material segregation at the interface are verified by analysis methods such as X-ray photoelectron spectroscopy. The permeation reduction factors of all thin films are determined in gas-driven deuterium permeation experiments. Corresponding to the three different microstructures, reduction factors of 25, 45, and 1100 are identified. Thus, the permeation reduction is strongly dependent on the Y2O3 microstructure. The measurement results suggest that a high density of grain boundaries leads to a high hydrogen permeation.
•All Y2O3 thin films show the same stoichiometry and feature a similar cubic phase after annealing.•For over 2 weeks the permeation reduction of all thin films is stable or slightly increasing.•No sample shows damage formation due to the annealing in the measurements.•3 different Y2O3 microstructures correspond to the reduction factors of 25, 45, and 1100.•The grain boundary diffusion through the thin films is preferred.
The findings of the EU 'Materials Assessment Group' (MAG), within the 2012 EU Fusion Roadmap exercise, are discussed. MAG analysed the technological readiness of structural, plasma facing and high ...heat flux materials for a DEMO concept to be constructed in the early 2030s, proposing a coherent strategy for R&D up to a DEMO construction decision. Technical consequences for the materials required and the development, testing and modelling programmes, are analysed using: a systems engineering approach, considering reactor operational cycles, efficient maintenance and inspection requirements, and interaction with functional materials/coolants; and a project-based risk analysis, with R&D to mitigate risks from material shortcomings including development of specific risk mitigation materials. Lessons learned from Fission reactor material development have been included, especially in safety and licensing, fabrication/joining techniques and designing for in-vessel inspection. The technical basis of using the ITER licensing experience to refine the issues in nuclear testing of materials is discussed.
Functionally graded (FG) iron/tungsten (Fe/W) composites are considered for stress-relieving interlayers in tungsten-steel joints, required in future fusion reactors. The macroscopic gradation of the ...two materials allows relaxation of thermally-induced stresses and hence extend the lifetime of the cyclic-loaded dissimilar materials joints. While many properties, e.g. thermal expansion and strength, of the as-manufactured Fe/W composites are promising with respect to the anticipated application, the temperature-induced microstructural changes and their effect on the material properties remain largely unexplored. Given that the thermodynamic system of FeW contains two types of intermetallic phases, understanding the microstructural changes in the FG Fe/W composites is crucial for long-term operation of fusion reactors.
In the present work, the microstructure of ultra-fast sintered Fe/W composites containing 50 and 75 vol% tungsten is studied via electron microscopy (SEM) and X-ray diffraction (XRD) in as-manufactured and thermal aged conditions (300, 500, and 800 °C for up to 72 h). The hardness and modulus of selected composites are measured via nanoindentation, and the fracture toughness of the FeW interfaces is tested via notched micro-cantilever bending tests. The results from microstructural and micromechanical analyses are discussed, and the materials are evaluated for their application in fusion reactors based on the microstructure-to-property relationship.
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•Fe/W composites aged at 300, 500, and 800 °C show fast precipitation of brittle intermetallics•Via a rule of mixture, hardness and modulus of individual phases on the microscale are correlated to effective properties•Interfaces between Fe and W volumes, studied via micro-cantilever bending, show very low toughness (KIc = 1.85 MPa m0.5)
The estimation of the hydrogen isotope flux through a fusion reactor wall component is important for the material selection and in order to guarantee a safe and economical reactor operation. Since ...the permeation flux through a component cannot be measured directly, due to the large size of such a component, the deuterium permeability of various fusion materials were investigated in the last years. In order to investigate, if an interface, meaning a change of materials in a component, has an influence on the permeation flux, combined material systems in laboratory scale are studied. The combined materials systems are produced by applying a thin W or Cu layer on a polished bulk steel or CuCrZr substrates by magnetron sputter deposition. In a previous study (Houben et al., 2022), the combined material system of Cu coated steel was investigated. The conclusion was that the interface has a minor influence on the permeation flux compared to the large influence of the layer microstructure. In order to investigate, if the minor influence of the interface is valid in general, the systems W coated steel and CuCrZr are studied in this publication. By heating the substrate during W sputter deposition, a crack-free W layer is produced and crack propagation in the W layer at elevated temperature is prevented. The W layer permeabilities are obtained for both W coated substrates and are similar, but compared to the W bulk permeability from literature the W layer permeabilities are several orders of magnitude larger.
The main conclusion from these studies is that in all investigated combined material systems the influence of the interface on the permeation flux is minor compared to the large influence of the microstructure. Therefore, for a reliable estimation of the permeation flux through a fusion reactor component it is crucial to characterize the applied materials. Especial for coatings the measurement of the layer permeability is important, since the layer permeability of a material can be very different compared to the permeability of a bulk material.
•Measurement of deuterium permeability in first wall materials of fusion devices.•W layers on steel and CuCrZr substrates are prepared, analyzed and compared.•The W layer permeability is several orders of magnitude higher compared to bulk W.•The layer microstructure strongly influences the permeability.•Minor influence of interface on the permeation flux.
•Yttrium improves the self-passivation of the investigated W-Cr-Y alloys.•Yttrium reduces the oxidation rates by one order of magnitude compared to the W-Cr system.•The effects of yttrium are the ...suppression of mixed oxides and pores.•The optimal composition found in this study is of W- 12 wt.%Cr-0.6 wt.%Y.•An idealized simulation shows that the loss of alloying elements due to diffusion and sputtering is small.
Tungsten is a prime material candidate for the first wall of a future fusion reactor. In the case of a loss-of-coolant accident (LOCA) wall temperatures of about 1450 K could be reached lasting about 30–60 days due to nuclear decay heat. In the worst case scenario combining LOCA with air ingress, the formation and release of highly volatile and radioactive tungsten trioxide (WO3) into the environment can occur. Smart self-passivating tungsten alloys preventing the formation of WO3 can be a way to mitigate this release.
In this contribution we present the studies of a new yttrium-containing W-Cr-Y alloys. The extent up to which yttrium acts as an active element improving the adherence and stability of the protective Cr2O3 layer formed during oxidation is assessed. The approach is similar to the one taken for high-temperature steels where active elements stabilize the oxide layers at a substantially reduced thickness by changing the oxygen diffusion and improving the adherence of the protective oxide layer by e.g. avoiding of pores. Further, simulations on mobilized material for the case of a LOCA are developed. In addition, the loss of alloying elements during normal operation of a reactor is estimated. This is done by modelling a thermally activated diffusion, using a diffusion coefficient which is extrapolated from experimental data at higher values.
The oxidation behaviour of magnetron sputtered and therefore alloyed at the atomic level W-Cr-Y alloys is tested in a thermo-gravimetric facility. The isothermal oxidations are performed in a gas mixture, containing 20 kPa oxygen and 80 kPa argon under ambient pressure at temperatures of 1273 K and 1473 K, respectively. Experiments with W-Cr-Y show a parabolic oxidation rate of kp=3·10−6mg2cm−4s−1 which is more than five orders of magnitude lower than that of pure tungsten at 1273 K. Investigations using X-ray diffraction analysis and focused ion beam cross-sections in combination with scanning electron microscopy and energy dispersive X-ray spectroscopy are conducted. A protective Cr2O3 layer is detected on the surface with a thickness between 100 and 300 nm.
DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions ...pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.
The retention and release of deuterium is compared for clean metallic beryllium and beryllium oxide. Deuterium energies of 600eV per atom are applied and implantation fluences are above the threshold ...for supersaturation. Desorption experiments subsequent to implantation into metallic Be are extrapolated to a perfectly homogeneous implantation spot. It is assessed that more than 80% of the D retained after implantation with homogeneous high fluences are released below 500K. All retained D is released below 800K. The oxide layer is grown by stepwise oxidation of Be in oxygen atmosphere and characterized by Rutherford backscattering spectrometry. The oxide thickness is large enough to guarantee deuterium implantation completely within the oxide layer. The release of deuterium from BeO proceeds at a slow rate over the whole temperature range from 350 to 850K. At this temperature only 75% of the amount retained after implantation has desorbed.
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