[en] In this study, samples of Eurofer 97 steel were subjected to processing by hydrostatic extrusion (HE) to a total true strain of about 4. HE processing resulted in a significant grain refinement. Microstructural changes caused by extrusion were observed using transmission and scanning electron microscopy. Thin foils for these observations were prepared using focus ion beam and electrochemical thinning. The microstructure of Eurofer 97 in the as-received state is characterized by ferrite grains, with some presence of martensite, and a significant density of carbides. The microstructures were quantitatively described in terms of the grain and particle size and their distributions. The average ferrite grain size in the as-received state was about 400 nm and the average size of carbides - 111 nm. After HE the grain size dropped to 86 nm and the average size of carbides to 75 nm HE also resulted in a more uniform grain size and spatial distribution of carbides. Various carbides were identified including TaC of the diameter of 20 nm. (au)

[en] Highlights: • Grain structure and crystallographic phase remain unchanged during the annealing in the permeation measurement. • The filling of the traps takes ∼20 h measurement time with exposure to deuterium. • The permeation reduction factor of 30 is the minimum value, after the filling of the traps was completed. • Over the measurement time of 7 days, the permeation barrier performance is degraded by a factor of 35. • For shorter measurement times than 20 h the apparent permeation reduction factor can be in the range of 1000. - Abstract: 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.

[en] The hydrogen retention in fusion reactors can be significantly influenced by the presence of plasma impurities. Earlier studies showed that helium can reduce the retention in tungsten wall materials. This paper gives the results of experiments on this topic in the linear plasma generator PSI-2. Exposures of polycrystalline tungsten samples to a deuterium plasma were performed at low temperatures (380 K) under the variation of the impurity species (He, Ar) and concentration (0–5%). For the experiments with He, the total deuterium fluence was varied between 2 ⋅ 10"2"4 m"−"2 and 2 ⋅ 10"2"6 m"−"2. Subsequently, the surface morphology and deuterium retention were investigated. The results show a reduction of the deuterium retention by a factor of 3 for helium, and an increase by up to 30% for argon. A diffusion model for the helium case was developed, in which a shallow layer of porous helium nanobubble structures reduces the total deuterium content

[en] Over the last few years, oxide dispersion strengthened (ODS) ferritic steels have emerged as one of the major materials for fusion reactors. Despite the progress made in their technology, currently available properties still do not meet all the expectations. As a result these steels remain the subject of extensive research, recently centred on a possible improvement due to their nanometric engineering. In particular, technologies are developed for grain size refinement down to nanometres and strengthening by nanooxides. This, in turn, calls for nanoscale investigations of the microstructures, which can be efficiently carried out only with the use of high-resolution transmission electron microscopy and spectroscopy. The present paper illustrates the results of investigations of a model alloy of ODS steel using a recent high-resolution scanning transmission electron microscope equipped with a spherical abberation Cs-correction system, energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy spectrometers. It allows bright field on transmitted electron imaging and secondary electron imaging, as well as high-angle annular dark field imaging, the so-called Z-contrast imaging. The results obtained in dark field prove that modern microscopy techniques are a necessity to provide new information on the nanostructure of the ODS steels relevant to the understanding and shaping of their properties.

[en] Highlights: • New reduced activation ferritic steel investigated for nuclear fusion applications. • PSI-2 D₂ plasma exposures up to 1150 K and modelling investigations. • ppm level of D retention and W enrichment after exposure. • Low impurity and C contents lead to low long-term activity. • Component modelling shows at least 1.5 MW/m² load limit. - Abstract: Materials are the most urgent issue in nuclear fusion research. Besides tungsten, steels are considered for unifying functional and structural materials due to their cost and mechanical advantages over tungsten. However, the fusion neutrons impose a strong constraint on the ingredients of the steel in order to avoid long lasting activation, while the material has to pertain sputtering resistance, low hydrogen retention, and long-term mechanical stability. In this proof-of-principle, we demonstrate the interesting properties of the new material HiperFer (High performance Ferrite) as a material suitable for fusion applications. The investigation covers neutron activation modelled by FISPACT-II, plasma sputtering and deuterium retention experiments in PSI-2, thermo-mechanical properties and component modelling. The material was found to feature a low nuclear inventory. Its sputtering yield reduces due to preferential sputtering by a factor 4 over the PSI-2 D₂ plasma exposure with possible reductions of up to 70 indicated by SD.Trim.SP5 modelling. The exposure temperature shows a strong influence on this reduction due to metal diffusion, affecting layers of 1 µm in PSI-2 at 1150 K exposure for 4 h. Deuterium retention in the ppm range was found under all conditions, together with ∼10 ppm C and N solubility of the ferritic material. The creep and cyclic fatigue resistance exceed the values of Eu-97 steel. As an all HiperFer component, heat loads in the order of 1.5 MW/m² could be tolerated using water-cooled monoblocks. In conclusion, the material solves several contradictions present with alternative reduced-activation steels, but its applications temperatures >820 K also introduce new engineering challenges.

[en] Highlights: • Y₂O₃ was deposited by reactive magnetron sputtering in two deposition modes. • As-deposited thin films retain metastable structure. • Annealing has been verified as an effective structure stabilizing process. • Different deposition modes give various application options as a nuclear material. • Potential solutions are substrate heating and deposition conditions combination. - Abstract: Yttrium oxide thin films were prepared by reactive magnetron sputtering in different deposition condition with various oxygen flow rates. The annealing influence on the yttrium oxide film microstructure is investigated. The oxygen flow shows a hysteresis behavior on the deposition rate. With a low oxygen flow rate, the so called metallic mode process with a high deposition rate (up to 1.4 µm/h) was achieved, while with a high oxygen flow rate, the process was considered to be in the poisoned mode with an extremely low deposition rate (around 20 nm/h). X-ray diffraction (XRD) results show that the yttrium oxide films that were produced in the metallic mode represent a mixture of different crystal structures including the metastable monoclinic phase and the stable cubic phase, while the poisoned mode products show a dominating monoclinic phase. The thin films prepared in metallic mode have relatively dense structures with less porosity. Annealing at 600 °C for 15 h, as a structure stabilizing process, caused a phase transformation that changes the metastable monoclinic phase to stable cubic phase for both poisoned mode and metallic mode. The composition of yttrium oxide thin films changed from nonstoichiometric to stoichiometric together with a lattice parameter variation during annealing process. For the metallic mode deposition however, cracks were formed due to the thermal expansion coefficient difference between thin film and the substrate material which was not seen in poisoned mode deposition. The yttrium oxide thin films that deposited in different modes give various application options as a nuclear material.

[en] Tungsten is a candidate material for plasma-facing components in nuclear fusion reactors. In operation it will face temperatures >800 K together with an influx of helium ions. Previously, the evolution of special surface nanostructures called fuzz was found under these conditions in a limited window of surface temperature, ion flux and ion energy. Fuzz potentially leads to lower heat load tolerances, enhanced erosion and dust formation, hence should be avoided in a fusion reactor. Here the fuzz growth is reinvestigated in situ during its growth by considering its impact on the surfaces infrared emissivity at 4 μ m wavelength with an infrared camera in the linear plasma device PSI-2. A hole in the surface serves as an emissivity reference to calibrate fuzz thickness versus infrared emissivity. Among new data on the above mentioned relations, a lower fuzz growth threshold of 815 ± 24 K is found. Fuzz is seen to grow on rough and polished surfaces and even on the hole’s side walls alike. Literature scalings for thickness, flux and time relations of the fuzz growth rate could not be reproduced, but for the temperature scaling a good agreement to the Arrhenius equation was found. (paper)

[en] Highlights: • Combined steady-state He-plasma and thermal shock exposure of W. • Strong modification of the W-fuzz, if combined with thermal shocks. • He induced nanostructures grow in cracks and enhance the risk of erosion. • Formation of He-bubbles near the surface reduces the thermal conductivity. • Alteration of laser absorption due to W-fuzz influences the thermal shock behavior. - Abstract: Experiments were performed in the linear plasma device PSI-2 in order to investigate the synergistic effects of combined steady-state He-plasma and thermal shock exposure. Tungsten produced according to the ITER material specifications by Plansee SE, Austria, was loaded sequentially and simultaneously by steady-state He plasma and transient thermal loads induced by a laser beam. All tungsten samples were exposed to helium plasma for 40 min at a base temperature of ca. 850 °C and a flux of ca. 2.8 × 10²² m⁻²s⁻¹. Before, during and after the plasma exposure 1000 thermal shock pulses with a pulse duration of 1 ms and a power density 0.76 GW/m² were applied on the samples. The thermal shock exposure before and after plasma exposure was done at room temperature in order to investigate helium induced surface effects also within cracks. The obtained results show that the combination of He plasma with transient thermal shock events results in a severe modification such as reduced height or agglomeration of the sub-surface He-bubbles and of the created nanostructures, i.e. W-fuzz.

[en] The influence of helium and argon impurities on the deuterium retention in tungsten is investigated by a numerical diffusion model, which treats diffusing depth profiles for deuterium and helium or argon in tungsten, taking into account the suggested effects of helium or argon. With helium, a helium nanobubble layer builds up at the surface of the sample, with depths higher than the penetration depth of the incident helium and deuterium ions. The nanobubbles form a porous network, which allows the release of trapped deuterium by surface recombination and diffusion through the pores to the surface. For argon, only a shallow layer of argon-induced defects exists, which also act as trapping sites for deuterium. A number of experiments with tungsten samples were conducted at the linear plasma device PSI-2 in support of the model. Helium and argon were admixed to deuterium plasma in ratios of up to 8% for otherwise similar exposure conditions. In addition, a variation of ion fluences was performed for investigation of the onset and evolution of the effects of impurities. The model shows that the influence on the deuterium retention both for helium nanobubbles as well as for argon-induced defects depends strongly on the ratio between the thickness of the helium- or argon-affected layer and the penetration depth of deuterium ions. (paper)

[en] The severe particle, radiation and neutron environment in a future fusion power plant requires the development of advanced plasma-facing materials. At the same time, the highest level of safety needs to be ensured. The so-called loss-of-coolant accident combined with air ingress in the vacuum vessel represents a severe safety challenge. In the absence of a coolant the temperature of the tungsten first wall may reach 1200 °C. At such a temperature, the neutron-activated radioactive tungsten forms volatile oxide which can be mobilized into atmosphere. Smart tungsten alloys are being developed to address this safety issue. Smart alloys should combine an acceptable plasma performance with the suppressed oxidation during an accident. New thin film tungsten–chromium–yttrium smart alloys feature an impressive 10⁵ fold suppression of oxidation compared to that of pure tungsten at temperatures of up to 1000 °C. Oxidation behavior at temperatures up to 1200 °C, and reactivity of alloys in humid atmosphere along with a manufacturing of reactor-relevant bulk samples, impose an additional challenge in smart alloy development. First exposures of smart alloys in steady-state deuterium plasma were made. Smart tungsten–chroimium–titanium alloys demonstrated a sputtering resistance which is similar to that of pure tungsten. Expected preferential sputtering of alloying elements by plasma ions was confirmed experimentally. The subsequent isothermal oxidation of exposed samples did not reveal any influence of plasma exposure on the passivation of alloys. (paper)