The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load ...capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme’s main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments.
Material issues pose a significant challenge for future fusion reactors like DEMO. When using materials in a fusion environment a highly integrated approach is required. Damage resilience, power ...exhaust, as well as oxidation resistance during accidental air ingress are driving issues when deciding for new materials. Neutron induced effects e.g. transmutation adding to embrittlement are crucial to material performance. Here advanced materials such as tungsten fibre-reinforced tungsten Wf/W and fibre-reinforced copper Wf/Cu composites could allow the step towards a fusion reactor. Recent developments in the area Wf/W mark a possible path towards a component mock-up early enough for utilisation in DEMO. High heat-flux tests show that having short fibres at the exposed surface leads to their selective erosion and melting. Initial tests in the linear plasma device PSI-2 confirm this behaviour.
•Based on the presented tests for PM-Wf/W with W-foil it can be said that the manufacturing path for Wf/W has been further improved.•The presented approach utilising a W-Foil mitigates the ...embrittlement of the constituent fibres during FAST processing.•Based on these results it can be seen that improved pseudo-ductile behaviour can be achieved for PM-Wf/W. It is planned to utilise this new route in developing prototype components for applicationin existing fusion devices.•In order to also establish material performance under irradiation PM - Wf/W samples are earmarked for irradiation in a nuclear reactor starting in 2018.
For the first wall of a fusion reactor unique challenges on materials in extreme environments require advanced features in areas ranging from mechanical strength to thermal properties. The main challenges include wall lifetime, erosion, fuel management and overall safety. For the lifetime of the wall material, considerations of thermal fatigue due to transient heat loading are crucial as severe mechanical and thermal loads during operation are expected.
Tungsten (W) is the main candidate material for the first wall of a fusion reactor as it is resilient against erosion, has the highest melting point of any metal and shows rather benign transmutation behavior under neutron irradiation. However, Tungsten has an issue related to intrinsic brittleness as well as operational embrittlement. To overcome this, a W-fiber enhanced W-composite material (Wf/W) incorporating extrinsic toughening mechanisms has been developed. Recently progress has been made in the powder metallurgical routes towards fully dense multi short-fiber Wf/W. For reasonable performance with respect to mechanical properties and hydrogen retention a fully dense pseudo-ductile Wf/W with is crucial. The properties of the used fibres are crucial. For the composite mechanisms to work a level of strength of the used fibres is required. In this contribution the change in ductility of the fibres is studied.
In this contribution it is shown that excluding or minimising the impact of carbon impurities during the sintering process can significantly improve the mechanical properties of the fibres. New test results on the behaviour of PM Wf/W with and without a diffusion barrier during the sintering show a clear benefit as the fibres can retain ductility. Not the grain growth during sintering but the carbon present during sintering is clearly identified as determining the mechanical properties of the fibres.
The potential application of tungsten as a structural material has been strongly restricted by inherent brittleness. The hitherto metallurgical efforts to improve tungsten toughness seem to be still ...less matured. The authors have been exploring a novel toughening technique based on reinforcement by tungsten wires. Toughness is supposed to be enhanced through the energy dissipation at the wire/matrix interfaces which is caused by the controlled crack deflection and friction. In this work, we focus on two kinds of copper coatings for interface engineering, namely, copper single-layer and copper/tungsten multi-layer. Single-filament composites were fabricated using magnetron sputtering and CVD process. The interfacial parameters were identified by means of fiber push-out test and microscopic fracture features were investigated. In this paper the results from the extensive push-out experiments are presented together with the fractographs. Finite element simulation was also carried out to estimate the plastic strain of the copper layer. Essential role of the significant plastic deformation in the overall failure behavior is highlighted.
To overcome the intrinsic brittleness of tungsten (W), a tungsten fiber-reinforced tungsten-composite material (Wf/W) is under development. The composite addresses the brittleness of W by extrinsic ...toughening through the introduction of energy dissipation mechanisms. These mechanisms allow the reduction of stress peaks and thus improve the materials resistance against crack growth. They do not rely on the intrinsinc material properties such as ductility. By utilizing powder metallurgy (PM) one could benefit from available industrialized approaches for composite production and alloying routes. In this contribution the PM method of hot isostatic pressing (HIP) is used to produce Wf/W samples containing W fibers coated with an Er2O3 interface. Analysis of the matrix material demonstrates a dense tungsten bulk, a deformed fiber and a deformed, but still intact interface layer. Metallographic analysis reveals indentations of powder particles in the interface, forming a complex 3D structure. Special emphasis is placed on push-out tests of single fiber HIP samples, where a load is applied via a small indenter on the fiber, to test the debonding and frictional properties of the Er2O3 interface region enabling the energy dissipation mechanisms. Together with the obtained experimental results, an axisymmetric finite element model is discussed and compared to existing work. In the HIP Wf/W composites the matrix adhesion is rather large and can dominate the push-out behavior. This is in contrast to the previously tested CVD produced samples.
► Tungsten-doped amorphous carbon films were eroded in deuterium plasmas. ► Initial C removal rate decreases with increasing W concentration. ► Erosion rate decreases with plasma exposure duration ...due to W accumulation. ► Porous W-rich layer forms at the surface during erosion. ► W erosion only if ion energy higher than threshold for physical sputtering.
Tungsten-doped amorphous carbon films with 0–9.5at.% W concentration were produced by magnetron sputtering and exposed to deuterium plasmas applying different ion energies and fluences. The partial C and W erosion rates were determined from the C and W areal density changes, respectively, measured by Rutherford backscattering spectrometry. For W-doped films the erosion rate decreases with increasing W concentration and incident fluence. During deuterium plasma exposure carbon is preferentially eroded while tungsten atoms accumulate at the surface leading to the formation of a W-rich layer, which decreases the removal efficiency and leads to a continuous decrease of the erosion rate. At 30eV/D incident energy a relatively compact W-rich layer is formed on films with higher (⩾5%) W concentration which protects the carbon underneath from further erosion. For films with lower (⩽2.5%) W concentration the erosion rate decreases, but the erosion process does not stop because the W-rich layer has a high porosity. Reactive neutral species can penetrate through this porous layer and react with carbon atoms below it. At 100eV/D incident energy the erosion rate is significantly higher compared with 30eV/D. In addition, slight tungsten sputtering is observed due to the presence of a small fraction of D+ plasma ions which impinge with 300eV/D.
For the fabrication of 600 actively cooled finger elements for the Tore Supra pump limiter in operation since 2001 it was necessary to rely on two different batches of the CFC N11 grade (Carbon Fibre ...reinforced Composite) namely so-called SEP N11-92 (fabricated in 1992) and N11-98 (fabricated in 1998). It came out during the incoming inspection of the fingers that the bonding quality was degraded for the 98-batch so that an important number of tiles had to be repaired. Due to the coming upgrade of the Tore Supra heating system, two high heat flux test campaigns were performed on the neutral beam GLADIS facility (IPP Garching, Germany) including micro-structural analyses in order to evaluate, compare and understand the fatigue behaviour of 92- and 98-batch finger elements.
A facility for testing the cooling structure to ensure the quality of the target element heat sink is under construction at IPP Garching. This test bed has been built up to do a proof of concept ...study with a hydraulic mock up, built from the same material as the target elements, but can be opened for artificial gap manufacture.
A bypass in the cooling structure was the main reason for the overheating of CFC tiles, which resulted in a defect by cracking of the CFC–AMC-Cu interlayer in pre-series 3. The cooling structure is built from two half-shells, in both a half pipe is milled
1. To correlate the thermal response function to the artificial bypass gaps the time constant of the cool down was used. The results of the measurements are presented and compared with the calculated results.
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