► Tungsten and CFC were subjected to transient high cycle heat loads. ► Tungsten showed fatigue damage (roughening, cracking, melting). ► Roughening occurred as precursor for cracking. ► Underlying ...mechanism depended on surface temperature. ► CFC showed 3× higher damage threshold and no long term fatigue mechanism.
Within the ITER divertor lifetime millions of transient events are expected during H-mode operation due to edge localized modes (type I ELMs). These will deposit their energy on plasma facing materials that are pre-heated to various surface temperatures, depending on the steady state heat load (SSHL) at the respective location, leading to synergistic effects. An electron beam facility was used to simulate ELM-like heat loads with ITER relevant power densities (≈0.5GW/m2) and pulse duration (0.5ms). At the same time additional SSHL was applied to obtain different base temperatures. Experiments were performed on actively cooled pure tungsten and the carbon fiber composite (CFC) NB41, applying 103–106 pulses of 0.5ms duration with a power density of 0.14–0.55GW/m2 and 0.55–0.68GW/m2 on tungsten and CFC, respectively. Surface temperatures were about 200°C, 400°C and 700°C for tungsten and about 450°C for CFC. Crack formation in tungsten was preceded by roughening due to plastic deformation. In case of Tsurf≈200°C cracks propagated comparably fast (brittle material), while slow propagation and recrystallization around the crack edges indicated fatigue damage at higher temperatures. Compared to tungsten, CFC showed a higher damage threshold.
ITER grade stainless steel (SS) 316L(N)-IG was tested in QSPA-T (photons) and JUDITH (electrons) under heat loads relevant to those expected from photon radiation on the ITER diagnostic first wall ...(DFW) during disruptions mitigated by massive gas injection. Repeated pulses slightly above the melting threshold eventually lead to a regular, “corrugated” SS surface, with hills and valleys spaced by 1–2mm. The negligible mass loss observed after the heat pulses indicates that hill growth (growth rate of ∼1–2μm per pulse) and SS plate thinning in the valleys is a result of melt-layer redistribution. A Similar behavior is observed on SS samples exposed in QSPA-T (pulse length 0.5ms) and JUDITH (pulse length⩽3.0ms) at the same heat flux factor. The combined data suggests, for the total lifetime, a surface roughening of ⩽1.5mm on parts of the ITER SS DFW exposed to the highest transient photon loads, after 1200 mitigated disruptions at high stored energy. The results also indicate that the surface roughness increase may be significantly reduced by variation of the SS impurity composition. This experimental observation is supported by a proposed theoretical mechanism for the surface roughness formation based on the growth of capillary waves in the melt layer.
The chosen materials for plasma facing components for the deuterium/tritium phase of ITER are beryllium and tungsten. These materials have already been widely investigated in various devices like ion ...beam or electron beam tests. However, the operation of this material combination in a large tokamak including plasma wall interaction, material degradation, erosion and material mixing has not been proven yet.
The ITER-like Wall, which has been recently installed in JET, consists of a combination of bulk tungsten and tungsten coated CFC divertor tiles as well as bulk beryllium and beryllium coated INCONEL in the main chamber. The experiments in JET will provide the first fully representative test of the ITER material choice under relevant conditions.
This paper concentrates on material research and developments for the materials of the JET ITER-like Wall with respect to mechanical and thermal properties. The impact of these materials and components on the JET operating limits with the ITER-like Wall and implications for the ongoing scientific program will be summarised.
Beryllium is one of the most interesting materials to be used in nuclear fusion devices due to its promising properties. However, testing of beryllium components is quite challenging because of its ...toxicity and the required safety procedures.
Within this paper an overview of high heat flux thermal fatigue tests carried out in the electron beam facility JUDITH 2 (Juelich Divertor Test Facility in the Hot Cells) at Forschungszentrum Juelich (FZJ) on actively cooled beryllium mock ups is given. The performance of the mock ups was documented by temperature measurements as well as optical inspections before and after testing. Special attention was paid to the temperature diagnostics during the tests for two different reasons. Firstly, local overheating of the beryllium tiles, which are the proof of bonding failures, is detected by temperature measurements. Secondly, temperature information is exceptionally important to meet the required safety standards because of the toxicity of beryllium. In order to avoid evaporation of beryllium, a temperature limit for an automatic safety shut down of the electron beam facility JUDITH 2 was set to a certain value. Finally examples of the temperature distribution under different loading conditions for the beryllium mock ups are presented within this paper.
Ferromagnetic pebbles are investigated as high heat flux (q∥) plasma facing components in fusion devices with short power decay length (λq) on a conceptual level. The ability of a pebble concept to ...cope with high heat fluxes is retained and extended by the acceleration of ferromagnetic pebbles in magnetic fields. An alloying concept suited for fusion application is outlined and the compatibility of ferromagnetic pebbles with plasma operation is discussed.
Steel grade 1.4510 is chosen as a well characterized candidate material to perform an analysis of the heating process. Scaling relationships as a function of q∥ for maximum and optimal pebble diameter, allowed exposure time, and removal time safety margin are obtained numerically for spherical pebble geometry. The acceleration of ferromagnetic pebbles in a tokamak resulting from magnetic gradients is studied and operation parameters for an ITER-based reactor are outlined. Counter-intuitively, it is found that ferromagnetic pebbles perform better for narrow λq profiles, making them an attractive heat exhaust concept for next step devices and thus an option to be investigated in detail.
The key results of this study are that very high heat fluxes are accessible in the operation space of ferromagnetic pebbles, that ferromagnetic pebbles are compatible with tokamak operation and current divertor designs, that the heat removal capability of ferromagnetic pebbles increases as λq decreases and, finally, that for fusion relevant values of q∥ pebble diameters below 100μm are required.
•Ferromagnetic pebbles are investigated as high heat flux plasma-facing components in fusion devices.•Steel grade 1.4510 is chosen as a candidate material due to the Fe-Cr composition and the availability of material data.•Optimum pebble diameter and exposure time for given q|| are obtained from numerical transient thermal simulations.•Inward acceleration of the ferromagnetic pebbles due to magnetic gradients in a tokamak is used to exchange the pebbles.•For narrow λq profiles very high heat fluxes>500MW/m2 are accessible in the operation space of ferromagnetic pebbles.•Ferromagnetic pebbles are compatible with tokamak operation and current divertor designs
The chosen materials for plasma facing components for the deuterium/tritium phase of ITER are beryllium and tungsten. These materials have already been widely investigated in various devices like ion ...beam or electron beam tests. However, the operation of this material combination in a large tokamak including plasma wall interaction, material degradation, erosion and material mixing has not been proven yet. The ITER-like Wall, which has been recently installed in JET, consists of a combination of bulk tungsten and tungsten coated CFC divertor tiles as well as bulk beryllium and beryllium coated INCONEL in the main chamber. The experiments in JET will provide the firstfully representative test of the ITER material choice under relevant conditions. This paper concentrates on material research and developments for the materials of the JET ITER-like Wall with respect to mechanical and thermal properties. The impact of these materials and components on the JET operating limits with the ITER-like Wall and implications for the ongoing scientific program will be summarised.
The application of multiphase steels in the automotive industry has been rapidly increased according to economic, environmental and safety reasons. To determine an optimal combination of high ...strength and good formability of multiphase steels by using the FE modelling, their complex microstructures have to be considered. Two‐dimensional Representative Volume Elements (RVEs) were currently developed based on real microstructures for dual phase (DP) steels. In general, the microstructure of DP steels contains hard martensite particles and a soft ferritic matrix. The strain hardening behaviour of the individual phases was described in the model taking the microstructural constituents and the carbon partitioning during intercritical annealing into account. Two dual phase microstructures with same martensitecontent but different martensite distributions were investigated in experiment as well as in FEM simulation by means of the RVE. The resulting mechanical properties of these steels are strongly influenced by the phase distribution and interaction. As validation, calculated flow curves were compared with the experimental results from quasi‐static tensile tests. In addition, the local stress and strain partitioning between both phases depending on the spatial phase distribution and morphology is discussed.
The thermal shock behavior of ceramic refractory materials is of particular interest. These materials are usually tested in downward thermal shocks by cooling hot specimens in water or air. In ...contrast to this method, the use of electron beam material testing facilities offers an alternative approach to perform thermal shock tests on heating. The method of thermal shock testing by an electron beam is introduced for ceramic refractory materials within this article. Finally, the material degradation of a carbon bonded MgO refractory material due to thermals shocks, applied with the electron beam, is presented.
The possibility of applying new high‐strength steels with excellent forming behaviour (TRIP, TWIP and LIP steels) in automotive manufacturing is a significant potential for improvement in the area of ...reducing weight while simultaneously increasing crash safety. The present work investigates endogenous inclusions in some high‐alloy TRIP and TWIP steels because the most stringent product requirements are tightly related to cleanness. The expected formation of inclusions is discussed based on thermodynamic observations made with ThermoCalc. The solidification conditions were varied in experiments with the so‐called SSCT (submerged split chill tensile) apparatus. Furthermore, different treatment times were set in order to investigate this influence on the inclusions. A catalogue of endogenous inclusions in these new steel grades is currently being created with the help of the automated SEM/EDX inclusion analysis system at voestalpine Stahl GmbH in Linz. Further studies will follow to systematically determine the interactions between steel, slag and refractory materials.
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