The electron beam device JUDITH 1 was used to establish a testing procedure for the qualification of tungsten as plasma facing material. Absorbed power densities of 0.19 and 0.38 GW m−2 for an edge ...localized mode-like pulse duration of 1 ms were chosen. Furthermore, base temperatures of room temperature, 400 °C and 1000 °C allow investigating the thermal shock performance in the brittle, ductile and high temperature regime. Finally, applying 100 pulses under all mentioned conditions helps qualifying the general damage behaviour while with 1000 pulses for the higher power density the influence of thermal fatigue is addressed. The investigated reference material is a tungsten product produced according to the ITER material specifications. The obtained results provide a general overview of the damage behaviour with quantified damage characteristics and thresholds. In particular, it is shown that the damage strongly depends on the microstructure and related thermo-mechanical properties.
Tungsten and tungsten alloys are actually the primary choice as plasma facing materials for future fusion reactors. Thereby, the material’s response to the different loading conditions occurring in a ...tokamak is strongly depending on the material properties and therefore the material’s microstructure. This is on the one hand controlled via the manufacturing process and/or the material’s composition and on the other hand by the operational conditions causing recrystallization and melting, and subsequently not only a modified microstructure but also locally a modified composition. The influence of the variation in microstructure is addressed and the pros and cons for using the respective materials and tungsten in general in a fusion environment with steady state and transient thermal loads are outlined. While roughening and the related cracking can hardly be avoided, melting will thwart all efforts to establish a high end microstructure with defined directional properties.
Crack formation in an ITER-reference tungsten grade was examined under single thermal shock loading. Typically two sorts of cracks, major cracks and microcracks, were observed at the loaded surfaces. ...The microstructures were quantified and the formation mechanisms were discussed. The major cracks were generated due to the brittleness of the tungsten material and microcracks were formed in a process which was initiated by plastic deformation at high temperature. The plastic deformation caused also surface elevation of the loaded area. At more intense thermal shock loading conditions, the microcracks disappeared and surface modifications due to recrystallization was observed.
The increasing importance of tungsten and tungsten alloys for future nuclear fusion facilities and reactors requires an increasing understanding of the materials behavior under operational loading ...conditions. Among others this comprises the thermal shock and thermal fatigue resistance of the material before and after recrystallization.
This study in particular aims for the qualification of deformed tungsten forged in two orthogonal directions intending to obtain a dense and nearly isotropic grain structure. The material is investigated in its stress relieved and recrystallized state and acts as a reference material for a better understanding of the cracking process when exposed to transient thermal loads, e.g. Edge Localized Modes (ELMs: f⩾1Hz, t=hundreds of μs). This is addressed by multiple thermal shock loadings in the electron beam facility JUDITH at different base temperatures. Hereby the influence of the DBTT on the onset of crack formation is verified and the latter qualified and quantified by metallographic means.
One potential plasma facing material candidate for future nuclear fusion facilities and reactors like ITER and especially DEMO is potassium doped tungsten, better known from lighting industry. ...Herein, in particular the potassium doped tungsten grade WVMW produced by PLANSEE AG is qualified in its as-received and annealed/recrystallized state (annealing at 1800
°C) and compared with a standard W-rod.
One qualification criteria is the crack formation under fusion relevant short transient events which is addressed by single and multiple thermal shock loading in the electron beam facility JUDITH at different power densities and base temperatures. Therein annealed WVMW is showing the best performance with the lowest transition temperature from crack formation to no crack formation between 150 and 200
°C. This might be also taken as a measure for the ductility and the DBTT of the material. For a better understanding of the cracking process as well as for the establishment of a data base for finite element modelling tensile tests are performed up to 2000
°C and at different speed (0.2 and 42
mm/min).
Industrial pure tungsten grades, manufactured by using a variety of manufactured techniques, are available worldwide in many different types of semifinished products, i.e. rods, wires, ribbons, and ...sheets. Thereby, the recrystallization temperature varies depending on the applied degree of deformation but also depending on the materials composition, i.e. the materials purity and in particular the level of certain impurities.
In order to compare different available industrial tungsten grades and a newly developed PIM-W grade, on the one hand recrystallization studies at three different temperatures from 1573 to 2073K for 1h were performed using microstructural analyses and Vickers hardness testing. On the other hand, the thermal shock induced low cycle thermal fatigue response of the material in its different recrystallization stages was done using high heat flux tests at 1273K base temperature, applying 1000 shots with 1ms and 0.38GW/m2 and post-mortem characterization, i.e. profilometry and metallography. The obtained results are related to the microstructural and mechanical features of the materials as well as the chemical composition of the individual tungsten grades.
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This paper describes the progress of the DEMO Design Activities in Europe and particularly the work done to address critical design integration issues that affect the machine configuration and ...performance, the plant concept layout and the selection of system design and technologies. Work continues to be primarily focused on the design integration of a pulsed baseline DEMO reactor concept, but a number of alternative configurations (e.g., a double-null divertor and a snowflake divertor as well as a ‘flexi’ DEMO that operates initially in an inductively driven pulsed regime, with the possibility to be upgraded to a long-pulse or steady-state machine with a greater reliance on auxiliary current drive, etc.) are under preliminary study, especially to evaluate their DEMO reactor relevance. Some initial considerations are given on the strategy to implement a structured design and technology down-selection, that progressively reviews and narrows options to arrive at the DEMO plant concept that addresses major system integration risks and offers the best probability to satisfy all stakeholder mission requirements. Finally, some recent technical achievements are highlighted.
Plasma facing components in future thermonuclear fusion devices will be subjected to intense transient thermal loads due to type I edge localized modes (ELMs), plasma disruptions, etc. To exclude ...irreversible damage to the divertor targets, local energy deposition must remain below the damage threshold for the selected wall materials. For monolithic tungsten (pure tungsten and tungsten alloys) power densities above ≈0.3 GW m
−2
with 1 ms duration result in the formation of a dense crack network. Thin tungsten coatings for the so-called ITER-like wall in JET, which have been deposited on a two-directional carbon–fibre composite (CFC) material, are even less resistant to thermal shock damage; here the threshold values are by a factor of 2 lower. First ELM-simulation experiments with high cycle numbers up to 10
4
cycles on actively cooled bulk tungsten targets do not reveal any cracks for absorbed power densities up to 0.2 GW m
−2
and ELM-durations in the sub-millisecond range (0.8 ms); at somewhat higher power densities (0.27 GW m
−2
, Δ
t
= 0.5 ms) cracks have been detected for 10
6
cycles.
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.
In this work two simulation methods for transient events on tungsten are compared. ELM-like thermal shock tests were performed with the electron beam device JUDITH 1 and a pulsed Nd:YAG laser on pure ...tungsten. The major difference between these two simulation methods is the surface near volumetric loading in case of the e-beam while the penetration depth of photons is negligible. Beside the definition of damage threshold values special attention was paid to the analysis of induced damages like surface modifications and crack formation. The obtained results indicate that there are no qualitative differences in the damage behaviour and the damage threshold values of tungsten for both simulation techniques. Slight differences of crack parameters are a result of geometric effects due to differences of the exposed areas. A more distinct impact results from the fast scanning of the sample (e-beam) in contrast to the homogenous loading (laser) which results in different surface modification.
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