Behavior/performance of tungsten as a wall material for fusion reactors

Gago Jiménez, German Mauricio; Krüger, Manja (Thesis advisor); Unterberg, Bernhard (Thesis advisor); Krupp, Ulrich (Thesis advisor)

Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2023)
Book, Dissertation / PhD Thesis

In: Schriften des Forschungszentrums Jülich. Reihe Energie & Umwelt = Energy & environment 612
Page(s)/Article-Nr.: X, 120 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2023

Abstract

Nuclear fusion is the most common source of energy in the universe. Heat and light are generated inside stars through nuclear fusion. Since the 1940s, many attempts have been made to harness the power of fusion energy, but this has not yet been achieved in a commercially viable manner. The largest-ever nuclear fusion reactor is now being built in Cadarache, southern France, and it is expected to achieve its first plasma in 2025. ITER, or "the way" in Latin, is expected to be the first fusion reactor in the world to produce more energy than it consumes, and it will, as its name suggests, pave the way towards commercial fusion energy in the future. One of the main issues expected in ITER is that of power exhaust. Enormous quantities of energy will be produced which need to be extracted. The divertor region in ITER will be mostly responsible for this task and will, thus, be the region exposed to the highest loads in the reactor. Due to its favorable thermal and mechanical properties, such as a high melting point, high thermal conductivity and low erosion rate, tungsten has been chosen as the best candidate for plasma facing material (PFM) in the ITER divertor. This work focuses on the analysis of the behavior of tungsten under ITER-relevant steady plasma and transient heat loads in order to understand and predict the effects the conditions in the ITER divertor will have on the PFMs. To achieve this, the tungsten samples were tested in the linear plasma device PSI-2. Two kinds of samples were utilized, one with needle-like grains transversal to the sample surface, which is the preferred microstructure for the ITER divertor, the other with larger, isotropic grains obtained after recrystallization of transversal samples at 1600 °C for 1 h. Moreover, samples were first exposed to each kind of load separately to study the effects independently of each other. Samples were then exposed to both loads simultaneously to analyze the possible synergistic effects of such loads. By exposing the samples to plasma loads the formation of nanotendrils on the surface, what is known as tungsten fuzz, was observed, without any indication of further surface modification or damage. The influence of only the transient heat loads was then investigated. It was observed that the fatigue stress caused by the laser pulses has a larger effect on the damage observed than the plasma particles. 105 laser pulses of 0.2 GWm-2 caused no observable cracking in transversal samples, and very slight cracking in recrystallized ones. At this power density, crack networks formed only after applying 106 pulses to the samples. With higher Pabs, of 0.4 and 0.8 GWm-2, a crack network already starts forming after exposure to 105 pulses. Once the effects of the separate loads were determined, the influence of the simultaneous exposure to both loads was investigated. In all cases, the cracking observed was exacerbated by the synergy between both loads. Hydrogen embrittlement and the formation of helium bubbles deteriorate the material properties, which cause the increased cracking and plastic deformation of the material. The formation of bubbles in the material can be of particular importance for the material behavior in PFMs. It was observed that applying pulses of 0.8 GWm-2 substantially accelerates the growth of helium bubbles near the surface of the material. A further increase in bubble size was observed by increasing the plasma fluence applied. Additionally, the hardness of the material was analyzed via nanoindentation. An increase in hardness was observed in the area of the material affected by both kinds of loads. This effect decreases with depth and is not observed in areas affected only by plasma. Furthermore, by analyzing the residual stresses of the samples via the sin2Ψ it was observed that as-received transversal samples have significant compressive stresses in the surface. This explains the higher damage threshold displayed by the transversal material compared to recrystallized samples. Residual stresses are relaxed after exposure to heat and plasma loads. These tests have revealed that despite a transversal microstructure is preferred for the tungsten PFMs in the ITER divertor, there is no significant difference in the material behavior at higher loads. This suggests that in areas of the ITER divertor where loads are lower, the transversal microstructure might be conserved and have superior performance to other microstructures. However, where the highest loads are expected, such as at the strike points, the initial microstructure might prove to be irrelevant, as widespread recrystallization should be expected. The results presented in this work have, furthermore, corroborated how vital ELM mitigation and control is for the success of ITER. A much lower number of ELM-like events was tested than what is expected in the lifetime of ITER, and widespread cracking was already observed. Cracking is not, in and of itself an issue for the functioning of ITER, but it might lead to other problems, such as the increased erosion of tungsten, which would, in turn, cause a cool down of the plasma. It also creates thermal barriers for heat dissipation, which might lead to local high temperature areas, which can exacerbate the material damage, eventually leading to a catastrophic failure of the material.

Institutions

  • Division of Materials Science and Engineering [520000]
  • Chair of Materials Engineering of Metals and Department of Ferrous Metallurgy [522110]

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