Degradation of tungsten components exposed to deuterium plasmas in JET: an investigation into component processing, mechanical behaviour and Be–W deposits

Thesis

The mechanical behaviour and structural integrity of tungsten components, specifically Langmuir probes and lamellae from the JET divertor are evaluated both in their as-received state and after exposure during the ITER-like wall campaigns. The results reveal significant disparities in the as-received microstructures of these components. The lamellae exhibit fine grains suited for high-heat flux conditions, whereas the probes display larger equiaxed grains. Electrical discharge machining (EDM) induced cracks up to 50 µm and voids in both component types, raising concerns about the mechanical integrity of these surfaces.

Advanced characterisation techniques, including atom probe tomography and transmission electron microscopy, identified significant impurity concentrations in the as-received samples, particularly phosphorus and iron at grain boundaries, which likely contributed to embrittlement. EDM-generated cracks were shown to widen and deepen after exposure, propagating up to 1 mm into the bulk of one Langmuir probe. Additionally, some EDM-generated cracks contained embedded debris, including elements such as carbon and oxygen. This debris poses potential safety risks, including beryllium retention and associated toxicity, as well as tritium retention issues.

Exposure to deuterium plasmas induced distinct microstructural differences between the Langmuir probes placed next to tiles 3 and 6 and the lamellae taken from stacks C (rows 3, 4, 12 and 13) and D (row 12). The Langmuir probes underwent recrystallisation and void formation with an increase in hardness of up to 0.40 GPa due to cascade damage from the incident deuterium. Conversely, the lamellae were more resilient under plasma loading, with no change in hardness or signs of melting or recrystallisation.

Beryllium and beryllium oxide were found to accumulate on the surface of lamella C3, particularly near W droplets, raising concerns about droplet detachment and increased erosion rates during reactor operation. The surface of Lamella C3 also contained stratified deposits, consisting of alternating layers of beryllium oxide and sputtered tungsten, which raises concerns about impurity accumulation and its impact on component performance. This study underscores the need for further research into the effects of EDM and D-T plasma exposure on tungsten, particularly in light of the decision to transition ITER to a tungsten first wall for the nuclear operational phase.

Authors: Kerr, RW

• DOI: 10.5287/ora-bdo6koz6b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: TEM; PFCs; fusion; dust; nanoindentation; tungsten; JET; ITER; impurities; deposits; beryllium; mechanical properties; EDM
• Subjects: Plasma-wall interactions; Tokamaks; Micromechanics; Electron microscopy