ITER will operate with a tungsten divertor, a material featuring surface morphology changes when exposed to helium plasmas, in particular the formation of the so called tungsten fuzz under specific conditions. Investigating interactions between tungsten plasma facing components and helium plasmas in a tokamak environment is therefore a key point to consolidate predictions for the ITER divertor performance and lifetime. To this end, a dedicated helium campaign was performed in the full tungsten WEST tokamak, cumulating ∼2000 s of repetitive L mode discharges. It is shown that conditions for tungsten fuzz formation, as derived from linear devices experiments (incident helium energy E _inc > 20 eV, helium fluence >10²⁴ He/m², surface temperature T _surf > 700 °C), were met in the outer strike point (OSP) area of the inertially cooled tungsten divertor elements of WEST. Preliminary inspection of the components after the campaign did not show visible signs of surface modification, but points to significant net erosion in the OSP area. An extensive post mortem analysis is now ongoing to confirm these first indications. These results underline that the complex balance between erosion/redeposition (in particular linked to impurities) and tungsten fuzz formation needs to be taken into account in tokamak conditions.

The net erosion and deposition patterns in the inner and outer divertor of WEST were determined after different experimental campaigns (C3 and C4) of the first operational phase using ion beam analyses and scanning electron microscopy techniques. The analyses were performed on four entire tiles from inertially cooled, W-coated divertor units with an additional Mo marker coating covered with a further W coating. Strong erosion occurred at the expected location of the inner and outer strike line area with a campaign-averaged net erosion rate of >0.1 nm s⁻¹. On the high field side of the inner strike line area, thick deposited layers were found (>10 μm; growth rate >1 nm s⁻¹), mainly composed of B, C, O, and W. Additionally, strong arcing was observed in this region. At the end of the C4 campaign, He discharges were performed to study the He-W interaction. Although the conditions for nanotendrils, i.e. fuzz formation were fulfilled around the outer strike line position, neither nanotendrils nor He bubbles (>10 nm) were observed at this area.

The International Thermonuclear Experimental Reactor (ITER) is an international project aimed at the production of carbon-free energy through the use of thermonuclear fusion. During ITER operation, in case of a loss-of-vacuum-accident, tungsten nanoparticles (W-NPs) could potentially be released into the environment and induce occupational exposure via inhalation. W-NPs toxicity was evaluated on MucilAir™, a 3D in vitro cell model of the human airway epithelium. MucilAir™ was exposed for 24 h to metallic ITER-like milled W-NPs, tungstate (WO₄²⁻) and tungsten carbide cobalt particles alloy (WC-Co). Cytotoxicity and its reversibility were assessed using a kinetic mode up to 28 days after exposure. Epithelial tightness, metabolic activity and interleukin-8 release were also evaluated. Electron microscopy was performed to determine any morphological modification, while mass spectrometry allowed the quantification of W-NPs internalization and of W transfer through the MucilAir™. Our results underlined a decrease in barrier integrity, no effect on metabolic activity or cell viability and a transient increase in IL-8 secretion after exposure to ITER-like milled W-NPs. These effects were associated with W-transfer through the epithelium, but not with intracellular accumulation. We have shown that, under our experimental conditions, ITER-like milled W-NPs have a minor impact on the MucilAir™ in vitro model.