Study of dust produced in tokamaks and potentially mobilized during a loss of vacuum accident

During the normal operating condition of the future ITER tokamak, a massive production of particulate matter (dust) in the toroidal vacuum chamber is expected. This dust, originating from the erosion of internal walls of the torus by the plasma, would be mobilized to some extent during a loss of vacuum accident (LOVA). For safety reasons, it is essential to quantify, both experimentally and theoretically, the fraction of formerly deposited dust that can be potentially mobilized during such an event. We collected dust particles deposited in the vacuum vessel of the AUG tokamak. Albeit much smaller than ITER, AUG design includes a characteristics particularly relevant for our study: its plasma facing components are made of tungsten, which will also be the component making up the ITER divertor. The particles were collected representatively, i.e., by means of a device (the so called "duster box", [1]) which mimics the resuspension of particles from a surface by a controlled air-flow, corresponding to an air velocity up to 9.5 m.s -1 (friction velocity: 0.70 m.s -1 ). The particles are counted on line (from 0.3 up to 10 µm into 6 bins), and collected on a Nuclepore  membrane. The performance of the duster box, in terms of resuspension efficiency, was carefully investigated theoretically using the resuspension model dubbed Rock'n Roll [2]. The main uncertainty in such a modelling stems from the poorly known adhesion force of the particle on the tungsten surface. This point will be carefully addressed in our contribution. The particles collected were then characterized by means of an optical microscope (Morphologi G3 Malvern) combined with an acquisition system which allows picture analysis. These results will be presented in our paper, mainly, the particle size distribution and the morphology of the particles. The microscope detected 70391 particles showing a bimodal distribution with a fine mode composed of flakes at 0.7 µm and a coarse mode composed of spherical particles at 1.9 µm. These first results are consistent with those of the literature review conducted. This in situ study of the resuspension of particles representative of the one expected in ITER and potentially transferable by air ingress in a tokamak is a first and paves the way for a better analysis of the consequences of a LOVA.