Fuel retention and fuel removal from first wall components in tokamaks
Abstract: Fuel inventory and generation of carbon and metal dust in a tokamak are perceived to be serious safety and economy issues for a steady-state operation of a fusion reactor, e.g. ITER. These topics have been explored in the on-going Ph.D. work in order to contribute to the better understanding and development of methods for controlling and curtailing fuel accumulation and dust formation in controlled fusion devices. The work was carried out with material facing fusion plasmas in three tokamaks: TEXTOR in Forschungszentrum Jülich (Germany), Tore Supra in Cadarache (France) and JET in Culham Centre for Fusion Energy (United Kingdom). This thesis provides an account on studies of fuel removal techniques from plasmafacing components (PFCs) and on consequences of dust formation. Following issues are addressed: (a) properties of carbon and metal dust formed in the TEXTOR tokamak; (b) dust generation associated with removal of fuel and co-deposited layers from carbon PFCs from TEXTOR and Tore Supra; (c) surface morphology of wall components after different cleaning treatments; (d) surface properties of diagnostic mirrors tested at JET for ITER. The study dealt with carbon, tungsten and beryllium, i.e. with the three major elements being used for PFC in present-day devices and foreseen for a next-step machine. Some essential results are summarised by the following. (i) The amount of loose dust found on the floor of the TEXTOR liner does not exceed 2 grams with particle size range 0.1 mm – 1 mm. The presence of fine (up to 1 mm) crystalline graphite in the collected matter suggests that brittle destruction of carbon PFC could take place during off-normal events. Carbon is the main component, but there are also magnetic and non-magnetic metal agglomerates. The results obtained strongly indicate that in a carbon wall machine the disintegration of flaking co-deposits on PFC is the main source of dust: (ii) The fuel content in dust and co-deposits varies from 10% on the main limiters to 0.03% on the neutralizer plates as determined by thermal desorption and ionbeam methods: (iii) Fuel removal by annealing in vacuum or by oxidative methods disintegrates codeposits. In the case of thick layers, the treatment makes them brittle thus reducing the adherence to the target and, as a consequence, this leads to the formation of dust: (iv) Application of thermal methods for fuel removal from carbon-rich layers is effective only at high temperatures (above 800 K), i.e. in the range exceeding the allowed baking temperature of the ITER divertor: (v) Photonic cleaning by laser pulses effectively removes fuel-rich deposited layers, but it also produces debris, especially under ablation conditions: (vi) Photonic cleaning of mirrors exposed in JET results in partial recovery of reflectivity, but surfaces are modified by laser pulses. The presentation of results is accompanied by a discussion of their consequences for the future development and the application of fuel and dust removal methods in a next-step fusion device.
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