Criteri di scelta di materiali per impieghi tribologici nel reattore a fusione ITER

Design of ITER (International Thermonuclear Experimental Reactor) involves critical problems concerning selection, testing and validation criteria of materials for components operating under severe environmental conditions. This paper presents the main results of a study aimed at selecting materials for tribological components of the ITER divertor. The main functions of the divertor are to control the purity of plasma and to exhaust helium and solid particles. The divertor is constituted by 60 cassettes (about 20 t each) realised in AISI 316L stainless steel. A remote replacement of cassettes is designed at 1 year intervals to allow maintenance operations. The cassettes are displaced by movers running on rails by wheels. The movers, in turn, are driven by a rack-and-pinion mechanism. In emergency conditions, the movement must be performed under sliding conditions; therefore, the cassettes must be equipped with sliding pads. Due to the operating conditions of ITER, the materials for the pad-rail couple must display low magnetic permeability, low-friction and adhesion, high wear resistance, as well as good toenail, mechanical and irradiation resistance. The operating and maintenance conditions, in particular temperature and and pressure suggest the use of dicalcogenides solid lubricants, such as MoS2 or WS2, applied as thin films on sliding surfaces or incorporated into composite materials. The sputtering technique allows to obtain lubricant films wear resistant and adherent to the substrate which, especially in vacuum, also display a very low friction coefficient.