Materials Requirements for Experimental Fusion Reactors
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objectives, more conventional materials with more extensive data bases will be used where possible. Even so, a substantial materials development program is required to support a test reactor like ITER. This article provides an overview of the critical materials requirements for ITER and the candidate materials for the various applications. Figure 1 is a poloidal view of the tokamak which illustrates the main systems of a reactor. The major components include the divertor and first wall, blanket, shield, magnets, and heating and current drive systems. The plasma is surrounded by the first wall/impurity control system (plasma facing components), which is directly exposed to radiation and energetic particles escaping from the plasma. The blanket system, behind the first wall, captures most of the neutrons produced by the fusion reaction and contains the tritium breeding material. Surrounding the blanket, the shield reduces the radiation to an acceptable level for the magnet system. The magnets serve to raise the temperature and current in the plasma (ohmic heating coils) and to control the shape and position of the plasma (toroidal and equilibrium field coils). Auxiliary components include the vacuum system, plasma heating and current drive systems (e.g., radio-frequency or ion beam systems), plasma fueling systems, and diagnostic systems. The following discussion focuses on the materials for the plasma facing components and the firstwall/blanket systems, since the requirements are most unique for the fusion application.
Plasma Facing Components The plasma facing components include the divertor system for impurity control, the first-wall armor which shields the structure from the plasma, and other auxiliary components that are directly exposed to the plasma. Divertor A divertor is proposed on ITER as the impurity control system for removing helium ash from the plasma. This component interacts most directly with the hot plasma during normal operation and, therefore, is exposed to the most i severe environment of all the components. The divertor materials that interact with the plasma edge are exposed to relatively high fluxes of energetic plasma particles (deuterium, tritium, helium, and impurities), 14 MeV neutrons, and electromagnetic radiation emitted from the plasma. In addition, the divertor is expected to receive a large fraction of the plasma energy in the event of a disruption. Under these conditions the divertor must operate for acceptable lifetimes and must not be the source of excessive plasma contamination. The proposed general divertor configuration consists of an armor tile as the plasma facing material brazed to a structural substrate that contains water coolant for heat removal (see Figure 2). Leading candidates for the divertor tile are graphite or a carbon composite and tungsten. Leading candidates for the structural substrate are certain copper , alloys and selected refractory metal alloys such as molybdenum, tantalum, and vanadium. Table I lists the desired materials properties for the plasma facin
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