Radiation Resistant Insulation Systems for the ITER Toroidal Field Coils

Insulation systems for fusion magnets have to operate under intense radiation, mainly fast neutrons and γ-radiation. At the ITER design fluence of 1x10 22 m -2 (E>0.1 MeV), conventional epoxy resins, which were used, e.g., for the ITER TF model coil, start to disintegrate. As a consequence, research programs were launched under EFDA, F4E and ITER contracts in order to find radiation harder resins as well as to investigate the properties of cyanate esters (CE) and CE/epoxy blends. The CE content was varied in the range between 100 % and 20 %. Test materials, consisting of wrapped R-glass fiber / polyimide tapes and VPI impregnated with various resins, were produced by European industry and tested at 77 K. Tensile and short beam shear tests were carried out to characterize the mechanical properties prior to and after irradiation to fast neutron fluences of up to 5x10 22 m -2 (E>0.1 MeV) in the TRIGA reactor (Vienna). In addition, tension-tension fatigue measurements were performed in the load-controlled mode to simulate the pulsed operation conditions of ITER. It turned out that these CE based insulation systems offer excellent properties at the ITER design fluence including an adequate safety margin, especially when a mixing ratio of 40% CE and 60 % epoxy resin is employed. Slight modifications of the resin, mainly regarding an extension of the pot-life to more than 100 hours, were made in view of the dimensions and the complexity of the ITER TF coils. The corresponding results confirm their excellent performance and will be reported. Radiation resistant materials have also been developed by US and Japanese Industry. The qualification of these insulation systems is currently under way and comprises 40:60 CE / epoxy blends from different suppliers (Huntsman, CTD). Test materials were fabricated under the same conditions in order to provide a comparable data base. The results will be presented and discussed.