Thermionic fuel element sheaths, seal and intercell insulators, and end restraints were irradiated in a fast neutron spectrum and examined. Samples were irradiated at temperatures ranging from 1110 K to 1200 K to fast fluences from 3.4×1022 n/cm2 to 6.0×1022 n/cm2. Sample examinations included visual, photographic, dimensional, electrical resistance to temperatures of 1175 K, helium leak rates, and metallography. Examinations of the end restraints and intercell insulators, which were limited to visual and photographic examination, showed no adverse effects from the irradiation. Alumina and yttria have been identified as insulator materials which meet design requirements.

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