DIMENSIONAL STABILITY AND STRENGTH OF NEUTRON-IRRADIATED SIC-BASED FIBERS

A variety of SiC-based fibers were characterized by measuring their le ngth, density, and tensile strength in the unirradiated, thermal annea led, and irradiated conditions. The irradiation was conducted in the E BR-II to a dose of 43 dpa-SiC (185 EFPD) at a nominal irradiation temp erature of 1000 degrees C. The annealed specimens were held at 1010 de grees C for 165 days to approximately duplicate the thermal exposure o f the irradiated specimens. In general, the results of this study indi cate the fibers that perform best in an irradiation environment are th ose that approach stoichiometric and crystalline SiC. Hi-Nicalon exhib ited negligible densification, accompanied by an increase in tensile s trength after irradiation. Nicalon CG possessed a higher tensile stren gth than Hi-Nicalon in the unirradiated condition, but was significant ly weakened in the annealed and irradiated conditions. In addition, Ni calon CG exhibited unacceptable irradiation-induced shrinkage. While t he irradiation stability of Hi-Nicalon was promising, other fibers wit h compositions closer to stoichiometric SiC may perform even better. T his potential was suggested by the MER99 fiber, which displayed excell ent dimensional stability. The principal drawback for the fully crysta lline and stoichiometric fibers such as MER99 and Crystalline SIC is t heir low strength and flexibility caused by high flaw concentrations.