The effects of fast-neutron irradiation on tensile strengths of carbon fibers

Two types of commercially available carbon fibers (high tensile strength, HTS, and high modulus, HMS) were irradiated in the Ground Test Reactor in environments of air and of liquid nitrogen (LN₂). The tensile strength of HTS fibers irradiated in air increased sharply above a fast-neutron fluence of 6 × 10¹⁷ n/cm² (E> 1 MeV) and was 17 per cent greater than the strength of unirradiated control fibers at a fluence of 8.5 × 10¹⁷ n/cm², but then the strengrh began to decrease for additional neutron exposure in air and fell 25 per cent below the control strength at the highest fluence of 4.5 × 10¹⁸ n/cm². However, when irradiated in LN₂ where surface oxidation did not take place, the room-temperature strength of HTS fibers continued to increase beyond 8.5 × 10¹⁷ n/cm² and became almost 30 per cent greater than the control strength for a fluence of 3 × 10¹⁸ n/cm². The tensile strength of HMS fibers irradiated in air increased slowly but steadily with neutron exposure and was only 4 per cent greater than the control strength at the highest fluence of 4.5 × 10¹⁸ n/cm²; the room-temperature strength of the HMS fibers decreased by 13 per cent when irradiated to a fluence of 3 × 10¹⁸ n/cm² in LN₂.