Accumulation of thermal resistance in neutron irradiated graphite materials

A nuclear graphite, H451, and two high thermal conductivity graphite composites have been irradiated in the temperature range of 310-710 °C in the high flux isotope reactor and their thermal conductivities monitored in situ. Data were measured continuously up to a fast neutron dose of approximately 1 × 10²⁵ n/m² (E > 0.1 MeV). Data are interpreted in terms of the added thermal resistance and materials compared on this basis. Following this analysis it is shown that for the three materials studied, which have significantly different initial thermal conductivity values, the accumulation of thermal resistance is greater for the materials with lower initial thermal conductivity. Given that vacancies dominate phonon scattering at these irradiation temperatures and dose levels, these data clearly indicate that materials of higher perfection have a slower rate of stable vacancy accumulation during irradiation.