Review of the thermal conductivity of nuclear graphite under HTGR conditions
Experimental data on the thermal conductivity of needle-coke and isotropic graphites irradiated between 500 and 1600 deg C are reviewed. Irradiation reduces the room-temperature thermal conductivity; as the fluence increases, the rate of reduction declines and the conductivity approaches a saturation level which increases as the irradiation temperature increases. Eventually, when irradiation-induced expansion starts, the conductivity again decreases. The time constant for approach to saturation appears to increase linearly with irradiation temperature, while the conductivity after saturation increases exponentially with irradiation temperature. Irradiation reduces the dependence of the thermal conductivity on measurement temperature. Theoretical treatments of thermal conduction in irradiated graphite based on the lattice dynamics of hexagonal crystallites in the presence of crystallite boundaries and point defects are fairly well developed. Single vacancies and small interstitial clusters are believed to control the thermal conductivity of graphite irradiated below 300 deg C; between 300 and 650 deg C vacancies alone dominate, while above 650 deg C vacancy loops play an increasingly important role. The main practical application of the theoretical model is that it enables the temperature dependence of the conductivity of irradiated graphite to be predicted once measurements have been made at a single temperature. Design-basis curves for the thermal conductivity of irradiated H-327 and H-451 graphites were derived from the lattice dynamics model, assuming that the important irradiation-induced defects are vacancies or vacancy loops which are small compared with phonon wavelengths. The necessary parameters were obtained from analysis of experimental measurements on unirradiated H-327 and H-451 graphite, together with published data on the room-temperature conductivity of irradiated needle-coke graphites and Gilsocarbon graphites. (32 figures, 9 tables, 32 references) (auth)
- Research Organization:
- Gulf General Atomic Co., San Diego, Calif. (USA)
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-29-016368
- OSTI ID:
- 4337908
- Report Number(s):
- GULF-GA-A--12615
- Country of Publication:
- United States
- Language:
- English
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