Thermal conductivity in nuclear graphite is usually determined by measuring thermal diffusivity using the laser flash method at ^30 °C. The mechanism for thermal conductivity in graphite over the tem­peratures of interest in nuclear reactors is lattice vibration (phonon) conductance. There is a pro­nounced reduction in thermal conductivity with increased temperature attributed to phonon — phonon scattering. At low irradiation fluence, there

9
6
5
CD О
4
О
3 1 0-1———— 0 10

Weight loss (%)

Figure 42 Coefficient of thermal expansion of thermally oxidized graphite. Modified from Hacker, P. J.; Neighbour, G. B.; McEnaney, B. J. Phys. D Appl. Phys. 2000, 33, 991-998.

is a significant decrease in thermal conductivity due to fast neutron irradiation, attributed to an increase in scattering in the damaged lattice. This decrease in thermal conductivity (or increase in thermal resis­tivity) saturates in the medium fluence range. At very high fluence, there is a secondary decrease attributed to microcracking due to high crystallite strains. There is also evidence of change in temper­ature dependence with irradiation.

The thermal conductivity in crystallite basal plane is much larger than that perpendicular to basal plain; thus, Ka ^ Kc. The thermal resistivity can be described by the equation below:

1 _ 1 1 1

Kg_ KB+KU+KD

• U — Umklapp scattering (German for turnover/ down) or phonon-phonon scattering (due to increase in temperature)

• D — scattering due to defects (caused by irradiation)

• B — boundary scattering (structural effects)

Changes to these resistances will be reflected in the thermal conductivity of polycrystalline graphite. Ther­mal conductivity is significantly decreased by radio­lytic oxidation. Data is usually presented as the reciprocal of conductivity, that is, thermal resistivity.

4.11.16.1 Pile Grade A

Although PGA is significantly anisotropic over the range of interest to the Magnox reactors, the change in thermal resistivity can, for practical purposes, be considered as invariant to grain direction. Changes
in thermal resistivity in PGA graphite are given in Figure 43. There is a significant change in the rate of increases in thermal resistivity between 250 and 300 °C, giving a similar trend to the change in crystal growth rates between these two temperatures. The increase in thermal resistivity is significant (a factor of 100) at 150 °C, for a relatively low fluence. The low irradiation temperature data for PGA in Figure 43 do not reach a high enough fluence to saturate. However, at the higher temperatures, data is near saturation.