Theoretical expressions are derived for the phonon thermal conductivit
y of single crystals of graphite in the basal plane at room and at ele
vated temperatures. The phonons are treated by a two-dimensional Debye
model in the frequency range from 4 to 46 THz. Mean free paths are ca
lculated for anharmonic three-phonon interactions, scattering by point
defects and scattering by grain boundaries. The calculated intrinsic
thermal conductivity of 19.1 W/cm/K at 300 K agrees with existing data
on highly oriented pyrolytic graphite. The spectral distribution of t
he intrinsic phonon heat current has a maximum at 4 THz and decreases
with increasing frequency, making the thermal conductivity sensitive t
o grain size. Estimates are given for the reduction in conductivity by
vacancies and Frenkel pairs, and for grain boundary and point defects
in combination. Observed reductions due to neutron irradiation are di
scussed in terms of Frenkel pairs or residual vacancies. Thermal condu
ctivity reductions are estimated in terms of observed fragmentation of
basal planes as a result of prolonged electron irradiation.