  The diffusive thermal conductivity of neutrons in dense matter [$\rho \sim (1
- 8) \times 10^{14}$ g cm$^{-3}$] of neutron star cores is calculated. The
contribution from neutron--neutron and neutron--proton collisions is taken into
account. We use the transition probabilities calculated for symmetric dense
nucleon matter on the basis of the Dirac--Brueckner approach to the in-medium
effects and the Bonn model of bare nucleon--nucleon interaction. The diffusive
thermal conductivity of neutrons in the presence of neutron and proton
superfluidities is analyzed in a microscopic manner; the effects of
superfluidity are shown to be significant. The low temperature behavior of the
thermal conductivity appears to be extremely sensitive to the relation between
critical temperatures of neutrons and protons. The results are fitted by simple
analytic expressions. In combination with the formulae for the electron and
muon thermal conductivities, obtained earlier, the present expressions provide
a realistic description of the full diffusive thermal conductivity in the
neutron star cores for normal and various superfluid phases.
