  We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of $\pi $ and $\sigma $ bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of $\pi $ than of $\sigma $ bands, as
suggested by a recent calculation [1].
