The outer layers of a neutron star are supposed to be formed of a solid Coulomb lattice of neutron rich nuclei. At densities above neutron drip density (about one thousandth of nuclear saturation density), this lattice is immersed in a neutron fluid. Bragg scattering of those dripped neutrons by the nuclei which has been usually neglected is investigated, within a simple mean field model with Bloch type boundary conditions. The main purpose of this work is to provide some estimates for the entrainment coefficients, as required for hydrodynamical two fluid simulations of neutron star crust, which relate the momentum of one fluid to the particle currents of the other two fluids. The implications for the equilibrium neutron star crust structure are also briefly discussed.