A general relationship between two-group fluxes and normal currents on the surface of a core surrounded by a homogeneous reflector is derived. The relationship is an integral one derived directly from the group diffusion equations for the homogeneous reflector material and hence depending only on group parameters associated with the reflector material. Approximate homogeneous, algebraic boundary conditions relating group fluxes to group currents at the core-reflector interface are then derived, and these are applied to three sizes of pressurized water reactors (PWRs). Application to a large PWR at the interface between core shroud and reflector yields particularly excellent results for criticality and flux shapes in the core. The savings in computer running time over that required if the reflector is accounted for explicitly is ∼40%.