  We investigate electrical transport in a three-dimensional massless Dirac
fermion model that describes a Dirac semimetal state realized in topological
materials. We derive a set of interdependent diffusion equations with eight
local degrees of freedom, including the electric charge density and the spin
density, that respond to an external electric field. By solving the diffusion
equations for a system with a boundary, we demonstrate that a spin Hall effect
with spin accumulation occurs even though the conventional spin current
operator is zero. The Noether current associated with chiral symmetry, known as
the axial current, is also discussed. We demonstrate that the axial current
flows near the boundary and that it is perpendicular to the electric current.
