Using atomistic simulations based on the creation-relaxation algorithm, we explore the evolution of microstructure in irradiated zirconium over a broad range of radiation exposure. In agreement with experimental observations, we find that at relatively low temperatures, microstructure evolves towards an asymptotic dynamic steady state forming at doses close to 1 dpa. Simulations show the spontaneous formation of a-type interstitial dislocation loops, gradually transforming into an a-type extended dislocation network and giving rise to macroscopic anisotropic dimensional changes in a textured material. A fully developed a-type interstitial/vacancy and c-type vacancy dislocation microstructure corresponds to the highest degree of irradiation growth anisotropy and vanishingly small volumetric swelling of the material.