We simulate effects of irradiation on nanocrystalline tungsten in the athermal high dose limit using the creation-relaxation algorithm, where microstructural evolution is driven not by thermally activated diffusion, but by fluctuating stresses resulting from the production and relaxation of defects. Over the entire interval of radiation exposure spanning several dpa, samples with smaller grains swell less than those with larger grains, featuring defect denuded zones resulting from the continuously fluctuating atomic configurations at grain boundaries. The grain size distribution broadens as a function of exposure, and the average grain size increases due to coarsening of grain microstructure if a low visibility criterion is applied to small grains.