An operational puff diffusion model has been developed at Risø National Laboratory to provide risk and safety assessments in connection with nuclear installations. The computer model releases a sequence of puffs with individual pollutant and heat contents, then calculates the time-dependent concentration field, which is provided by the collection of puffs. The puffs are advected through a three-dimensional grid on the basis of a sequence of either measured or simulated horizontal wind vectors. In one case study where the time duration of a pollutant release was varied, the puff model predicted a Gaussian dose distribution only when the source duration was relatively short. For use at distances up to ∼1 km from the release point, experimental observations of nonstationary smoke plume diffusion seem to justify a puff advection scheme, where all the puffs in each time step are advected with the instantaneous velocity vector measured at the release point.