Description of the Risø Puff Diffusion Model

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.