The current geopolitical situation implies an increased risk of detonation of nuclear weapons. Given the highly energetic nature of such detonations, the radioactive material released during the explosion is quickly distributed into a stabilized mushroom-shaped cloud which may have significant vertical and horizontal extent on the order of kilometers. As such, initialization to account for this is necessary for accurate modeling of the dispersion of the radioactive material from a nuclear detonation e.g. for emergency management purposes. We have implemented a description of the stabilized mushroom cloud into the Danish Emergency Response Model of the Atmosphere (DERMA), which is the long-range dispersion model in operation at the Danish Meteorological Institute (DMI). The model is a Lagrangian puff-particle model using random-walk to simulate the turbulent mixing in the atmospheric boundary layer. The geometrical parameters of the stabilized cloud are based on the empirical KDFOC3 parametrizations which provide a full geometrical description given the yield and altitude of detonation—parameters which are likely to be available, derived from observations. The geometrical description is coupled with a simple activity distribution. Following a discretization of the continuous cloud into puffs, these are advected by DERMA using available Numerical Weather Prediction (NWP) model data, and the concentration and deposition fields are calculated. The initial horizontal puff dimension is defined by the horizontal resolution of the NWP model, whereas the optimal vertical separation between the puffs in the stabilized cloud was chosen based on testing to limit inter-run variability in the deposition field.