This article presents a new model for the initial dispersion phase of energetic instantaneous expansion following a catastrophic vessel rupture. This model has been implemented as a new sub-model in the Phast dispersion model and is based on more sound physical principles than the old Phast sub-model. It allows for both vapor and two-phase releases. In the case of two-phase releases, droplets are assumed to expand radially, potentially leading to time-varying droplet rainout and the formation of a spreading evaporating liquid pool. For ground-level vapor or two-phase releases the correctness of the numerical predictions is confirmed against an analytical solution. The model has been validated against previously published experimental data for pressurized releases with and without rainout. This includes ground-level releases for nitrogen vapor and flashing liquid propylene, and elevated flashing liquid releases for Freon 11, Freon 12, propane and butane. Overall the old model tends to under-predict the cloud radius and cloud speed versus time, while the new model more closely agrees with small-scale experimental data. Therefore the new model produces smaller concentrations and doses, and is less conservative. For two-phase releases the new model predicts an increased amount of rainout, which is more in line with the experimental data. (c) 2017 American Institute of Chemical Engineers Process Saf Prog 36: 307-313, 2017