The work presented in this paper forms part of the EC FP7 CO2PipeHaz project which aims to address the fundamentally important and urgent issue regarding the accurate prediction of fluid phase, discharge rate, emergency isolation and subsequent atmospheric dispersion during accidental releases from pressurised CO2 pipelines. Such pipelines are considered to be the most likely method for the transportation of captured CO2 from power plants and other industries to subsequent sequestration sites, and their safe operation is of paramount importance as their inventory is likely to be large. The developments presented describe a state-of-the-art, multi-phase heterogeneous discharge and dispersion model capable of predicting the near-field fluid dynamic and phase behaviour in such CO2 releases. Predicting the correct fluid phase during the discharge process in the near-field is of particular importance given the very different hazard profiles of CO2 in the gas and solid states. Validation of the model is undertaken using recently obtained experimental data from discharge tests. Model predictions are found to describe the experimental observations very well, with a high level of agreement between the two. The study clearly demonstrates that models addressing accidental releases of CO2 must include shock-capturing methods and complete three-phase formulations for prediction of the physical and thermodynamic properties of CO2 in order to accurately predict the discharge and dispersion phenomena of interest in risk assessments.