•A novel multi-phase model capable of predicting CO2 releases is presented.•Measurements of temperatures and concentrations of CO2 releases are presented.•Validation of mathematical models is undertaken against new experimental data.•Excellent level of agreement with measurements of jet characteristics obtained.•Tools presented to aid evaluation of hazard consequences of CO2 pipeline failure. The deployment of a complete carbon capture and storage chain requires a focus upon the hazards posed by the operation of CO2 pipelines and intermediate storage vessels, and the consequences of accidental release. The aim of this work is the construction of a computational fluid dynamic model capable of accurately representing the complex physics observed in such a release, essential if dispersion phenomena are to be accurately predicted. The interacting thermo-physical processes observed include those associated with the rapid expansion of a highly under-expanded jet, leading to an associated sonic flow structure. In such a release, it is also possible for three phases to be present due to the expansion and subsequent Joule–Thomson cooling, and a suitable equation of state is required to elucidate a system's composition. The primary objective of this work is the consideration of these physical processes, and their integration into a suitable numerical framework which can be used as a tool for quantifying associated hazards. This also incorporates the validation of such a model using data available in the literature and also using that recently obtained, and presented here for the first time. Overall, the model has provided an excellent level of agreement with experimental data in terms of fluid and sonic structure and temperature measurements, and good agreement with respect to composition data.