This UKCCSRC (UK Carbon Capture and Storage Research Centre) Call 1 project involved the development, testing and validation of a two-fluid transient flow model for simulating outflow following the failure of high pressure CO2 pipelines is presented. The project made use of experimental data and used experimental data available from other UK/EC funded projects. The model developed accounts for thermal and mechanical non-equilibrium effects during depressurisation by utilising simple constitutive relations describing inter-phase mass, heat and momentum transfer in terms of relaxation to equilibrium. Pipe wall/fluid heat exchange on the other hand is modelled by coupling the fluid model with a finite difference transient heat conduction model. This paper describes the model, the details of its numerical solution and its validation as well as parametric analysis of relevant parameters. http://www.sciencedirect.com/science/article/pii/S1750583614002394, DOI: 10.1016/j.ijggc.2014.08.013. UKCCSRC grant UKCCSRC-C1-07.