In the near future large quantities of CO2 will be transported over a large distance from Carbon dioxide Capture plants to onshore and off-shore underground Storage (CCS) sites. The risk assessments for the existing CO2 pipelines show distances to harmful threshold concentrations from 1 to 7.2 km. Such large differences in safety distances are not acceptable. For the design, construction and operation of new high pressure CO2 pipelines through populated areas it will be necessary to have a validated risk assessment model. This paper describes the applied models for the outflow and dispersion and the causes of the uncertainties in the outflow and dispersion of CO2 after an accidental release from a high pressure pipeline. © 2011 Published by Elsevier Ltd.