A model was developed to investigate carbon dioxide droplet size distribution and the size of solid particles formed during horizontal rapid fluid expansion. These two parameters are crucial for risk assessment when constructing carbon capture and storage (CCS) facilities. The model was an integration of two sub-models: a CFD model to obtain temperature and velocity profiles, and a mathematical model to calculate droplet and particle sizes. The model was validated using experimental data of CO2 expansion, and was able to describe the formation of solid CO2 particles with sufficient accuracy. The model also found that when rapid CO2 expansion occurred under supercritical storage conditions, the solid particles formed were too small to develop a rainout pool. © Copyright 2014, AIDIC Servizi S.r.l.