Within the overall objective to support the development of fast reactor technology and to extend the predictive capabilities of existing simulation tools for design and safety analysis, this publication aims to improve the understanding of key phenomena involving radioactive material transport and reduce uncertainties in the estimation of potential releases to the environment. It arises from an IAEA coordinated research project (CRP), launched to perform modelling and simulation of the source term for sodium cooled fast reactors under hypothetical severe accident conditions. The publication presents the results and conclusions of the CRP to verify and validate modelling the in-vessel and in-containment source terms. The technical aspects addressed by the CRP are divided into three main parts: the in-vessel source term estimation, the primary system/containment interface source term estimation and the in-containment phenomenology. This publication documents the models, simulations, results and discussions.