This document is an extension of the NEA/SEN/SIN/AMA(2006)2 report produced by Writing Group 3 on the extension of computational fluid dynamics (CFD) to two-phase flow safety problems. Writing Group 3 was formed following the recommendations made at the “Exploratory Meeting of Experts to Define an Action Plan on the Application of Computational Fluid Dynamics (CFD) Codes to Nuclear Reactor Safety Problems” held in Aix-en-Provence, France in May 2002. Extension of CFD codes to two-phase flow may provide insights to smaller scale flow processes which were not seen by present tools. Using such tools as part of a safety demonstration my provide a better understanding of physical situations, more confidence in the results and an estimation of safety margins.