[en] Computational Fluid Dynamics (CFD) is to an increasing extent being adopted in nuclear reactor safety analyses as a tool that enables specific safety relevant phenomena occurring in the reactor coolant system to be better described. The Committee on the Safety of Nuclear Installations (CSNI), which is responsible for the activities of the OECD Nuclear Energy Agency that support advancing the technical base of the safety of nuclear installations, has in recent years conducted an important activity in the CFD area. This activity has been carried out within the scope of the CSNI working group on the analysis and management of accidents (GAMA), and has mainly focused on the formulation of user guidelines and on the assessment and verification of CFD codes. It is in this GAMA framework that a first workshop CFD4NRS was organized and held in Garching, Germany in 2006. Following the CFD4NRS workshop, this XCFD4NRS Workshop was intended to extend the forum created for numerical analysts and experimentalists to exchange information in the field of Nuclear Reactor Safety (NRS) related activities relevant to Computational Fluid Dynamics (CFD) validation, but this time with more emphasis placed on new experimental techniques and two-phase CFD applications. The purpose of the workshop was to provide a forum for numerical analysts and experimentalists to exchange information in the field of NRS-related activities relevant to CFD validation, with the objective of providing input to GAMA CFD experts to create a practical, state-of-the-art, web-based assessment matrix on the use of CFD for NRS applications. The scope of XCFD4NRS includes: - Single-phase and two-phase CFD simulations with an emphasis on validation in areas such as: boiling flows, free-surface flows, direct contact condensation and turbulent mixing. These applications should relate to NRS-relevant issues such as: pressurized thermal shocks, critical heat flux, pool heat exchangers, boron dilution, hydrogen distribution, thermal striping, etc. Discussion of validation of the CFD tool, use of systematic error quantification and Best Practice Guidelines (BPGs) was encouraged and considered in the paper review process. - Experiments providing data suitable for CFD validation, specifically in the area of NRS. These should focus on local measurements using multi-sensor optical or electrical probes, laser-doppler velocimetry, hot-film/wire anemometry, particle image velocimetry and laser induced fluorescence. There were over 140 participants to the XCFD4NRS workshop to hear 5 invited talks, 3 talks on OECD-CSNI activity related to CFD, 44 technical papers, and to see 15 posters. The objectives that 2/3 of the papers be concerned with two-phase issues and 1/3 dedicated to experimental techniques and CFD grade experimental data were reached. Selected papers from the workshop were published in a special issue of the Nuclear Engineering and Design (NED) Journal. The other papers and the slides of the presentations are available in this document