[en] The realistic simulation of the complex mixing flow phenomena in the primary circuit of light water reactors is an important topic in reactor safety assessments. For example in case of a postulated loss-of-coolant accident the distribution of the emergency coolant within the reactor pressure vessel (RPV) and its degree of mixing are crucial to the investigation of possible pressurized thermal shock (PTS) scenarios or boron dilution processes. Well-established one-dimensional (1D) thermal-hydraulic system analysis codes are used to model long-term transient and accident scenarios. These codes offer the possibility to simulate entire thermal-hydraulic systems, employing a wide range of single- and multiphase models, but only with limited capabilities to account for the 3D-mixing phenomena. The German thermal-hydraulic system code ATHLET (Analysis of Thermal-hydraulics of Leaks and Transients) is being continuously developed by GRS for the analysis of the entire spectrum of operational transients, design-basis accidents and beyond design-basis accidents anticipated in nuclear energy facilities. The code provides a wide range of specific models for the simulation of light water reactors of both western and Russian designs as well as for Generation III, III+ and IV nuclear energy systems. Currently, the code's 1D, 6-equation, two-fluid model is being extended towards a fully 3D set of conservation equations. The capabilities of the new multidimensional equations have been investigated based on the experimental results from the Rossendorf Coolant Mixing (ROCOM) test facility. The ROCOM facility features a 1:5 scaled model of a German KONVOI-type Pressurized Water Reactor. The Test 2.1, carried out in the frame of the OECD PKL-2 project dedicated to the fast cool-down transient induced by a main steam line break, was selected for this investigation. The calculations performed with ATHLET have employed different RPV simulation models covering various grid resolutions for the downcomer and the lower plenum, using both the 1D and the 3D equation systems. In general the simulation results show a good agreement with the experimental data. Moreover the analysis highlights the advantages of the newly developed 3D model of ATHLET for the simulation of flow mixing processes. This document is made up of an abstract and the slides of the presentation. (authors)

[en] The realistic numerical prediction of transient fluid-dynamic scenarios including the complex, three-dimensional flow mixing phenomena occurring in the reactor pressure vessel (RPV) both in normal or abnormal operation are an important issue in today's reactor safety assessment studies. Both Computational Fluid Dynamics (CFD) tools as well as fluid-dynamic system analysis codes, each with its advantages and drawbacks, are commonly used to model such transients. Simulation results obtained with the open-source CFD tool-box OpenFOAM and the German thermal-hydraulic system code ATHLET (Analysis of THermal-hydraulics of LEaks and Transients), the later developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) for the analysis of the whole spectrum of operational transients, design-basis accidents and beyond design basis accidents anticipated for nuclear energy facilities, are compared against experimental data from the ROssendorf Coolant Mixing (ROCOM) test facility. In the case of the OpenFOAM CFD simulations the influence of various turbulence models and numerical schemes has been assessed while in the case of the system analysis code ATHLET a multidimensional nodalization recommended for real power plant applications has been employed. The simulation results show a good agreement with the experimental data, indicating that both OpenFOAM and ATHLET can capture the key flow features of the mixing processes in the Reactor Pressure Vessel (RPV). (author)

[en] The European Nuclear Young Generation Forum (ENYGF) is the event organised every 2 years within the European Nuclear Society - Young Generation Network (ENS-YGN) for European young professionals and students. It consists in 3 days of conferences (plenary sessions, workshops, panel sessions, technical and poster session), 1 day of technical tours and 1 day of cultural visits. ENYGF 2015 is dedicated to the dual aspect of the relationship between nuclear power and environment: the impact of nuclear activities on the environment and the contribution of nuclear energy to fight climate change. A great deal of this document is composed of the slides of the presentations