Some progresses in system code modelling validation and application including 3D modelling

International audienceSystem thermalhydraulic (SYS-TH) codes are extensively used for Design Basis Accidents (DBA) simulations in the frame of safety analyses and licensing studies. The second generation of SYS-TH codes based on the two-fluid model reached a good maturity for industrial use in the 90s. Then several ways were followed for further improvements of the capabilities, including dynamic interfacial area and turbulence modelling, multi-field modelling and 3D modeling in the porous body approach. This paper present the current status of the R&D performed in the last twenty years with the rationale of each research topic, the current achievements and the perspective for future. The challenges related to system scale simulations are first listed including the issues related to the need to address the complex system geometry with a coarse nodalization. Then the limitations of the 2-fluid model are identified to introduce the advanced 1-D models. The porous 3D pressure vessel (PV) models are now used with a finer nodalization which allows a more complete modelling of small scale geometrical effects. Ongoing progress is significant in this domain, which is illustrated by the application to loss of coolant accidents (LOCAs)