Status of Simulation Capability of 3D System-scale Thermal-hydraulic (TH) Analysis Codes

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This report aims to establish the status of current 3D capabilities in T/H system codes, covering all aspects and limitations, including the equations and simplifications considered, the time and space averaging procedures, the closure models and available or needed experimental support. These aspects are organised sequentially. The reasons for developing multidimensional capabilities in T/H system codes are first discussed to illustrate the variety of situations to be covered. The multidimensional phenomena and the general approach for modelling them are then described. These descriptions clarify the general picture, after which the report presents the current situation regarding codes, experiments and validation. The report presents a gap analysis between what is available and what is desirable. Finally, the main challenges are summarised and some proposals on how to tackle them are suggested.

Multidimensional phenomena that can have a significant impact occur in the course of asymmetric design basis accident scenarios, such as a feed water line break (FWLB), main steam line break (MSLB) and loss-of-coolant accident (LOCA). Furthermore, 3D phenomena can play a key role under natural circulation flow regimes, in which flow mixing and stratification are predominant, similar situations occur when passive systems are involved.