3-D SYS-TH: an OECD/NEA activity on multi-dimensional capabilities of thermalhydraulic system codes

The evaluation model and computational capabilities required for engineering design and safety analyses of nuclear installations have shown significant progress compared to the first tools established in the 60s. Regarding thermalhydraulics, first generation system codes were based on simple one-dimensional models associated with conservatisms intended to cover lack of knowledge, simplifications and limited computational capabilities and limited experimental support available at that time. The second generation, mainly one-dimensional with some limited multi-dimensional capabilities, implemented the more advanced two-fluid sixequationmodel, and adopted the best–estimate approach, benefited from an extensive experimental program, and showed large improvement compared to the first generation tools. However, the current tools still have limitations that both industries and regulatory bodies would like to address. Next generation codes are being developed to achieve an improved thermal-hydraulic analysis capacity. Within the Working Group on Analysis and Management of Accidents (WGAMA) of the OECD/NEA, an activity has been initiated in 2016 aiming at establishing a state of the art of current 3D capabilities in thermal hydraulic system codes whichcovers all aspects and limitations, from the equations and simplifications considered, time and space averaging hypotheses with unavoidable use of relatively coarse meshes, closure models up to available or needed experimental support. The main findings of this activity are presented in this paper