An implicit code coupling of 1-D system code and 3-D in-house CFD code for multi-scaled simulations of nuclear reactor transients

•We coupled an in-house 3-D CFD code, CUPID, and a 1-D reactor system code, MARS.•We verified the coupling with forced convection flows and manometer flow oscillations.•We applied the coupled code to the simulation of the Rossendorf Core Mixing test 1.1.•We can use the coupled CUPID/MARS code in multi-scaled reactor safety analyses.

A coupled multi-dimensional thermal–hydraulic system code, CUPID/MARS, has been developed by consolidating the MARS and CUPID codes. This enables users to take advantage of the very general, versatile features of MARS and the realistic three-dimensional features of the CUPID code. A calculation set of forced convection flows in a straight channel and manometer oscillations shows that mass is conserved at the interface cells of the MARS and CUPID, and the CUPID/MARS multi-scale coupling is successful in two-way direction in two phase flows as well as in single phase flows. The validation calculation for the Rossendorf Core Mixing (ROCOM) test, where the pressure vessel is calculated by the CUPID and the other components such as hot and cold legs are simulated by the MARS, shows that the multi-scale approach is cheap and convenient. By using the coupled code, we can simulate a specific part of a thermal–hydraulic system with CUPID and the others with the system code, which can provide the system code with a local zoom function.