Improvement of MARS-KS sub-channel module using CTF code

•Realistic prediction of a multi-dimensional and multi-component nuclear system.•Implicit pressure coupling of CTF sub-channel code with MARS-KS system code.•Setting 1D and 3D interfacing cells as conventional components of codes.•Developed code was well verified and validated against selected problems and tests.•Developed code has good capability of predicting nuclear core thermal hydraulics. In this work, the sub-channel analysis module of MARS-KS nuclear safety analysis code was upgraded using CTF sub-channel analysis code for realistic prediction of the multi-component and multi-dimensional nuclear core thermal hydraulics. The semi-implicit pressure coupling method was used to couple these two codes. Both codes use the finite difference method and semi-implicit numerical scheme and hence they were coupled together by exchanging volume/cell pressures through the boundary interfaces between 1D (i.e., system) and 3D (i.e., sub-channel) computational regions via implicit phasic velocities at the boundary. The pressure linkage was simply accomplished by designating the 1D and 3D regions as implicit sinks in the CTF sub-channel module and time dependent volumes in the MARS-KS code, respectively. The heat structure component and point kinetics models of the MARS-KS code were also coupled with the CTF core sub-channel module for the analysis of the whole reactor system. Automatic initialization of the 3D vessel was performed with CTF null-transient calculation to initialize all computational cells using a single set of initial and boundary conditions. The improved MARS-KS code has been verified and validated using various problems and tests. This paper selectively presents the results of three analyses, including a conceptual manometer oscillation, a blowdown test, and a 3 × 3 hot rod sub-channel experiments, with the improved code. All the analyses showed good predictions and this confirms the semi-implicit pressure coupling as well as the capability of the improved MARS-KS code. In the future, parallel processing should be implemented for the improved MARS-KS core sub-channel module to perform full core sub-channel analysis.