Multi-scale system–subchannel coupling for PWR thermal-hydraulic analysis: Development and Validation

A multiscale coupled code, VITARS-SC, was developed by externally coupling the system code VITARS (Visual Interactive Transient Analysis code for nuclear Reactor System) with the core subchannel code SACOS (Subchannel Analysis Code Of Safety) through a spatial overlapping-domain coupling strategy. The coupled framework incorporates system-level component models for key equipment such as pipes, pumps, and valves, while also enabling high-fidelity subchannel resolution within the reactor core. Specifically, VITARS provides the core inlet and outlet boundary conditions (inlet temperature/enthalpy, inlet mass flow and outlet pressure) to the subchannel code; in turn, SACOS performs detailed core calculations and feeds back a corrected core pressure drop to the system model. The accuracy and reliability of VITARS-SC were validated against the transient data of the LOFT L9-1 test, which corresponds to a loss-of-flow accident (LOFA) scenario. The predicted trends of key loop thermal–hydraulic parameters show excellent consistency with the experimental data, and the coupled code yields a more accurate prediction of the transient event sequence than the standalone VITARS calculation. Furthermore, a representative main steam line break (MSLB) scenario for a typical pressurized water reactor (PWR) was simulated as an application case. The calculated core inlet and outlet temperatures, primary-system pressure, primary-system mass flow rate, and break steam flow rate are physically reasonable and well-reflect the characteristic thermal-hydraulic behaviors of MSLB accidents. In addition, the high-fidelity predictions of core enthalpy and void fraction distributions are consistent with theoretical physical expectations. These results demonstrate that VITARS-SC has preliminary capability for full-loop multiscale thermal–hydraulic transient analyses of PWRs, offering a valuable numerical tool for PWR design optimization and safety assessment.