Multi-Physics Analysis of SMART Small Modular Reactor Benchmark Problems Using the Coupled McCARD/MATRA Code System

In this study, a high-fidelity multi-physics framework was established through the direct link between the continuous energy Monte Carlo (MC) neutronics code McCARD and the subchannel thermal-hydraulic (T/H) analysis code MATRA. A relaxation scheme suppressed numerical oscillations due to the stochastic nature of the MC method and ensured stable convergence of the Picard iteration. The SMART benchmark problems served to evaluate the accuracy and numerical stability of the developed system through a stepwise verification procedure. First, closed-channel calculations assessed the physical consistency of the fundamental T/H models in the two codes. The subsequent open-channel calculations quantified the effects of cross-flow and turbulent inter-channel exchange. The numerical results show that explicit consideration of lateral heat and mass transfer between adjacent subchannels substantially reduces local coolant-temperature peaks and produces temperature distributions that more closely represent the relevant physical phenomena. The coupled framework also captures distinct pin-wise power variations, particularly near the periphery, due to subchannel-scale T/H feedback that the single-physics model does not account for. Consequently, this study demonstrates that the McCARD/MATRA external multi-physics code system can represent the multi-physics phenomena required for high-fidelity safety analyses and provides a reliable technical basis for SMR design optimization.