Validation of the one-dimensional gas-liquid model in the GAMMA+ code for gravity-driven drainage in molten salt reactors

Molten salt reactors (MSRs) typically employ a drain tank to collect liquid fuel during accidents or maintenance. The drainage of liquid fuel from the primary loop involves two-phase phenomena: liquid discharges into the drain tank while the gas initially inside the tank is displaced upward. This process induces various flow regimes and complex phenomena within the primary loop, including bubbly, slug, transitional, and annular flows, as well as countercurrent flow limitation (CCFL). To describe these multiphase behaviors, we validated the one-dimensional gas-liquid model in the system code GAMMA+ (General Analyzer for Multi-component and Multi-dimensional Transient Application). The code solves five coupled equations for pressure, phase velocities, void fraction, and temperature. We demonstrated the capability of the GAMMA+ gas-liquid model using eight fundamental multiphase problems relevant to the drainage process: 1) fill-drain, 2) gravity-driven drain, 3) concurrent two-phase flow (air-water), 4) concurrent two-phase flow (steam-water), 5) countercurrent two-phase flow, 6) CCFL, 7) drain between tanks (open system), and 8) drain between tanks (closed system). For each case, model predictions were compared with analytical solutions, experimental measurements or other computational results. The comparisons demonstrate that the one-dimensional gas-liquid model in GAMMA+ robustly predicts key thermal-hydraulic parameters for each flow regime and CCFL phenomena associated with the drainage process, providing a reliable tool for the safety analysis of MSRs.