The use of liquid metal alloy as a coolant in nuclear systems receives more and more attention in recent years, such as Dual-functional Lithium Lead Test Blanket Module (DFLL-TBM) for ITER, Lead-bismuth spallation target for accelerator driven sub-critical system (ADS) and Lead-alloy-cooled fast reactor (LFR). A system safety analysis code is an important tool for the liquid metal nuclear system safety analysis. In order to analyze some of the basis accidents, there is the need to simulate the mixing of liquid metal and noncondensable gas. While the current system safety code RELAP/SCDAPSIM/MOD4.0 which was initially designed to predict the behavior of light water reactor systems is incapable of modeling the mixture of liquid metal fluids and noncondensable gas. This paper first briefly introduce the two-fluid model in RELAP5/MOD4.0 and the reason for its incapability of modeling liquid metal in contact with a non-condensable gas. Then, a solution to solve the problem and the modification of the RELAP5/MOD4.0 code is proposed. Last, several typical problems in DFLL-TBM system were simulated and the results demonstrate the feasibility and validity of the modified RELAP5/MOD4.0 in modeling the mixing of liquid metal and non-condensable gas. The modified code provides a powerful tool for liquid metal nuclear system designs and safety analysis.

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