Modeling Co-Current and Counter-Current Flow: A Performance Evaluation of the TRACE Condensation Model with Non-Condensable and Light Gases

Efficient steam condensation is crucial for ensuring the secure functioning of nuclear power plants (NPPs) by mitigating the potential dangers associated with excessive pressure and overheating. Nonetheless, the existence of non- condensable gases (NCGs) can obstruct this essential process, forming thermal resistance layers, impeding steam diffusion, and hindering condensation on the system's surfaces. Consequently, the objective of this study is to enhance our comprehension of steam condensation by assessing its effectiveness in the presence of non-condensable gases. The current work utilizes a condensation setup in KAIST's passive containment cooling system (PCCS) facility, chosen for its validation accessibility. Unlike prior validations focused on pure steam and air using RELAP5/MOD3.2, this study employs TRACE validation to explore the effects of non-condensable gases, including air, nitrogen, hydrogen, and helium. The research generates diverse scenarios, correlating them with TRACE parameters to understand the impact of gases. Using the user-friendly Symbolic Nuclear Analysis Package (SNAP) graphical interface, a geometric model is built and verified in the TRACE code, aligning well with experimental data. Subsequently, 2,100 TRACE cases are generated to evaluate the condensation heat transfer coefficient (HTC). Pearson correlation coefficients from the data highlight that mass fraction has the most significant adverse impact on the HTC, followed closely by the gas type based on molecular weight. Given these notable negative effects, the initial focus is on how mass fraction and molecular weight influence the liquid generation rate in co-current flow condensation. Results demonstrate that a higher mass fraction and molecular weight reduce the liquid generation rate compared to pure steam. Examining their impact on drain mass flow rate in steam/NCG mixture variations reveals their role in triggering counter- current flow limitations (CCFL). Findings suggest that reaching the steam/NCG mixture flow rate threshold stops condensation, initiating CCFL. Moreover, increased mass fraction and a shift to gases with higher molecular weights lead to an earlier onset of CCFL.