Numerical investigation on steam condensation and heat transfer in an emergency condenser tube with the thermo-hydraulic system code ATHLET

Based on a series of COSMEA experiments conducted in the scope of the PANAS project, the paper presents results of post-test simulations performed with a standard version of ATHLET as well as with an improved version of the code. After an overview on the research topics, the paper discusses the heat transfer phenomena in an emergency condenser pipe, the corresponding physical models implemented in ATHLET and the input deck developed to simulate the COSMEA tests. First results of the simulations with version 3.3 of ATHLET showed a significant underestimation of the heat transfer, an overestimation of the condensate outlet temperature and, depending on the experimental conditions, an underestimation or overestimation of the condensation rate. An indepth analysis of the results helps to identify possible reasons for the deviations. The second set of simulations with an improved version of the code shows much better agreement with the experimental data. The corresponding part of the paper describes the modifications to the source code in the convection and condensation heat transfer models, the physical reasons behind these modifications and the improvements obtained with the updated models. In comparing three condensation models against experimental outcomes, including the DobsonChato, KONWAR, and Thome models, the Thome model demonstrated the highest degree of congruence with experimental data. Additionally, the study revealed that heat transfer correlations tailored for the rough flow regime are essential to elucidate the heat transfer phenomena on the secondary side of the COSMEA test section, leading to their subsequent integration into the ATHLET code.