Steam condensation heat transfer inside reactor containment during the initial transient of a severe accident

During the Loss of Coolant Accident (LOCA) situations, a significant amount of steam comes inside the containment. Heat transfer is highly transient during the initiation of the accident (initial blow-down period) and occurs in the presence of high content of Non-Condensable Gases (NCGs), primarily air. This initial blow-down period sets the tone for the local thermo-hydrodynamics conditions in the post blow-down period. Therefore, it is essential to verify the existing empirical/semi-empirical correlation(s), mostly developed using steady-state experimental data, in correctly estimating the transient condensation flux at the early stage of the accident. In this background, condensation experiments are performed in the presence of initially present air inside a closed enclosure under well-controlled boundary conditions at two bulk pressures (2.6 bar abs. and 4.1 bar abs.) and two wall-subcooling (281 K and 305 K). Consequential thermal transience is recorded using the micro-thermocouples, and a mass spectrometer system is used to estimate the time-varying mixture compositions inside the test enclosure. A numerically validated and experimentally benchmarked inverse heat conduction based measurement technique, developed in our laboratory at IIT Kanpur, is used to non-intrusively estimate the transient condensation flux. We found that the steam condensation studies performed in the presence of air at steady-state (situations where liquid and gas boundary layers are generally full-developed), poorly represent the very initial transience where boundary layers are still under development. However, after a very brief time, the existing correlations developed using steady-state experimental data do predict the transient condensation flux quite well. (author)