Experiments on Passive Heat Removal of Fully and Partially Immersed Containments by Laminar and Turbulent Convection at Very High Rayleigh Numbers

Abstract

Passive heat removal systems are essential components in modern small nuclear reactor (SMR) designs, aimed at enhancing overall plant safety during incidents. Technical details vary depending on the design and positioning of the containment in relation to the ultimate heat sink. In some configurations, steam condensation occurs at the containment wall, which is fully or partially immersed in water pools during a hypothesized loss of coolant accident.

The heat transfer from the outer containment wall to the surrounding water depends on the Rayleigh number, assuming either single-phase laminar or turbulent natural convection. Experimental data for validating numerical models and correlation approaches for free convection are available up to Rayleigh numbers of Ra ~ 10¹². However, for vertically aligned cylindrical containments with great lengths, Rayleigh numbers may reach up to Ra ~ 10¹⁶. Therefore, the application of established correlations for Ra > 10¹² in safety analysis codes requires validation through experimental data.

To achieve Rayleigh numbers in the order of Ra = 10¹⁵, steam was injected into a stainless-steel containment simulator (9 m high, 0.4 m wide, 0.128 m thick) submerged in water within the PAD vessel of the THAI+ facility. Temperature measurements were taken in both the fluids and structures, and velocity profiles in the water were measured using PIV. The experimental results are presented, providing insight into the performance and behavior of passive heat removal systems in scenarios involving vertically aligned cylindrical containments of significant height.