Experimental and numerical investigations of natural circulation phenomena in passive safety systems for decay heat removal in large pools

Modern concepts of nuclear power reactor systems are equipped with passive systems for decay heat removal. In different passive systems the energy is transferred by natural circulation into large pools which are considered as near infinite heat sink. Examples include the pool of the emergency condenser (BWR-1000) or the pool of the ESBWR. The questions arise: How reliable are the underlying physical mechanisms to act in the designated way? Are they understood completely? Are actual models able to describe the phenomena?The paper deals with experiments and with Computational Fluid Dynamics (CFD) simulations to investigate the capability of actual CFD codes to describe these phenomena. At the TOPFLOW facility of FZ Dresden-Rossendorf heating-up tests of an emergency condenser were performed. The main goal of these experiments was the determination of the heat transfer capability of the emergency condenser under different thermal hydraulic conditions. During these tests, the temperature histories on the secondary side of the pool were also recorded. The data recording comprises periods starting from single-phase liquid until the occurrence of steam on the secondary side of the pool. During these experiments, temperature stratification phenomena were observed, which can compromise the heat transfer. These phenomena were found also in earlier small-scale tests, which are also described here. The lower experimental and computational effort of the small-scale tests enables their comprehensive experimental and theoretical investigations. A detailed CFD analysis of these experiments was performed. An explanation of the observed phenomena on the basis of the small-scale tests and the CFD simulations is presented.Current CFD codes are capable of the qualitative description of the key phenomena of single and two-phase flow. They can contribute to a better understanding of the processes and give hints for design improvements to avoid undesired effects.This work contributes to an IAEA coordinated research project (CRP) on “Natural Circulation Phenomena, Modelling, and Reliability of Passive Systems That Utilize Natural Circulation” (Cleveland et al., 2005 and Choi et al., 2010).