Study of alkaline coolant boiling in the fuel assemblies of fast reactors in emergency modes with natural convection

The article presents the results of experiments conducted by the IPPE JSC on heat exchange and circulation stability during boiling of alkaline liquid metals in the models of single and parallel fuel assemblies (FAs) under natural circulation conditions. It is shown that the complex structure of liquid metal boiling in FAs is formed under the influence of various factors and is characterized by both stable and oscillatory modes, as well as possible transition to departure from nucleate boiling. The influence of the fuel rod surface roughness on the heat exchange and flow modes of a two-phase liquid metal flow in bundles is shown. The presented results of experiments on the heat exchange during sodium boiling in an FA model with a sodium cavity above the reactor core demonstrate the possibility of long-term cooling of fuel rod simulators in FAs. The dependence on the heat transfer during boiling and a map of the modes for a two-phase liquid metal flow in the FA are presented. The agreement between the results of computational simulation and experiments on alkaline metal boiling in a single model FA and a system of parallel FAs is shown.