Testing of Reflood Heat Transfer under Very-Low-Flooding-Rate Conditions at the NRC-PSU RBHT Facility

While numerous reflood tests have been conducted in various test facilities in the past, very few experiments have examined very-low inlet flowrates, which are more likely in a Small Modular Reactor (SMR) long term cooling scenario. In this study, various sets of data were collected in the Rod Bundle Heat Transfer (RBHT) test facility located at the Pennsylvania State University focusing on very-low-flooding-rate (0.25<Vin<2.3 cm/sec) test conditions subjected to different system pressures and degrees of inlet subcooling. These benchmark data sets will further expand the ability of thermal-hydraulics codes to evaluate nuclear reactor safety, and additionally confirm the facility is capable of further testing in this very-low reflood rate condition. The very-low-flooding-rate test conditions were carefully chosen to identify the effects of inlet subcooling, system pressure, bundle power, and reflooding rate. The test series included a demonstration of test repeatability with a change in droplet laser camera location to observe the effects of bundle position on droplet entrainment characteristics. Droplet entrainment data will be presented in a future paper. It was observed that the very-low reflood condition extends overall bundle quench and decreases liquid carryover and droplet entrainment substantially when compared to other reflood rate conditions. Decreased inlet subcooling extends the bundle quench time and increases liquid carryover. It is anticipated that the data will expand the capability of thermal-hydraulics codes down at these very-low reflood conditions. Further testing will be possible now that this facility has been benchmarked at these very-low reflood conditions.