Experimental study on pressurization characteristics of a water droplet entrapped in molten LBE pool

Abstract

In a postulated Steam Generator Tube Rupture (SGTR) accident of pool-type Lead-cooled Fast Reactor (LFR), highly-pressurized water from the secondary circuit will be discharged into the primary vessel containing low-pressure molten lead-based alloy (commonly pure lead or low-melting-point Lead-Bismuth Eutectic (LBE)). One of the dangers of such accident is the formation of a multiphase flow structure consisting of water droplets inside the molten metal pool, which could result in energetic Coolant-Coolant Interaction (CCI). In this study, to ascertain the pressurization characteristics underlying the CCI process, a series of experiments is conducted through releasing water lump of different volumes and shapes within a molten LBE pool based on the PMCI (Pressurization characteristics in Melt-Coolant Interaction) experimental facility at the Sun Yat-sen University. To obtain comprehensive understandings, a variety of experimental parameters are employed such as the temperature (subcooling), volume and geometry of water lump along with the temperature and depth of the molten LBE pool. After detailed analyses, it is verified that within the thermal interaction zone, there is the possibility for the steam explosion to occur that generally produces more violent pressure buildup. For the non-explosion cases, all our experimental parameters except the water subcooling are observed to have remarkable impact on the pressurization characteristics. Due to the restraining role of steam bubbles generated at the LBE-water interface, for given thermal condition (i.e. melt & water temperature) and LBE-pool depth, increasing water volume is confirmed to result in limited pressurization. Instead of LBE static pressure, the effect of pool depth on pressurization is supposed to be primarily caused by the varied easiness of the molten LBE penetration through the vapor interface. Regardless of steam explosion, the calculated mechanical energy conversion efficiency for present CCI experiments is found to be varied over a narrow scope of 2.5–5.5%. Current experiments provide a great amount of favorable database and insight for enhanced understanding on the mechanisms of SGTR accident as well as for the improved validations of LFR safety analysis codes in China.