An experimental study on the dynamics of melt-water micro-interactions in a Vapor explosion

Vapor explosion as a result of Molten Fuel-Coolant Interactions (MFCI) postulated to occur in certain severe accident scenarios in a nuclear power plant presents a credible challenge on the plant containment integrity. Over the past several decades, a large body of literature has been accumulated on vapor explosion phenomenology and methods for assessment of the related risk. Vapor explosion is driven by a rapid fragmentation of high-temperature melt droplets, leading to a substantial increase of heat transfer areas and subsequent explosive evaporation of the volatile coolant. Constrained by the liquid-phase coolant, such rapid vapor production in the interaction zone causes pressurization and dynamic loading on surrounding structures. While such a general understanding has been established, the triggering mechanism and subsequent dynamic fine fragmentation have yet not been clearly understood. A few mechanistic fragmentation models have been proposed, however, computational efforts to simulate such phenomena generated a large scatter of results. In order to develop a mechanistic understanding of thermal-hydraulic processes in vapor explosion, it is paramount to characterize dynamics of fragmentation of the hot liquid (melt) drop and vaporization of the volatile liquid (coolant). In the present study, these intricate phenomena are investigated by performing well-controlled, externally triggered, single-drop experiments, using advanced diagnostic techniques to attain visual information of the processes. The methodology’s main challenge stemming from the opaqueness of the molten material surrounded by the vapor film and rapid dynamics of the process, was overcome by employing a high-speed digital visualization system with synchronized cinematography and X-ray radiography system called SHARP (Simultaneous High-speed Acquisition of X-ray Radiography and Photography). The developed image processing methodology, focus on a separate quantification of vapor and molten material dynamics and an image synchronization procedure, consists of a series steps to reduce the effect of uneven illumination and noise inherited of our system, further segmentation, i.e. edge detection, and extraction of image features, e.g. area, aspect ratio, image center and image intensity (radiography). Furthermore, the intrinsic property of x-ray radiation, namely the differences in linear mass attenuation coefficients over the beam path through a multi-component system, which translates th