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

Abstract: 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 the image intensity to a transient projection of the molten material morphology, was exploited. A methodology for the quantitative analysis of the x-ray images, i.e. transient maps of the fragmented melt, was developed. Its uncertainties were evaluated analytically and experimentally pointing towards the need to minimize the X-ray scattering and noise inherited from the optical system, for a more accurate quantification and a larger calibrated thickness range.Analysis of the data obtained by the SHARP system and image processing procedure developed provided new insights into the physics of the vapor explosion phenomena, as well as, quantitative information of the associated dynamic micro-interactions.The qualitative analysis, based on the matched radiograph and photographic images, describe the bubble and melt interrelated progression granting information on the phenomenological micro-interaction of the vapor explosion process. The dynamics of the initially disturbed vapor film is composed by multiple cycles, where the vapor bubble grows to a maximum diameter and collapses. X-ray radiographs show that during the first bubble expansion, the melt undergoes deformation/pre-fragmentation but does not follow the bubble interface during the subsequent expansion; suggesting no mixing between coolant and melt. Coolant entrainment occurs when the expanded bubble collapses leading to fine fragmentation of the molten material due to explosive evaporation. The vapor bubble expansion, fed by these fragments at the boundary, reaches its critical size, and start collapsing. The remaining melt is accountable for the following cycle.Bubble dynamics analysis shows a strong correlation between energetics of the subsequent explosive evaporation and the high temperature molten material drop (tin) deformation/partial fragmentation during the first bubble growth. The data suggest that this pre-fragmentation may have been responsible in providing an adequate mixing condition that promotes coolant entrainment during the bubble collapse stage. The SHARP observations followed by further analysis leads to a hypothesis about a novel phenomenon called pre-conditioning, according to which dynamics of the first bubble-dynamics cycle and the ability of the melt drop to deform/pre-fragment dictate the subsequent explosivity of the so-triggered drop.