Research On Thermal Fragmentation Mechanism And Model Of Single Metal Drop In A Steam Explosion

Consequences of severe accidents in nuclear reactors are extremely bad. With the great development of science and safety of nuclear energy, it is eager to accurately predict and mitigate the consequences of severe accidents, which prompts the nuclear reactor severe accident research to more in-depth and detailed direction. Steam explosion is one of the consequences of fuel-coolant interactions in a severe accident. A steam explosion may threaten the containment integrity and cause the release of radioactive to the environment. However, the mechanisms of steam explosion are very complicated. Metal drop fragmentation, which is the key phenomenon during steam explosion, has not been clarified sufficiently which prevents predicting the explosion precisely.From the microcosmic view and using metal surface dynamics after direct contact heat transfer as the breakthrough point, molten metal drop fragmentation phenomenon is investigated by experimental and numerical methods, to overcome the deficiency of normal experiments which cannot obtain clear images of interaction after vapor film collapse. This paper analyses molten metal droplet thermal fragmentation mechanism from a new angle of view and direction. Then the molten metal droplets thermal fragmentation process is divided into several sub-processes. A molten metal droplets thermal fragmentation rate model is developed by constructing models to describe those sub-processes. Finally, the molten metal droplets thermal fragmentation rate model is applied into Jasmine PRO to carry out its application analysis.A small scale steam explosion device was designed and constructed. An experiment, in which subcooled water droplet impinged into a high temperature molten tin liquid in a thin stainless-steel vessel, was carried out to investigate the entire process of fuel coolant interaction using high-speed photography system. The thermal interaction phenomenon in molten metal environment was observed. Three typical modes were observed, i.e. no interaction mode, slight interaction mode and explosive interaction mode. The phenomena of these three interaction modes include water droplet behavior, vaporization process at the interface between the molten tin and water, and interaction area expansion are interpreted in detail. When contact surface temperature exceeds spontaneous nucleation temperature threshold, explosive interactions happen. For explosive interaction mode, the severity of thermal interaction is reflected by the interaction area expansion speed in the experiments. The effects of tin temperature, water temperature and Weber number on the interaction area expansion are discussed. Meanwhile, a direct contact heat transfer model based on thermal boundary layer is developed when contact surface temperature exceeds spontaneous nucleation temperature threshold. Based on experimental data, the coefficients in the model are modified.A 3D multi-phase flow thermal hydraulic calculation code based on simple algorithm is developed for investigating molten metal and water interaction phenomena. First, the code is used to simulate typical phenomena of molten metal and water interaction. The numerical simulation results agree with the experimental results which indicate that the code can be reasonably applied to simulate similar phenomena, such as molten metal interface, water evaporation, and heat transfer characteristics etc. Then, the fragmentation process of melt drop triggered by external pressure pulse is numerically analyzed to investigate the mechanism of fragmentation in steam explosion. The simulation results are similar to that Ciccarelli’s experiment data. The simulation results suggest that violent heat transfer, growth and breaking up of filaments are the essential mechanism of melt tin. Finally, based on the numerical simulation results, the fragmentation process can be divided into several stages, including vapor film instability, melt drop-coolant direct contact after vapor film collapse, molten metal surface instability, quick growth of filament around the molten metal drop and breaking up of the filament.Based on thermal fragmentation phenomenological model, a molten metal droplets thermal fragmentation rate model is developed by constructing sub-models to describe sub-processes correspondingly. The comparison between model and single drop fragmentation experiments shows that droplet thermal fragmentation mass rate calculations are in a reasonable range. At the same time, the result that effects of metal temperature, density, initial radius, direct contact heat transfer coefficient on the fragmentation mass rate comply the law of thermal fragmentation mechanism. The model is introduced into the JASMINE-PRO software and application analysis of thermal fragmentation model is carried out. The calculation of MIXA experimental results show that compared to the Carachalios model and Esprose model results, the accuracy of new fragmentation rate model result on the front position prediction of molten metal fragments, steam and coolant mixture has been greatly improved.It indicates that when relative velocity between metal and coolant is low, fragmentation mass calculated by hydraulics fragmentation model underestimates real thermal fragmentation mass.Based on the above research, the molten metal droplets thermal fragmentation physical mechanism is summarized. New steam explosion field research road and analysis tools are provided. The understanding of classical steam explosion is further explained and advanced. This study is of important instructive and practical value for the future research work.