| Extremely high rapid evaporation could occur when high-temperature particles contact with low-temperature liquid. This kind of phenomenon is associated with the engineering safety and the problems in high-transient multi-phase fluid and heat transfer process. During the past 15~20 years, a lot of researches have been performed, including both experimental and numerical investigations to study the interaction of molten fuel/ high-temperature particles with water. Fuel-coolant interactions (FCI) may occur during the course of a severe accident in a light water reactor. Fuel Coolant Interactions (FCIs) are habitual phenomena as the core melt accidents in the nuclear power station come out. Under this situation, the FCIs will develop to steam explosions, in which the structural parts and the security wall are destroyed and the radioactive matters flee outside. Therefore, the research on FCIs has played an increasingly significant part in nuclear reactor severe accident analysis in recent years.It is believed that steam explosions in nuclear reactor power station are based on the phenomena of fragmentation of high-temperature molten fuel, the rapid increase of heat transfer area, evaporation and steam pressure. The conditions of fragmentation are relate to heat transfer, liquid flow and vapor around hot particle. The movement of particles and behavior of film in coarse pre-mixing influence on the possibility and extent of expansion. The method of this study is that firstly, the effect of each experimental condition is studied (the method is that one experimental condition is changed while the others hold the line); secondly, the factors of deciding vapor explosion are analyzed; thirdly, though the general analysis, the critical condition of vapor explosion is concluded. The real reason of vapor explosion is revealed.This paper, which is supported by the national natural science foundation of china:"a study on thermodynamic characteristic around a high-temperature particle in film boiling under vapor exploration", studied the interaction of molten metal and coolant. A general theoretical model is deduced. By using of this model, the experimental phenomenon can be explained.An observable experiment facility was designed and built. A series of experiments were performed by pouring molten metal into a liquid pool. The observable experiment of molten metal and coolant interaction were conducted. Three experiments were performed by using this experiment facility, i.e., pre-mixing stage breakup experiments; vapor explosion experiments; single droplet bursting experiments. The effect of different experimental conditions on pre-mixing stage breakup, vapor explosion, single droplet bursting were studied. The experiment results showed that the experiment facility is available and meet the experiment requirements under different experimental conditions. The interaction of molten metal and coolant were recorded by high-speed camera, by which, a lot of experimental results were obtained. The experiment includes three processes: 1. Premixing-stage breakup experiment 50%Bi-25%Pb-25%Sn alloy as simulate material; 2. Vapor explosion tin-base alloy as simulate material; 3.Single droplet bursting tin-base alloy as simulate material.The experiment results showed that for the interaction of molten metal jet and coolant, the vapor explosion depends on the metal thermal diffusivity. The higher the metal thermal diffusivity is, the easier the vapor explosion takes place. Through the different coolant temperature experiment, it is showed the effect of debris equivalent diameter on the vapor explosion. The higher the coolant temperature is, the bigger the debris equivalent diameter is. That results in the decreasing of heat transfer surface. In the same case, the explosion intensity decreases. When coolant temperature approaches the saturated temperature, the vapor explosion disappears. This result verified the conclusion that that steam explosions in nuclear reactor power station are based on the phenomena of fragmentation of high-temperature molten fuel, the rapid increase of heat transfer area, evaporation and steam pressure . The author perform a simple experiment what verify triggering promote the vapor explosion but it is not crucial factor. In some case, the triggering restrains the vapor explosion. Through the synthetic experiment analysis above, the author put forward the explosion discriminate by which many experiment results can be explained.In pre-mixing stage breakup experiment, 50%Bi-25%Pb-25%Sn alloy is simulate material which thermal diffusivity is less than that of tin-base alloy. In the same case, the debris equivalent diameter of breakup has no obvious difference with that of vapor explosion. The experiment results showed that the molten metal temperature promotes the molten metal breakup for Marangoni effect. The improvement of coolant temperature restrains the molten metal breakup for the reason of Kelvin-Helmholz effect and critical Weber. The jet velocity increases the molten metal surface tangential stress which promotes the molten metal breakup.The aim of single droplet bursting verifies relation of the molten mental and explosion pressure impulse and the effect of Rayleigh–Taylor instability on the breakup. Though each pressure impulse of single droplet bursting is not same, the mean pressure impulse of 600℃droplet is at most. By using of the high-speed camera, the effect of Rayleigh–Taylor instability on the breakup can be observed distinctly.At the same time, based on the results of comprehensive experimental and theoretical studies, the mean velocity and thickness of vapor film and coolant sub-layer have been proposed from the momentum conservation equation. Though the energy conservation equation, the film boiling heat transfer can be obtained. And then the vapor production can be acquired. The theoretical value is agreed well with the experiment results.The Chapman-Jouquet theory is used to calculate the states before and after the shock wave and to predict the shock pressure and the temperature of the mixture.
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