【作者】 周源；

【导师】 程旭；

【作者基本信息】 上海交通大学 ， 核能科学与工程， 2014， 博士

【摘要】 核反应堆严重事故后果极其恶劣,随着科学技术进步和核能安全领域的发展,迫切希望能准确预测并缓解严重事故后果,这促使核反应堆严重事故研究向更加深入和细致的方向发展。蒸汽爆炸是核反应堆发生严重事故后熔融物与冷却剂相互作用的恶劣后果,产生的冲击波破坏周围结构,会增加放射性裂变产物释放到环境中去的风险。然而,蒸汽爆炸机理复杂,其中的关键现象熔融金属液滴热碎化至今仍然未获清晰统一的认识,阻碍对蒸汽爆炸后果进行准确预测。本研究从微观角度出发,以冷热液直接接触传热后液态金属界面行为为切入点,采用实验和数值相结合方式研究了熔融金属液滴热碎化现象,弥补了传统实验中无法获得熔滴周围汽膜塌陷后相互作用图像的不足,从一个新的视角和方向深入探析了蒸汽爆炸过程中关键现象熔融金属液滴热碎化的机理。在此基础上,对熔融金属液滴热碎化过程进行分段建模,构建了可以适用于蒸汽爆炸分析软件的熔融金属液滴热碎化速率模型,并将熔融金属液滴热碎化速率模型嵌入蒸汽爆炸分析程序进行应用性分析。1.在可视化实验研究方面,设计并搭建低温熔融金属蒸汽爆炸实验台,进行水滴注入高温液态金属池中发生接触式传热的小规模实验,研究融池环境下伴随水滴激烈蒸发的液态金属、水和蒸汽多相介质之间的运动及传热特性。随着金属温度的升高,实验依次观察到三种现象,即水滴弹性现象,轻微反应现象以及剧烈反应现象。当熔融金属与水直接接触界面温度接近或超过自发核化沸腾温度,剧烈反应现象发生的概率明显增加。在剧烈反应实验中,随着金属温度的增加,水滴与熔融金属反应剧烈程度呈现缓慢增加趋势。水滴的温度和We数对反应剧烈程度影响不明显。此外,从直接接触界面温度边界层出发,构建直接接触温度超过自发核化沸腾温度的传热模型,并结合实验数据,对直接接触传热模型系数进行修正和建议。2.采用基于交错网格的simple算法,开发三维直角坐标系的多相流热工水力计算程序,为研究熔融金属与水相互作用现象提供分析工具。首先,采用程序对熔融金属与水滴相互作用典型现象进行研究,数值模拟结果与实验结果基本吻合,说明程序能够对熔融金属界面,以及水和蒸汽多相介质之间的运动和传热特性进行合理模拟。然后,进一步对外部触发条件下的熔融金属液滴热碎化现象进行了数值模拟研究,模拟结果与Ciccarelli实验结果基本一致,计算结果表明冷热液直接接触剧烈传热和熔融金属液滴表面的金属细丝生长断裂是熔融液滴碎化的主要原因。最后,基于熔融金属液滴周围汽膜塌陷后热工水力行为数值模拟结果,将熔融金属液滴热碎化过程分为熔滴周围汽膜不稳定性、汽膜塌陷冷热液直接接触传热、熔滴表面不稳定性、熔滴表面细丝生长和断裂等四个阶段。3.基于熔融金属液滴在冷却剂中的热碎化现象的四个阶段划分方法,进行理论分析,构建熔融金属液滴热碎化速率模型。将熔融金属液滴热碎化速率模型计算结果与单个熔滴热碎化实验结果比较,结果表明熔滴热碎化质量速率计算结果合理。同时,金属温度、密度、初始半径、直接接触传热系数等参数对熔滴热碎化质量速率的影响符合熔滴热碎化现象的演化规律。将本研究模型嵌入到JASMINE-PRO软件中进行了应用性分析,对MIXA实验的计算结果表明,使用本研究发展的热力学碎化速率模型的计算结果,即熔融金属碎片、蒸汽及冷却剂混合物的前端位置,其准确度相比于Carachalios模型和Esprose模型计算结果有了较大提高。结果表明在熔融金属与冷却剂相对速度较小时,使用水力学碎化模型计算的碎化质量将小于实际热力学碎化质量。基于上述研究工作,本研究总结出了熔融金属液滴在冷却剂中热碎化的物理机理,提供了新的蒸汽爆炸领域研究思路和分析工具,拓展了经典蒸汽爆炸现象的研究内涵,对今后的研究工作有一定的启发和实用价值。更多还原

【Abstract】 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.更多还原