多维熔融物与冷却剂相互作用分析程序COSMETRIC的开发与验证

基于MCBA-SIMPLE算法开发了自主化的多维熔融物与冷却剂相互作用分析程序COSMETRIC。为验证该程序，针对熔融物与冷却剂相互作用实验KROTOS的典型工况进行了模拟计算。通过与KROTOS37实验结果对比，验证了程序模拟高温熔融物与冷却剂混合过程中熔融物液柱碎化、熔融物液滴迁移以及冷却剂蒸发的能力；通过与KROTOS21实验结果对比，验证了程序对蒸汽爆炸压力脉冲峰值及传播速度预测的合理性。在此基础上，对KROTOS21爆炸工况计算的初始空泡份额、熔滴水力学碎化无量纲时间和熔融物碎片初始直径等参数进行了敏感性分析，评估了这些参数对最终压力脉冲的影响。敏感性分析结果发现，较大的初始空泡份额会抑制压力峰值和传播速度；增大熔融物碎片初始直径和水力学碎化无量纲时间，会提升压力波传播速度，降低压力峰值。     

熔融液柱与冷却剂作用中液柱碎化预测

液柱碎化是熔融物和冷却剂相互作用(FCI)粗混合阶段的关键物理现象，在安全分析时需建立液柱碎化模型。本文将实验验证和理论分析相结合，开展了高温熔融液柱与冷却剂相互作用实验；建立了不同沸腾条件下的液柱表面膜态沸腾模型和液柱表面不稳定波生长模型；再考虑不稳定波断裂和熔融物的脱离，构建起完整的熔融液柱水力学碎化模型。用该水力学碎化模型对不同沸腾条件下的熔融液柱碎化行为进行了预测。预测结果表明，实验得到的碎片中位直径和碎裂长度与模型预测结果符合较好，且能进一步应用于典型反应堆原型材料FCI实验的液柱碎化预测。     

基于界面跟踪与两流体模型的耦合模型对金属液柱碎化的数值模拟

针对熔融物与冷却剂相互作用(FCI)过程中多尺度相界面共存的复杂流型,将基于流体体积法(VOF)的界面跟踪模型与两流体模型耦合在一套统一的数值求解框架下,得到一个新的多相流数值模型,可以模拟大尺度界面流体与小尺度界面流体共存的复杂多相流过程。该模型中,对于动量场,流体根据界面尺度分为连续相和离散相。连续相界面通过VOF/PLIC方法进行捕捉,离散相表面积浓度分布通过表面积输运方程模拟。耦合模型的控制方程通过MCBA-SIMPLE算法求解。使用该模型对金属液柱的流动和碎化过程进行模拟,并与实验观测结果进行对比,同时还对液柱碎化速率模型和金属液滴初始直径的影响进行了探讨。结果表明:原液柱碎化模型对液柱贯穿深度有所高估;金属液滴初始直径的选择将对熔融物的冷却效率造成显著影响。     

气腔夹带作用下射流碎化过程实验研究

熔融物射流落入水面后的碎化行为是决定蒸汽爆炸威力的重要现象,这一过程可能受气腔夹带的影响。本文通过可视化实验研究了气腔夹带作用下苯甲酸苄酯和水银2种液体射流的水力学碎化特性。开展了不同入射速度下射流碎化行为的定性定量研究。实验发现苯甲酸苄酯射流(与水密度比为1.118)的碎化过程中伴随明显的气腔夹带现象。气腔可以阻碍水与射流的接触,抑制碎化过程,导致碎化长度模型的预测值远低于实验值。而水银(与水密度比为13.6)的碎化过程中气腔夹带不明显,碎化长度符合碎化长度模型的预测。实验表明:气腔夹带现象会影响射流碎化过程,导致碎化长度模型无法准确模拟气腔夹带作用下的碎化长度。     

液态铅铋与水界面碎化行为的可视化实验

在中国铅基反应堆(CLEAR)换热器管道破口(SGTR)事故工况下,二回路高压水可能会直接与一回路铅铋共晶合金(LBE)接触,导致水/蒸汽混合物的急速沸腾,甚至发生蒸汽爆炸,从而危及反应堆的安全。为研究水与熔融LBE接触界面间的沸腾传热与蒸汽爆炸现象及机理,本文通过熔融LBE/水直接接触反应实验平台,依托高速摄像机记录熔融LBE入水爆炸或碎化过程。实验分析了LBE温度(250~500℃)、水温(25~80℃)对熔融LBE碎化行为的影响。结果显示,随着熔融LBE温度或水温的升高,LBE碎化质量中位粒径呈减小趋势;当熔融LBE与水接触界面温度大于水的均相成核温度时,蒸汽爆炸现象更易发生,碎化现象更明显。     

熔融铅锡合金与冷却剂相互作用热细粒化研究

堆芯熔融物与冷却剂相互作用(Fuel Coolant Interaction,FCI)是核反应堆严重事故下可能发生的严重问题之一。为进一步了解FCI现象及解明热细粒化过程的关键影响因素,本文通过可视化实验方法,采用铅锡合金模拟材料开展实验研究。采用高速摄像系统对反应过程进行图像采集,通过计算熔融物所占像素点的面积得到熔融物的截面积;收集反应碎片,从实验产物形貌、相互作用过程状态及熔融物周围气体分布三个方面对影响热细粒化过程的熔融物初始温度、质量及冷却剂温度展开研究。结果对比分析表明:熔融物温度升高,热细粒化程度先增加后减小;初始水体量一定的情况下,熔融物质量增加,可能导致熔融物细粒化程度降低;冷却剂过冷度增加,热细粒化程度增加。     

金属物性与冷却剂温度对蒸汽爆炸的影响

为了模拟研究核电站严重事故蒸汽爆炸,本研究设计建造了低温熔融金属入水碎化的可视化实验装置。采用高速摄像仪拍摄记录熔融金属液柱入水碎化的过程。实验研究了不同熔融金属材料,冷却水温度对蒸汽爆炸的影响。实验结果表明熔融金属热扩散系数对蒸汽爆炸有重要影响,热扩散系数越大,越容易发生蒸汽爆炸;冷却水温度则相反,冷却水温度的提高,使金属碎化颗粒增加,降低了熔融金属的传热,抑制了蒸汽爆炸。     

冷却剂欠热度对蒸汽爆炸的影响研究

为了研究蒸汽爆炸的爆炸机理,设计建造了低温熔融金属入水碎化的可视化实验装置,采用高速摄像仪拍摄记录熔融金属液柱入水爆炸或碎化的过程。实验研究了冷却水温度对蒸汽爆炸的影响,实验结果表明：冷却水温度对蒸汽爆炸起抑制作用。为了验证该结论,进行了碎化实验。结果表明：冷却水温度对碎裂后颗粒直径范围影响明显,熔融金属液柱在温度高的冷却水中遇到的阻力小,碎裂的颗粒体表比越大,换热能力越弱。后者实验结果与前者相符。     

蒸汽爆炸的影响因素分析

为研究各种影响因素对蒸汽爆炸的影响,设计建造了低温熔融金属入水碎化的可视化实验装置。采用高速摄像仪拍摄记录熔融金属液柱入水碎化的过程。实验研究了熔融金属热扩散系数、温度及冷却水温度对蒸汽爆炸的影响。实验结果表明,熔融金属热扩散系数和金属温度的乘积与蒸汽爆炸脉冲成正比;冷却水温度对蒸汽爆炸起抑制作用。熔融金属热扩散系数是影响蒸汽爆炸的重要因素。     

熔融锂液滴与冷却剂在不同温度下的相互作用实验研究

针对未来聚变装置中严重事故时可能发生的液态锂与冷却剂相互作用及爆炸过程,建立实验装置并在其上开展了熔融锂液滴与冷却剂相互作用实验研究。观测了不同初始温度下锂液滴与冷却剂相互作用的爆炸过程,对不同工况下的峰值压力进行了比较,并分析了熔融锂液滴初始温度和冷却剂初始温度对爆炸作用的影响。研究结果表明,熔融锂液滴与冷却剂接触面积的显著增大是产生压力峰值的关键因素,当熔融锂液滴温度超过300℃,冷却剂温度超过50℃时,熔融锂液滴与冷却剂相互作用爆炸强度明显增大;但是当冷却剂温度超过70℃时,爆炸反应反而受到了抑制。同时,在评估熔融锂液滴与冷却剂相互作用风险时,蒸汽爆炸作用的影响不可忽视。     

Vapor film collapse triggered by external pressure pulse and the fragmentation of melt droplet in FCIs

The fragmentation process of high-temperature molten drop is a key factor to determine the ratio heat transferred to power in FCIs,which estimates the possible damage degree during the hypothetical severe accident in the nuclear reactors.In this paper,the fragmentation process of melt droplet in FCIs is investigated by theoretic analysis.The fragmentation mechanism is studied when an external pressure pulse applied to a melt droplet,which is surrounded by vapor film.The vapor film collapse which induces fragmentation of melt droplet is analyzed and modeled.And then the generated pressure is calculated.The vapor film collapse model is introduced to fragmentation correlation,and the predicted fragment size is calculated and compared with experimental data.The result shows that the developed model can predict the diameter of fragments and can be used to calculate the fragmentation process appreciatively.     

Collapsing criteria for vapor film around solid spheres as a fundamental stage leading to vapor explosion

Following a partial fuel-melting accident, a Fuel-Coolant Interaction (FCI) can result with the fragmentation of the melt into tiny droplets. A vapor film is then formed between the melt fragments and the coolant, while preventing a contact between them. Triggering, propagation and expansion typically follow the premixing stage.In the triggering stage, vapor film collapse around one or several of the fragments occurs. This collapse can be the result of fragments cooling, a sort of mechanical force, or by any other means. When the vapor film collapses and the coolant re-establishes contact with the dry surface of the hot melt, it may lead to a very rapid and rather violent boiling. In the propagation stage the shock wave front leads to stripping of the films surrounding adjacent droplets which enhance the fragmentation and the process escalates. During this process a large quantity of liquid vaporizes and its expansion can result in destructive mechanical damage to the surrounding structures. This multiphase thermal detonation in which high pressure shock wave is formed is regarded as “vapor explosion”. The film boiling and its possible collapse is a fundamental stage leading to vapor explosion. If the interaction of the melt and the coolant does not result in a film boiling, no explosion occurs.Many studies have been devoted to determine the minimum temperature and heat flux that is required to maintain a film boiling. The present experimental study examines the minimum temperature that is required to maintain a film boiling around metal spheres immersed into a liquid (subcooled distilled water) reservoir. In order to simulate fuel fragments that are small in dimension and has mirror-like surface, small spheres coated with anti-oxidation layer were used. The heat flux from the spheres was calculated from the sphere's temperature profiles and the sphere's properties. The vapor film collapse was associated with a sharp rise of the heat flux during the cooling process—from values typical for film boiling to much higher values typical for nucleate boiling. Correlations for the minimum temperature and the minimum heat flux necessary to maintain film boiling were established in terms of the subcooling level, the size of the spheres and their material.The minimum temperature to maintain film boiling was used as the principle criteria for the occurrence of vapor explosion. Other criteria, for the intensity of the vapor film collapse was derived from the maximum heat flux following the vapor film collapse, and the audible sound (which is generated by the shock wave). It is assumed that a high intensity of the vapor film collapse will result in a more efficient propagation stage and enhancement of the vapor explosion.     

Visual study of ex-pin phenomena for SFR with metal fuel under initial phase of severe accidents by using simulants

In the present Korean sodium-cooled fast reactor (SFR) program, early dispersion of the molten metal fuel within a subchannel is suggested as an inherent safety strategy in the initiating phase of a hypothetical core disruptive accident (HCDA). This safety strategy provides a negative reactivity driven by the melt dispersion; therefore, it could reduce the possibility of occurrence of a severe recriticality event. In the initiating phase, the melt could be injected into the subchannel horizontally by the internal pressure of the fuel pin. Complex phenomena occur during intermixing of the melt with the coolant after the horizontal injection of the melt. It is rather difficult to understand the several combined mechanisms that occur that are related to the dispersion and fragmentation of the melt. Thus, it seems worthwhile to study the horizontal injection of melt at lower temperatures, which could help to observe the dispersion phenomenon and understand the fragmentation mechanism. In this work, for a parametric study, tests were performed under structural conditions, coolant void conditions, and boiling conditions. As a result, in some cases, the injected molten materials were stuck around the injection hole. On the other hand, the molten materials were dispersed upward sufficiently well under the boiling condition when R123 was used as the coolant. The built-up vapor pressure was found to be one of the driving forces for the upward dispersion of the molten materials.     

Experimental investigation on the bursting of single molten droplet in coolant

An experiment facility for observing low-temperature molten tin alloy droplet into water was es- tablished to investigate mechanisms of vapor explosion occurring in severe accidents of a fission nuclear reactor.The vapor explosion behaviors of the molten material were observed by a high-speed video cam- era and the vapor explosion pressures were recorded by a pressure transducer mounted under the water surface.The results showed that the pressure reached a peak value when the molten metal temperature was 600℃-650℃,and the coolant temperature had an obvious decreasing effect on the droplet breakups.A model for single droplet fuel/coolant interaction is proposed.It considers that in the case of Rayleigh-Taylor instability,the coolant that jets from opposite direction penetrates into the fuel and the vapor explosion occurs because of the rapid evaporation.This model explained the effect of metal droplet temperature and coolant temperature on vapor explosion.     

高温熔融液滴破碎特性实验研究

对高温熔融液滴在水中的破碎特性进行了实验研究，重点考察和分析了液滴材料的物理化学性质、熔化潜热和熔融液“粘糊状区域”(Mushy Zone)等因素对液滴破碎过程的影响。用共晶和非共晶的铅(Pb)-铋(Bi)合金作为液滴材料，用水作冷却剂，完成了约10个系列的实验工况．分别测量了液滴破碎后碎片的累积质量分布和质量平均尺寸。根据实验结果探讨了液滴材料的焓、熔化潜热、粘性等因素影响液滴变形与破碎过程的规律。     

Time-Optimal Power-Change Control for Coupled-Core Reactors

The numerical method used in this study is Moving Particle Semi-implicit (MPS) method which is based on moving particles and their interactions. Grids are not necessary. Large deformation of fluids can be calculated without grid tangling. A surface tension calculation model is developed to analyze droplet breakup. This model is verified by the simulation of vibration of an ethanol droplet. Two-dimensional numerical analyses of droplet breakup in liquid-liquid and gas-liquid systems are carried out. The correlation between the Weber number and the breakup mode observed in the calculations agrees with that in the experiments. Breakup behavior of a droplet surrounded by a vapor film is analyzed. Flow in the vapor film is considered, though boiling of water and solidification of the melt droplets are ignored. It is found that the breakup of a droplet is suppressed by the vapor film. The critical Weber number in the vapor film is obtained as 50. Molten core coolability is considered by using this result. The median diameter of stable droplets of the molten core is expected as 5 mm in a typical condition, which is consistent with FARO experiment. This result shows that in Advanced Boiling Water Reactor (ABWR) the debris bed up to 40% of the core can be cooled down in the lower head of the reactor pressure vessel.     

Fragmentation of continuous molten copper droplets penetrating into sodium pool

The progression of hypothetical core disruptive accidents (CDAs) in metal fuel cores is strongly affected by exclusion of molten metal fuel from the core region due to molten fuel–coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation characteristics of continuous molten copper droplets with a total mass from 20 to 50 g penetrating into a sodium pool. The results show that the fragmentation of the continuous molten copper droplets is sensitive to the change of the hydrodynamic and thermal conditions when the instantaneous contact interface temperature (T_i) is lower than the turning point (T_tp) and insensitive at T_i > T_tp. Compared with the fragmentation of a single droplet, the fragmentation of continuous droplets is accelerated and enhanced due to the collision between the droplets and the upward microjets. The present mass median diameter (D_m) or dimensionless mass median diameter (D_m/D₀) of continuous copper droplets shows a distribution with smaller values than those of single copper droplet, and larger values than those of copper jets under similar thermal and hydrodynamic conditions. These results are promising to assure the termination of accidents in CDAs and useful to the core design with enhanced safety in FBRs.     

Fragmentation of a single molten metal droplet penetrating into sodium pool. IV. Thermal and hydrodynamic effects on fragmentation in copper

To characterize the relationship between thermal and hydrodynamic effects on fragmentation of molten metallic fuels, with the interaction of the sodium coolant under a wide range of thermal and hydrodynamic conditions, in this paper, we focus on the fragmentation characteristics of a single molten copper droplet (1 and 5 g) with an ambient Weber number (We _a) from 102 to 614 and superheating conditions from 15 to 574°C, which penetrates into a sodium pool at an initial temperature from 298 to 355°C. In our experiments, fine fragmentations of the single molten copper droplets with a high We _a were clearly observed even under a supercooled condition that is well below the copper melting point of 1083°C. The dimensionless mass median diameters (D _m /D ₀) of molten droplets with a high We _a are less than the molten droplets with a low We _a under the same thermal condition. When We _a was approximately >200, the hydrodynamic effect on fragmentation became dominant over the thermal effect under a relatively low superheating condition. For a higher We _a range, the comparisons indicated that the fragment sizes of the molten copper droplets had similar distributions to those of copper and metallic fuel jets and stainless steel droplets even with different thermophysical properties and a 1000-fold mass difference, which implied the possibility that the fragment size characteristics of the molten metal jets could be evaluated by the interaction of a single droplet with the sodium coolant without consideration of dropping modes and mass.     

Study on fragmentation behavior of liquid lead alloy droplet in water

Fragmentation behavior of molten lead alloys droplet in water was investigated experimentally by releasing liquid LBE (45w%Pb-55w%Bi) and lead droplets into a pool of subcooled water. The fragmentation occurred when the temperature of the interface between a molten droplet and water was higher than the spontaneous nucleation temperature of water and lower than the minimum film boiling temperature. With increasing the droplet temperatures, the peak pressure in fragmentation of LBE droplet increased from 5 to 8 kPa, and for lead, the value remained around 2 kPa. With increasing the water subcooling, the peak pressure in fragmentation remained constant at 5 kPa for LBE droplet and at 2 kPa for lead droplet. The lead alloy fragmentation process in water was numerically simulated by embedding a semi-empirical fragmentation model for droplet fragmentation rate into the computer code of two-phase flow: JASMINE code. The corresponding results, such as pressure history and fragmentation peak pressure, agreed well with the experimental results.     

Molten fuel-coolant interaction phenomena with application to carbide fuel safety

This paper reviews the major phases occurring during an energetic molten fuel/coolant interaction (MFCI), the categories of interaction and modes of contact between molten fuel and liquid coolant, the film boiling destabilization and collapse mechanisms, and the important fragmentation mechanisms of the melt. Two major models that describe the processes involved in an MFCI event are discussed: the spontaneous nucleation model and the pressure detonation model. Finally, the MFCI experiments involving carbide fuel and liquid sodium are reviewed and the potential for an energetic interaction between molten carbide fuel and liquid sodium is discussed. Recommendations are given for future work on MFCI phenomena relative to the carbide fuel/sodium system.