Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Experimental study on evaporation characteristics of single and multiple fuel droplets

In this work, evaporation experiments of multiple droplets are carried out in a stagnant hot atmospheric environment (573, 673 and 773 K) using high-speed backlit image technique. Three fuel droplets with nearly same initial diameter are suspended at intersections of two 0.1 mm quartz fibers. The normalized droplet spacing (s/d₀) of three droplets is 2.25. The results show that the evaporation process of single, edge and central fuel droplet containing three stage: initial heating, unsteady evaporation and quasi-steady evaporation stage. Classical d² law is still suitable for edge and central droplet at quasi-steady evaporation stage. The third stage of edge and central droplet accounts for more than 60% of droplet lifetime at low temperatures and about 50% at high temperatures. The evaporation rate constant of edge and central droplet increases and droplet lifetime decreases with increasing ambient temperature. The evaporation time of edge and central droplet at first and third stage is higher than single droplet, but lower than single droplet in the second stage. More importantly, the evaporation interactions between droplets is significant at low temperature. Compared with single droplet, the lifetime of central droplet is increased by 31.8%, 18.6% and 25.9%, respectively.     

Transient characteristics of small molten salt reactor during blockage accident

This paper reports the results from a transient core analysis of a small molten salt reactor (MSR) when a duct blockage accident occurred. The focus of this study is a numerical model employed in order to consider the interaction among fuel salt flow, heat transfer, and nuclear reactions. The numerical model comprises continuity and momentum conservation equations for fuel salt flow, two‐group neutron diffusion equations for fast and thermal neutron fluxes, transport equations for six‐group delayed neutron precursors, and energy conservation equations for fuel salt and graphite moderators. The analysis results show the following: (1) the effect of the self‐control performance of the MSR on the effective multiplication factor and thermal power output of the reactor after the blockage accident is insignificant, (2) fuel salt and graphite moderator temperatures increase drastically but locally at the blockage area and its surroundings, (3) the highest fuel salt temperature after the blockage accident is 1,363 K; this value is lower than the boiling point of fuel salt and the melting temperature of the reactor vessel, (4) the change in the distributions of fast and thermal neutron fluxes after the blockage accident when compared with the distributions at the rated condition is very slight, and (5) delayed neutron precursors, especially the first delayed neutron precursor, accumulate at the blockage area due to its large decay constant. These results imply that the safety of the MSR is assured in the case of a blockage accident. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(6): 434–450, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20123     

Review of conceptual design and fundamental research of molten salt reactors in China

Molten salt reactor (MSR) as 1 candidate of the generation IV advanced nuclear power systems attracted more attention in China due to its top ranked in fuel cycle and thorium utilization. Two types of MSR concepts were studied and developed in parallel, namely the MSR with liquid fuel and that with solid fuel. Abundant fundamental research including the neutronics modeling, thermal‐hydraulics modeling, safety analysis, material investigation, molten salts technologies etc. were carried out. Some analysis software such as COUPLE and FANCY were developed. Several experimental facilities like high‐temperature fluoride salt experiment loop have been constructed. Some passive residual heat removal systems were designed, and 1 test facility is under construction. The key MSR techniques including the extraction and separation of molten salt and construction of N‐base alloy have been mastered. Based on these fundamental research, Chinese Academy of Sciences has completed the design of thorium‐based MSRs with solid fuel and liquid fuel and is promoting their construction in the near future. In China, future efforts should be paid to the material, online fuel processing, Th‐U fuel cycle, component design, and construction and thermal‐hydraulic experiments for MSR, which are rather challenging nowadays.     

Solidification characteristics of rising immiscible oil droplets in coolant

Solidification characteristics of hexadecane particles injected through a single nozzle into coolants of stagnant pure water, ethylene glycol 30 wt% water solution, and ethylene glycol 50 wt% water solution were investigated. Experimental parameters were varied in the ranges of 22.4 < Re_p < 492, 8.79 < Pr < 49.9, 0.0475 < Ste < 0.0741, and 0.0566 < Fo < 1.66. The investigation was focused on the solidification phenomena of hexadecane particles in coolants. The present study is summarized in the following. (1) Empirical equation correlating the drag coefficients of hexadecane particles was proposed, and (2) empirical equations correlating the solidification mass fraction of hexadecane particles were proposed for low injection velocity condition and for higher injection velocity condition independently.     

Adjoint-based sensitivity analysis of circulating liquid fuel system for the multiphysics model of molten salt reactor

The strongly coupled behaviors between neutronics and thermal-hydraulics of liquid-fueled molten salt reactors make it difficult to evaluate system behaviors, due to the transport of precursors along moving fuel. Extending an adjoint-based method on the multiphysics approach, different assumptions on temperature dependencies of nuclear and thermophysical properties of salt are included in the local sensitivity analysis of a circulating liquid fuel system. Local sensitivity of various types of system response in steady-state is analyzed for 39 parameters including coupling models, reactor design values, and kinetic constants of delayed neutron and decay heat precursors for a simplified 1D model of molten salt fast reactor. Extended adjoint-based sensitivity analysis method for MSR is successfully validated achieving 1.38% deviation on average between a recalculation and adjoint method, comparing local sensitivities to all parameters. Also, it takes 66.3 times less in computational time compared with the recalculation method for evaluating the sensitivity of the same type of system response. The importance of all the parameters to the system response is analyzed according to the assumptions on temperature dependencies to nuclear data and salt properties. The most influencing ones are fission energy-related terms, and their importance increases when temperature dependencies are taken into account, compared with constant properties. Changes of influences on the sensitivity are investigated from the relative changes of the parameter values in various system response types, and it implies the importance to consider the multiphysics modeling on the local sensitivity analysis.     

A study on thermal conductivity of a mixture of magnesia particles and molten nitrate by a transient hot wire method

In this study, the effective thermal conductivity of a mixture of magnesia particles and molten nitrate used in a high temperature thermal energy storage system was investigated by a transient hot wire method. The effective thermal conductivity of the mixture was around 2.0W/(m·K) in the temperature range of 423 K to 703 K, although it decreased about 5% with increasing temperature. This value was about 10 times larger than that of the packed bed of magnesia particles including air. The effective thermal conductivity increased about 3% with a 1% increase in the volume ratio of magnesia particles in the mixture with molten nitrate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(4): 245–253, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20116     

Operating characteristics of a molten carbonate fuel‐cell power‐generation system

This paper investigates the operating characteristics of a molten carbonate fuel‐cell power‐generation system by using dynamic simulations. The implementation of the system model and the evaluation of the system model performance are presented and the model effectiveness for improving system operation and design is demonstrated. Copyright © 1999 John Wiley & Sons, Ltd.     

Natural convection cooling of a hot vertical wall wet by a falling liquid film

The system studied is a plane channel in which one of the two vertical walls is kept at an arbitrary temperature profile and may be partially or completely wet by a falling liquid film, while the opposite wall is adiabatic. Air from the environment flows along the channel with a mass flow rate which depends on the balance between hydraulic resistances and buoyancy forces. These latter, in their turn, depend on the distribution of temperature and humidity (hence, density) along the channel and eventually on the heat and mass transferred from wall and film to the humid air.A simplified computational model of the above system was developed and applied to the prediction of relevant quantities, such as the total energy subtracted to the hot wall, as functions of the geometrical and physical quantities that characterize the problem (channel height and thickness, localized hydraulic resistance, hot wall temperature and its distribution, film flow rate, ambient air temperature and humidity). Conclusions were also drawn on the cooling strategy to be adopted in the case when only a limited amount of coolant is available.     

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part I: distributions of temperature,energy transfer and current density

This study examined the temperature distributions of the anode and cathode gases of the cell body as well as the current density distributions at each point of the direct internal reforming molten carbonate fuel cell (DIR‐MCFC) using numerical modelling. The model was based on assumptions and experimental data from a 5 cm × 5 cm sized unit cell operation. The results showed there was an approximately 13°C temperature difference between the initial point (0, 0) and end point (1, 1) of the cell body and the temperature increased steadily along with the direction of the anode gas flow. The temperature distribution of the anode gases showed a similar trend to those of the cell body. The temperature of the anode gases was an average 11°C lower than that of the cell body. The temperature distributions of cathode gases were relatively higher than those of the anode gases and the cell body. The temperature distributions at each point of the cell body, including the anode and cathode gases, could be explained by the different rates of the electrochemical, methane steam reforming and water–gas shift reactions at each point in the cell body. The current density distribution at the entrance of the cell was the highest at 290 mA cm^?2, and decreased steadily to 150 mA cm^?2 at the exit. These results were also confirmed by the amount of hydrogen reacted in the electrochemical reaction (referred to Part II). Finally, modelling simulations showed a non‐uniform distribution of the temperature and current density throughout the DIR‐MCFC were observed. In addition, it was confirmed that the distributions of the reaction rates and gas compositions at each point of the cell also showed a great deal of difference throughout the DIR‐MCFC. The non‐uniformity of these temperature distributions can lead to deterioration in the cell performance. These might provide the necessary information for solving these problems. Copyright © 2005 John Wiley & Sons, Ltd.     

Dynamic characteristics of hydrocarbon fuel within the channel at supercritical and pyrolysis condition

Regenerative cooling with fuel as the coolant is used in the scramjet engine.In order to grasp the dynamic characteristics of engine fuel supply processes,this article studies the dynamic characteristics of hydrocarbon fue lwithin the channel.A one-dimensional dynamic model was proved,the thermal energy storage effect,fuel volume effect and chemical dynamic effect have been considered in the model,the ordinmy differential equations were solved using a 4th order Runge-Kutta method.The precision of the model was validated by three groups of experimental data.The effects of input signal,working condition,tube size on the dynamic characteristics of pressure,flow rate,temperature have been simulated.It is found that cracking reaction increased the compressibility of the fuel pyrolysis mixture and lead to longer responding time of outlet flow.The responding time of outlet flow can reach 3s when tube is 5m long which will greatly influence the conmo1 performance of the engine thrust system.Meanwhile,when the inlet flow rate appears the step change,the inlet pressure leads to overshoot,the overshoot can reach as much as 100％,such highly transient impulse will result in detrimental effect on fuel pump.     

Thorium utilization in a small modular molten salt reactor with progressive fuel cycle modes

The thorium‐uranium (Th‐U) fuel cycle is considered as a potential approach to ensure a long‐term supply of nuclear fuel. Small modular molten salt reactor (SMMSR) is regarded as one of the candidate reactors for Th utilization, since it inherits the merits of both MSR and small modular reactor. The Th utilization in a 220‐MWe SMMSR with the once‐through fuel cycle mode is investigated first. Then, the SMMSR with batch and online fuel processing modes is investigated second for comparison, considering the progressive development of fuel reprocessing technology. To keep a negative temperature reactivity feedback coefficient (TRC), a configuration for fuel salt volume fraction (SVF) equal to 15%, with a mixed fuel of low enriched uranium (LEU) and thorium at an operation time of 5 years is recommended for the once‐through mode, corresponding to the Th energy contribution (ThEC) of 37.6% and natural U and Th utilization efficiency (UE) of 0.51%. Considering the solubility limit of heavy nuclide (HN) proportion (below 18.0 mol%) in the fuel salt, the total operation time of the SMMSR shall be less than 50 years for the batch reprocessing mode with a 5‐year reprocessing interval time. In this case, the ThEC and UE can be improved to about 47.4% and 0.99%, respectively. Finally, the Th utilization and fuel sustainability are analyzed at a lifetime of 50 years for the online reprocessing fuel cycle mode, including both the only online fission products (FPs) removing scheme and the fuel transition scheme from LEU to ²³³U. For the former scheme, the ThEC and UE can be further improved to 58.6% and 1.52%, respectively. For the latter scheme, ²³³Pa is extracted continuously from the core to breed and store ²³³U. If a total reactor lifetime of 50 years is assumed, the operation time using LEU as starting and feeding fuel for 6 years is required, and the bred ²³³U during this 6‐year operation can start and maintain the reactor criticality for the remaining 44 years. In this case, the ThEC is improved significantly to 89.1% corresponding to a UE of 2.74%.     

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part II: comparative distributions of reaction rates and gas compositions

This study examined the distributions of the three reaction rates and the compositions of the gases at each point of the unit cell in DIR‐MCFC using a numerical simulation. The electrochemical reaction rates at the anode gas entering position were almost two times faster than those at the anode gas outlet position and most of the feeding CH₄ reacted in the region from the position x=0 to the position x=0.3. In addition, the water–gas shift reaction became faster from near the half position of the unit cell to the gas outlet position. Therefore, in the rear position of the unit cell, the steam reforming reaction played an important role as a supplementary reaction for providing the H₂ needed in the electrochemical reaction. The rates of the two catalytic reactions in the case without the electrochemical reaction were relatively slower than those in the DIR‐MCFC. Unlike the distributions of temperature, the current density, gas compositions and the reactions rates at each point of the DIR‐MCFC cell, the exit gas compositions from the simulation in particular could be comparative to those of the experimental results. Although there was an approximately 10% difference between both of them, the extent of the difference was considered to be reasonable for this simulation considering the experimental values that could be included in this simulation such as the lower conversion of the reactions, the lower current density and any other values. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of molten metal spreading and solidification behaviors utilizing moving particle full-implicit method

To retrieve the fuel debris in Fukushima Daiichi Nuclear Power Plants (1F), it is essential to infer the fuel debris distribution. In particular, the molten metal spreading behavior is one of the vital phenomena in nuclear severe accidents because it determines the initial condition for further accident scenarios such as molten core concrete interaction (MCCI). In this study, the fundamental molten metal spreading experiments were performed with different outlet diameters and sample amounts to investigate the effect of the outlet for spreading-solidification behavior. In the numerical analysis, the moving particle full-implicit method (MPFI), which is one of the particle methods, was applied to simulate the spreading experiments. In the MPFI framework, the melting-solidification model including heat transfer, radiation heat loss, phase change, and solid fraction-dependent viscosity was developed and implemented. In addition, the difference in the spreading and solidification behavior due to the outlet diameters was reproduced in the calculation. The simulation results reveal the detailed solidification procedure during the molten metal spreading. It is found that the viscosity change and the solid fraction change during the spreading are key factors for the free surface condition and solidified materials. Overall, it is suggested that the MPFI method has the potential to simulate the actual nuclear melt-down phenomena in the future.     

Modelling and simulation of a molten salt loop of a solar tower power plant in a Modelica environment

CSP（concentrated solar power）系统通过聚焦太阳光进行光热发电,主要工作介质包括导热油及熔盐。由于太阳能的不连续性,为了提供更稳定的能量输出,发展出了采用熔盐为储热工质进行蓄热的系统,该系统同时也提供了与其它能源系统耦合的可行性。该系统的熔盐回路主要由3个部分构成,分别为集热器、高低温储热罐和蒸汽发生器。通过对50 MW塔式太阳能的热发电系统熔盐回路的各个组件的模型进行建立,设计一套与之相应的控制系统,并针对不同工况下,各个组件及整个系统的响应进行了模拟,得出熔盐回路的瞬态响应特性。由于其熔盐回路与熔盐反应堆具有一定的相似性,本研究也可为熔盐反应堆核能综合利用中储能系统提供参考。     

Phenomena identification and ranking table exercise for thorium based molten salt reactor-solid fuel design

Thorium based molten salt reactor-solid fuel (TMSR-SF) design is an innovative reactor concept that uses high-temperature tristructural-isotropic (TRISO) fuel with a low-pressure liquid salt coolant. In anticipation of getting licensed applications for TMSR-SF in the future, it is necessary to fully understand the significant features and phenomena of TMSR-SF design, as well as its transient behavior during accidents. In this paper, the safety-relevant phenomena, importance, and knowledge base were assessed for the selected events and the transient of TMSR-SF during station blackout scenario is simulated based on RELAP/SCDAPSIM Mod 4.0.The phenomena having significant impact but with limited knowledge of their history are core coolant bypass flows, outlet plenum flow distribution, and intermediate heat exchanger (IHX) over/under cooling transients. Some thermal hydraulic parameters during the station blackout scenario are also discussed.     

Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles

The burning characteristics of fuel droplets containing nano and micron-sized aluminum particles were investigated. Particle size, surfactant concentration, and the type of base fluid were varied. In general, nanosuspensions can last much longer than micron suspensions, and ethanol-based fuels were found to achieve much better suspension than n-decane-based fuels. Five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame, and aluminum droplet flame) were identified for an n-decane/nano-Al droplet, while only the first three stages occurred for an n-decane/micron-Al droplet. For the same solid loading rate and surfactant concentration, the disruption and microexplosion behavior of the micron suspension occurred later with much stronger intensity. The intense droplet fragmentation was accompanied by shell rupture, which caused a massive explosion of particles, and most of them were burned during this event. On the contrary, for the nanosuspension, combustion of the large agglomerate at the later stage requires a longer time and is less complete because of formation of an oxide shell on the surface. This difference is mainly due to the different structure and characteristics of particle agglomerates formed during the early stage, which is a spherical, porous, and more-uniformly distributed aggregate for the nanosuspension, but it is a densely packed and impermeable shell for the micron suspension. A theoretical analysis was then conducted to understand the effect of particle size on particle collision mechanism and aggregation rate. The results show that for nanosuspensions, particle collision and aggregation are dominated by the random Brownian motion. For micron suspensions, however, they are dominated by fluid motion such as droplet surface regression, droplet expansion resulting from bubble formation, and internal circulation. And the Brownian motion is the least important. This theoretical analysis explains the different characteristics of the particle agglomerates, which are responsible for the different microexplosion behaviors that were observed in the experiments.     

超临界二氧化碳再压缩布雷顿循环系统动态特性研究

超临界二氧化碳(S-CO₂)布雷顿循环因其环保性与高热电转换效率而被视为核能发电未来发展的重要方向。借助Simulink软件平台,建立了S-CO₂再压缩布雷顿循环闭环动态仿真系统,通过模拟结果与Sandia实验数据的比较,验证了系统模型的有效性。研究搭建的超临界CO₂再压缩布雷顿循环系统在稳态设计点条件下的预测热效率为31.85%,此外,还获得了热源功率和流量扰动条件下系统热力学参数的响应特征,发现热源功率的变化促使系统效率单调提升或降低,而改变系统流量未呈现类似变化趋势;扰动施加过程中,循环系统的参数对热源功率的变化非常敏感,热源功率减小15.00%,循环效率从31.85%降低到22.00%。最终基于仿真结果,获得多参量耦合关联下的变化规律与调控策略,可为S-CO₂再压缩布雷顿工程应用奠定基础。     

并网光伏发电系统动态特性的对比分析

大量光伏电站的并网,相应地带来并网稳定性、自身抗扰动性等相关问题。由于单级型光伏并网系统和双级型光伏并网系统的拓扑结构不同,其并网运行的动态特性也不尽相同。文章基于PSCAD/EMTDC电磁暂态仿真软件,分别搭建单级型和双级型光伏并网系统模型,并对比仿真了光照扰动与电网电压跌落扰动下两种光伏并网系统的动态特性,单级型光伏并网系统更适合于大中型光伏电站。     

Dynamic modeling of electrical characteristics of solid oxide fuel cells using fractional derivatives

An efficient controller is greatly important for the quick load-following response of solid oxide fuel cell (SOFC) power systems, which is vital for the fuel utilization and the life of the stack. To design such a controller, an accurate dynamic model of SOFC electrical characteristics is critical. Here an integer order dynamic model is established by a transient equivalent circuit, and then a fractional order dynamic model is done in the perspective of the fractional derivatives theory. The parameters of the dynamic models then are optimized via genetic algorithms according to electrochemical impedance spectroscopy (EIS) experimental data. Finally, the dynamic response experiments from the models are studied. The results show that the fractional order dynamic model has higher accuracy for representing the dynamics of the SOFC electrical characteristics, which lays a solid foundation for the controller based on the accurate model.