Modeling and optimization of steam dryer for removal of lead–bismuth droplets

Removal of lead–bismuth droplets from steam flow is a crucial issue in the direct contact boiling lead–bismuth cooled fast reactor. Droplets are generated due to the boiling of water directly in the reactor chimney, where steam for the turbine is generated. The droplets could severely damage the turbine and therefore a steam dryer is used for their removal. This paper presents an optimization of the main steam dryer geometrical parameters and steam inlet velocity. The Lagrangian method is used, in which first the steam flow field is developed using the CFD code FrontFlow/Red and then the particle motion is simulated. It was found that the reduction of the plate spacing can improve the steam dryer performance without a significant increase of pressure drop, the wane pitch has a value after which the steam dryer performance is not significantly improved, the number of wanes of 1.5 was selected at this point, however, a more detailed model is necessary to arrive at the final conclusion. The optimum steam inlet velocity should be found using a detailed economical assessment. Velocities between 2 and 4 m/s seem to be reasonable to achieve good removal efficiency and keep the pressure drop at reasonable values.     

Compatibility of surface-coated steels, refractory metals and ceramics to high temperature lead–bismuth eutectic

Compatibility of cladding material with lead–bismuth eutectic at temperature higher than 650 °C is one of the most crucial issues for feasibility of lead–bismuth-cooled fast reactors with cycle efficiency as high as 40%. In order to search for corrosion-resistant materials with lead–bismuth eutectic at temperature higher than 650 °C, surface-coated steels, some refractory metals and various ceramics were tested by means of stirred-type corrosion test. Lead–bismuth was heated up to 700 °C electrically in an alumina crucible, and oxygen concentration in the lead–bismuth was adequately controlled by injection of argon, steam and hydrogen gas mixture into the lead–bismuth. Specimens of aluminum–iron-alloy-surface-coated steels, refractory metals and ceramics including SiC/SiC composites were immersed in the stirred lead–bismuth for 1000 h. It was found that the surface-coated steels showed good compatibility with the lead–bismuth due to formation of a thin and stable protection layer on the surfaces. Tungsten and molybdenum exhibited high corrosion resistance. On the other hand, niobium is not a reliable material for the high temperature LBE. SiC and Ti₃SiC₂ also exhibited high corrosion resistance. On the other hand, the physical performance of the SiC/SiC composite must be improved especially by minimizing the porosity.     

Innovative nuclear technology based on modular multi-purpose lead–bismuth cooled fast reactors

Today's nuclear power is in the state of an intrinsic conflict between economic and safety requirements. This fact makes difficult its sustainable development.

One of the ways of finding the solution to the problem can be the use of modular fast reactors SVBR-75/100 cooled by lead–bismuth coolant that has been mastered in conditions of operating reactors of Russian nuclear submarines.

The inherent self-protection and passive safety properties are peculiar to that reactor due to physical features of small power fast reactors (100 MWe), chemical inertness and high boiling point of lead–bismuth coolant, integral design of the pool type primary circuit equipment.

Due to small power of the reactor, it is possible to fabricate the whole reactor at the factory and to deliver it to the NPP site in practical readiness by using any kind of transport including the railway.

Substantiation of the high level of reactor safety, adaptability of the SVBR-75/100 reactor relative to the fuel type and fuel cycle, issues of non-proliferation of nuclear fissile materials, opportunities of multi-purpose usage of the standard SVBR-75/100 reactors have been viewed in the paper.     

Simulation study of steels corrosion phenomenon in liquid lead–bismuth cooled reactors using molecular dynamics methods

Corrosion phenomena of stainless steel in liquid lead–bismuth as a coolant in nuclear fast breeder reactor are a field which is intensively investigated by the researchers in the recent year. We try to study this corrosion phenomena by computer simulation using molecular dynamics methods. The initial positions of the system were taken from the crystal structure data including the cell parameters and the types of the crystal. In this simulation, interatomic potential between Fe–Fe, Pb–Pb, Bi–Bi, Ni–Ni and Cr–Cr was assumed to follow Lennard–Jones potential. The Lennard–Jones potential parameters have been derived by fitting the data available in the literature. Nickel and chromium atoms were substituted into Fe crystal with the percentage of 10% and 16% to construct systems like SS 316. Molecular dynamics simulation has been done by interfacing iron and steel with liquid lead and liquid lead–bismuth in several temperatures. The result of this simulation showed that lead atoms can diffuse into Fe–10%Ni–16%Cr about 1.18 Å at 773 K while in Fe–10%Ni and Fe–16%Cr about 7.25 Å and 11.08 Å, respectively.     

A rail transportable lead–bismuth cooled reactor

A conceptual design of a small reactor cooled by lead–bismuth is developed. The main constraint on this reactor design is its transportability. The whole reactor module should be transportable on a rail cart. This imposes a volume envelope of approximately 4.5 × 4.5 × 24 m and the maximum weight of about 300 tons. Therefore, the reactor vessel is 3 m in diameter and 3.85 m tall. In order to satisfy the proliferation resistance requirements the reactor is sealed after the fuel is loaded and shall not be opened until it is shipped back after it reaches its end of lifetime after 15years. The reactor fuel is 11% and 13% enriched plutonium nitride. Reactor power is 50 MW_th which translates into 15 MW_e. Reactor pool is at nearly atmospheric pressure. Core inlet and outlet temperature are 350 and 365 °C, respectively. The reactor uses electromagnetic pumps to drive the primary coolant circulation. Secondary system consists of saturated steam cycle operating at 7 MPa and 290 °C. This reactor is well suited for secluded areas with the demand for electricity such as small islands.     

Boiling heat transfer behavior of lead–bismuth–steam–water direct contact two-phase flow

The boiling heat transfer behavior of lead–bismuth (Pb–Bi)–steam–water direct contact two-phase flow was experimentally investigated. Experimental study was performed using Pb–Bi–steam–water direct contact boiling two-phase flow loop. The heat transfer rate was estimated from data of one-dimensional flow direct contact boiling of water in Pb–Bi. It is assumed in the analysis that film boiling occurs at the surfaces of a small water droplet after water is injected into hot Pb–Bi flow, because of the large temperature difference between water and Pb–Bi (i.e. 493 K and 733 K for injection water and Pb–Bi temperature, respectively). The heat transfer occurs between Pb–Bi and steam without phase change after all water completely evaporates. The overall heat transfer coefficient decreased with the superheat at low injection flow rate and was nearly constant for high injection flow rate. The local heat transfer coefficient was higher than average one in the whole tube, which means that the direct contact boiling heat transfer coefficient was high and it decreased in the downstream direction. Almost all the water vaporized in the test tube at high pressure according to the local and total heat transfer rate.     

Development of Simplified Wave-type Vane in BWR Steam Dryer and Assessment of Vane Droplet Removal Characteristics

Air-water experiments were performed for the BWR steam dryer in order to elucidate droplet removal characteristics of the vane. Based on the results, a simplified vane was developed and its droplet removal characteristics were confirmed by air-water experiments using a whole dryer model.

Phase-Doppler anemometer was used to measure droplet diameter distributions. In the experiments of the current vane with four-wave stages and 120° bend angle, almost all of the droplets were found to be trapped in the first and second vane stages. For the air velocity of 3.1 m/s, 90% of the inlet droplets were trapped there and 4% were trapped in the third and fourth stages, resulting in 6% being carried over. Sauter mean diameters at the exit were 6 and 5μm while at the inlet they were 71 and 64μm for the respective air velocities of 1 and 3 m/s. Based on the Weber number evaluation, the possible mechanism for the fine droplet generation was considered to be the breakup of droplets due to impingement on the liquid film formed in the vane surface.

From the above results, the simplified vane with two-wave stages was developed. In order to remove fine droplets, the bend angle of the vane was reduced to 90°, in which a larger inertia force is exerted on finer droplets generated by breakup. Experiments for the whole dryer model showed that the developed vane could achieve the same droplet removal characteristics as the current vane without increasing the pressure drop of the dryer. A design having ten dryer bank rows and 1.2 m height, which is 0.8 m shorter than the current dryer, is possible for the dryer using the new simplified vane.     

Analysis of radioactivity releases in the MEGAPIE reference accident

A computer model for the MEGAwatt Pilot Target Experiment (MEGAPIE) was developed, verified and applied to the analysis of the reference accident, resulting from the hypothetical loss of a Pb–Bi inventory and subsequent cooling down of the target with free convection of the atmospheric air. The radioactivity releases caused by evaporation of activation products from the lead–bismuth films on the surfaces of the target inner structures are estimated. The maximum radioactivity releases were found to be from evaporation of the mercury and polonium estimated as 7.0 × 10¹¹ and 6.2 × 10⁸ Bq, respectively. Thermal conductivity and radiation heat transfer through the gas gaps were found to be more important mechanism of the target cooling down than the atmospheric air convection. The final conclusion made on the basis of the work is that MEGAPIE in the reference accident meets the 1 mSv criterion.     

Comparative study of ADS for minor actinides transmutation

The methodology of the efficiency comparison of the different ADS-burners is discussed. ADS with lead–bismuth coolant (fast neutron spectrum), molten salt ADS (intermediate spectrum) and heavy water ADS (thermal spectrum) are chosen as representatives for the comparison. The first results of the suggested approach are discussed.     

Lead-alloy coolant technology and materials – technology readiness level evaluation

Lead and lead–bismuth eutectic heavy liquid metal coolants are under wide-ranging international investigation and development for advanced nuclear systems for energy production and waste transmutation (reactor-based or accelerator-driven). This report reviews the major supporting international R&D programs, the key advances in the main areas of coolant technology and materials, the state of technology, and the strategic directions for further development. Based on this review, we conservatively evaluate the technological readiness level (TRL) for programmatic and industrial applications in high-temperature advanced reactors to be 7, “one-dimensional engineering-scale demonstration”, or the first level in the proof-of-performance category. A 3-D engineering-scale integral test and demonstration facility should be the next step toward the realization of a test and demonstration nuclear system (reactor or accelerator-driven). The recent success of MEGAPIE, a 1 MW class lead–bismuth eutectic spallation target operating at the Paul Scherrer Institute signals that for such applications of short to intermediate durations at moderate temperatures, the TRL is close to 9, meaning the technology is nearly ready for deployment.     

Design and safety optimization of ship-based small nuclear power reactors

Design and safety optimization of ship-based nuclear power reactors have been performed. The neutronic and thermo-hydraulic programs of the three-dimensional X–Y–Z geometry have been developed for the analysis of ship-based nuclear power plant. Quasi-static approach is adopted to treat seawater effect and quasi-static approach is also employed to treat neutronic aspect during safety analysis.

The reactors are loop type lead–bismuth-cooled fast reactors with nitride fuel and with relatively large coolant pipe above reactor core, the heat from primary coolant system is directly transferred to water–steam loop through steam generators. The power level is 100–200 MW th and excess reactivity is about 1$. Three types of core were investigated in the optimization process: balance, tall, and pancake with five values of Z–Y size ratio.

As the optimization results, the core outlet temperature distribution is changing with the elevation angle of the reactor system. The pancake core type has larger temperature distribution change as the elevation angle changes due to the sea wave. The natural circulation capability is good for safety. However, large driving head of natural circulation may cause large temperature fluctuation as the elevation angle changes.     

气流扰动下单液滴撞击单根干燥扁网丝特性数值研究

丝网分离器在工业中有着广泛地应用。本文针对液滴撞击网丝的动态过程,采用CLSVOF方法对单个液滴撞击干燥网丝的问题进行数值模拟,经过合理的简化,建立了气流扰动下单液滴撞击干燥扁网丝面的二维数学模型,分析了液滴撞击角和撞击位置对液滴撞击行为特性的影响。数值计算的结果表明:液滴撞击到干燥网丝上分为铺展和飞溅2个过程,撞击角越小,上铺展半径越大,下铺展半径越小,分离的二次液滴体积越大;液滴撞击网丝的位置离网丝边缘越近,越容易产生二次液滴,二次液滴的总体积越多。      

钠滴氧化燃烧特性的实验研究

液态钠泄漏和燃烧是钠冷快堆在运行中一多发的常见事故。本文主要针对液态钠滴在不同初始温度(140~370℃)和氧浓度(4%~21%)条件下的氧化燃烧行为进行实验研究。实验通过1套钠滴燃烧装置和高速摄像机使钠滴的氧化燃烧行为可视化。实验结果表明:钠滴的初始温度和氧浓度越低,钠滴表面产生的柱状氧化物越长;在相同氧浓度条件下,钠滴初始温度越高,越易着火燃烧;钠滴初始温度在200℃以下时很难点燃,当有扰动破坏了表面的氧化层结构时,钠滴也会逐渐燃烧;钠滴初始温度在140~370℃的条件下,氧浓度≥12%时,钠滴能燃烧充分,最高温度基本可升至600~800℃;氧浓度12%时,燃烧并不充分,燃烧的最高温度均在600℃以下。这些结果对柱状流及雾状钠火的研究有重要的指导意义。     

Rate of droplet deposition in steam-water annular flow and effect of a flow obstacle

Deposition rate of droplets in steam-water annular two-phase flow was measured using a 5 mm diameter vertical round tube as a test section. In the experimental conditions tested in this work, the droplet mass transfer coefficient decreased with an increase in the droplet concentration in the gas core flow and with an increase in the length of a deposition section. The dependence on these two parameters agreed fairly well with predictions by available correlations. Placing a small cylindrical tube concentrically in the test section round tube, the effect of a flow obstacle on the deposition rate of droplets was also experimentally investigated. It was found that the obstacle effect was significant and the deposition rate of droplets increased approximately three times in average. The obstacle effect measured in this work was compared with an empirical correlation and a simple mechanistic model that were developed using experimental results of air-water annular flows. Fairly good agreement was achieved in both cases, which would indicate that the mechanism of deposition enhancement induced by the flow obstacle is similar between air-water and steam-water flows.     

液滴撞击干燥倾斜壁面铺展实验研究

通过高速摄影技术及像素分析方法对液滴撞击干燥倾斜壁面进行实验研究，分析了不同倾斜角及韦伯数（We）对液滴飞溅及铺展的影响，结果表明，倾斜角的增加有利于抑制飞溅的产生；We一定时，液滴的前铺展因子随着倾斜角的增加而增加，后铺展因子随着倾斜角的增加而减小；液滴的前后初始铺展速度均大于液滴的撞击速度，且随着倾斜角的增加而减小。      

倾斜条件对喷淋液滴运动特性的影响

本文以倾斜条件下喷淋液滴在空气环境下运动特性为工程背景,建立倾斜条件下单个液滴在常温、常压空气环境中动量方程,分析倾斜角度、喷射角度、液滴直径、初始速度对液滴运动轨迹的影响机理。计算结果表明：液滴初始喷射角度为45°、倾斜角度为30°时,距初始位置垂向距离为50 m的中心剖面的横向跨度约为5.5 m;倾斜角度越大、液滴直径越小、液滴初始速度越小,液滴覆盖范围越小;液滴初始喷射角度越大、液滴直径越小、液滴速度越小,倾斜角度对液滴喷射覆盖范围的影响越小;倾斜角度对倾斜同向和反向的液滴运动轨迹的影响相对大小,与液滴初始喷射角度、液滴直径和初始速度密切相关。研究还表明,液滴离开喷淋头后,按照正向和负向最大位移运动的两个液滴,液滴直径对液滴横向位移由初始状态到最大值的时间影响十分显著,而初始速度、喷射角度和倾斜角度对其影响有限。     

蒸汽发生器一级汽水分离器内流场数值模拟

以压水堆核电厂蒸汽发生器一级汽水分离器为研究对象，采用基于计算流体动力学（CFD）的计算软件ANSYS Fluent对湿蒸汽进入汽水分离器后的流场特性和汽水分离性能进行模拟，在模拟过程中采用了欧拉多相流模型和k-ε Realizable湍流模型相结合的计算模型。对工质流经汽水分离器的模拟结果表明，在汽相与液相经由汽水分离器流至各自出口时，出现明显的分层现象。对比不同切向出口和不同液滴粒径下的模拟结果表明，出口面积越大，汽水分离器对液滴的分离效果越好；在0.01~0.10 mm的粒径范围内，液滴粒径越大，分离效果越好。对不同负荷条件下汽水分离器分离效率的模拟结果表明，分离效率随机组负荷升高略有降低。      

Analysis of the operating parameters of a vortex electrostatic precipitator

A vortex electrostatic precipitator(VEP) forms a vortex flow field within a precipitator by means of the vertical staggered layout of the double-vortex collecting plate facing the direction of the gas flow.The ion concentrations within the precipitator can be significantly increased.Correspondingly,the charging and coagulation rates of fine particles and particle migration velocity are significantly improved within the VEP.Since it can effectively collect fine particles and reduce precipitator size,VEPs represent a new type of electrostatic precipitator with great application potential.In this work the change curve of the external voltage,gas velocity,row spacing and effective collecting area influencing the precipitation efficiency were acquired through a single-factor experiment.Using an orthogonal regression design,attempts were made to analyze the major operating parameters influencing the collecting efficiency of fine particles,establish a multiple linear regression model and analyze the weights of factors and then acquire quantitative rules relating experimental indicators and factors.The regression model was optimized by MATLAB programming,and we then obtained the optimal factor combination which can enhance the efficiency of fine particle collection.The final optimized result is that:when gas velocity is 3.4 ms~(-1),the external voltage is 18 kV,row spacing is 100 mm and the effective collecting area is 1.13 m~2,the rate of fine particle collection is 89.8867%.After determining and analyzing the state of the internal flow field within the VEP by particle image velocimetry(PIV),the results show that,for a particular gas velocity,a vortex zone and laminar zone are distinctly formed within the VEP,which increases the ion transport ratio as well as the charging,coagulation and collection rates of fine particles within the precipitator,thus making further improvements in the efficiency of fine particle collection.     

Spectroscopic and microscopic study of the corrosion of iron–silicon steel by lead–bismuth eutectic (LBE) at elevated temperatures

The performance of iron–silica alloys with different silicon composition was evaluated after exposure to an isothermal bath of lead–bismuth eutectic (LBE). Four alloys were evaluated: pure iron, Fe–1.24%Si, Fe–2.55%Si and Fe–3.82%Si. The samples were exposed to LBE in a dynamic corrosion cell for periods from 700 to 1000 h at a temperature of 550 °C. After exposure, the thickness and composition of the oxide layer were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS), including sputter depth profiling. Particular attention was paid to the role, spatial distribution, and chemical speciation of silicon. Low-binding-energy silicon (probably silicates or ) was found in the oxide; while elemental silicon (Si) was found in the metal as expected, and silica (SiO₂) was found at the bottom of the oxide layer, consistent with the formation of a layer between the oxide and the metal. Alloys with low concentrations of Si contained only silicate in the oxide. Alloys with higher concentrations of Si contained a layer of silica at the boundary between the oxide and the bulk metal. All of the alloys examined showed signs of oxide failure. This study has implications for the role of silicon in the stability of the oxide layer in the corrosion of steel by LBE.     

Experimental observation of the droplet size change across a wet grid spacer in a 6 × 6 rod bundle

During the reflood phase of a postulated loss of coolant accident in a nuclear reactor, entrainment of liquid droplets can occur at a quench front of reflooding water. It is widely recognized that the behavior of the entrained droplets crucially affects the reflood heat transfer phenomena by decreasing the superheated steam temperature and interacting with a rod bundle and spacer grids. For this reason, various experimental and numerical studies have been performed to examine droplet behavior such as the droplet size, velocity and droplet fraction inside a rod array. In this study, an experiment on the droplet behavior inside a heated rod bundle has been performed. The experiment was focused on the change of droplet size induced by a spacer grid in a rod bundle geometry, which results in the change of the interfacial heat transfer between droplets and superheated steam. A 6 × 6 rod bundle test facility in Korea Atomic Energy Research Institute was used for the experiment. Steam was supplied by an external boiler into the bottom of the test channel, and a droplet injection nozzle was equipped instead of simulating a quench front of reflooding water. The major measuring parameters of the experiment were the droplet size and velocity, which were measured by a high-speed camera and a digital image processing technique. A series of experiments were conducted with various flow conditions of a steam injection velocity, heater temperature, droplet size, and droplet flow rate. The experiments provided the data on the change of the Sauter mean diameter of droplets after collision with a wet grid spacer depending on flow conditions.