Radio Frequency Induction Plasma Spraying of Molybdenum

Radio frequency (RF) induction plasma was used to make free-standing deposition of molybdenum (Mo). The phenomena of particle melting, flattening, and stacking were investigated. The effect of process parameters such as plasma power, chamber pressure, and spray distance on the phenomena mentioned above was studied. Scanning electron microscopy (SEM) was used to analyze the plasma-processed powder, splats formed, and deposits obtained. Experimental results show that less Mo particles are spheroidized when compared to the number of spheroidized tungsten (W) particles at the same powder feed rate under the same plasma spray condition. Molten Mo particles can be suf[iciently flattened on substrate. The influence of the process parameters on the flattening behavior is not significant. Mo deposit is not as dense as W deposit, due to the splash and low impact of molten Mo particles. Oxidation of the Mo powder with a large particle size is not evident under the low pressure plasma spray.     

射频感应等离子体制备球形Ti粉的工艺

采用射频(RF)感应等离子体球化颗粒形状不规则的Ti颗粒。研究了加料速率、物料分散方式、Ti颗粒大小等因素对球化率的影响。电子扫描显微镜(SEM)观察的球化效果以及对钛粉振实密度的测定表明:当钛原粉以极短暂时间快速穿越等离子体炬时,钛粉颗粒因受热而熔化成液滴,快速冷却,形成球形固态颗粒。     

Kinetic study of key species and reactions of atmospheric pressure pulsed corona discharge in humid air

In this study, we examined the key particles and chemical reactions that substantially influence plasma characteristics. In summarizing the chemical reaction model for the discharge process of N2–O2–H2O(g) mixed gases, 65 particle types and 673 chemical reactions were investigated. On this basis, a global model of atmospheric pressure humid air discharge plasma was developed, with a focus on the variation of charged particles densities and chemical reaction rates with time under the excitation of a 0–200 Td pulsed electric field. Particles with a density greater than 1% of the electron density were classified as key particles. For such particles, the top ranking generation or consumption reactions (i.e. where the sum of their rates was greater than 95% of the total rate of the generation or consumption reactions) were classified as key chemical reactions. On the basis of the key particles and reactions identified, a simplified global model was derived. A comparison of the global model with the simplified global model in terms of the model parameters, particle densities, reaction rates (with time), and calculation efficiencies demonstrated that both models can adequately identify the key particles and chemical reactions reflecting the chemical process of atmospheric pressure discharge plasma in humid air. Thus, by analyzing the key particles and chemical reaction pathways, the charge and substance transfer mechanism of atmospheric pressure pulse discharge plasma in humid air was revealed, and the mechanism underlying water vapor molecules' influence on atmospheric pressure air discharge was elucidated.     

Analysis of the plasma-particle interaction during the plasma spraying of beryllium

Beryllium plasma-spray technology is being investigated as a method for coating plasma facing surfaces inside the international thermonuclear experimental reactor (ITER). This study investigated the plasma and plasma-particle interactions that occur during the plasma-spraying of beryllium. To evaluate the effect of the chamber pressure on the temperature, velocity, and trajectory profiles of the injected beryllium particles, the particles were numerically analyzed at three operating pressures (i.e., 101.3, 66.6 and 46.6 kPa) under a fixed operating condition. The thermal plasma was numerically modeled to predict the gas dynamics of the plasma column and plume. This information was then used as boundary conditions to solve the plasma—particle interaction problem at the various operating pressures. Calculations were performed for beryllium particles that ranged in diameter between 4 and 38 μm. Results of the numerical simulations describing the particle temperatures, velocities, and trajectories are discussed.     

Thermal conductivity of U–Mo/Al dispersion fuel: effects of particle shape and size,stereography, and heat generation

This paper describes the effects of particle sphericity, interfacial thermal resistance, stereography, and heat generation on the thermal conductivity of U–Mo/Al dispersion fuel. The ABAQUS finite element method (FEM) tool was used to calculate the effective thermal conductivity of U–Mo/Al dispersion fuel by implementing fuel particles. For U–Mo/Al, the particle sphericity effect was insignificant. However, if the effect of the interfacial thermal resistance between the fuel particles and Al matrix was considered, the thermal conductivity of U–Mo/Al was increased as the particle size increases. To examine the effect of stereography, we compared the two-dimensional modeling and three-dimensional modeling. The results showed that the two-dimensional modeling predicted lower than the three-dimensional modeling. We also examined the effect of the presence of heat sources in the fuel particles and found a decrease in thermal conductivity of U–Mo/Al from that of the typical homogeneous heat generation modeling.     

Dynamics of Dust in a Plasma Sheath with Magnetic Field

Dynamics of dust in a plasma sheath with a magnetic field was investigated using a single particle model. The result shows that the radius, initial position, initial velocity of the dust particles and the magnetic field do effect their movement and equilibrium position in the plasma sheath. Generally, the dust particles with the same size, whatever original velocity and position they have, will locate at the same position in the end under the net actions of electrostatic, gravitational, neutral collisional, and Lorentz forces. But the dust particles will not locate in the plasma sheath if their radius is beyond a certain value.     

Room Temperature Oxidation of UN Powder

Fresh UN powder of a medium particle size (0.3 m²/g) prepared by nitridation of U powder was exposed to O₂ at room temperature in a constant volume apparatus. Oxidation was followed by pressure measurement. Under O₂ pressures higher than 3 mmHg the powder ignited instantaneously, probably due to local heating in the surface layers of the powder particles. When the O₂ pressure was lower, oxidation proceeded in two clearly distinct stages, without N₂ release. The first stage terminated instantaneously. Irrespective of differences in powder conditions and pressure, the amount of O₂ absorbed in the first stage was found almost invariably to be about 1.7 cm³ STP/m², which corresponds to the formation of an oxidized layer somewhat thicker than 18Å. The powder particles covered with this layer did not ignite even under 1 atm O₂. This oxide coating could be of practical utility for protection against ignition. The second stage proceeded very slow and could be completely suppressed by cooling.     

Experimental investigation of coolability behaviour of irregularly shaped particulate debris bed

In case of a severe nuclear reactor accident, the core can melt and form a particulate debris bed in the lower plenum of the reactor pressure vessel (RPV). Due to the decay heat, the particle bed, if not cooled properly, can cause failure of the RPV. In order to avoid further propagation of the accident, complete coolability of the debris bed is necessary. For that, understanding of various phenomena taking place during the quenching is important. In the frame of the reactor safety research, fundamental experiments on the coolability of debris beds are carried out at IKE with the test facility “DEBRIS”. In the present paper, the boiling and dry-out experimental results on a particle bed with irregularly shaped particles mixed with stainless steel balls have been reported. The pressure drops and dry-out heat fluxes of the irregular-particle bed are very similar to those for the single-sized 3 mm spheres bed, despite the fact that the irregular-particle bed is composed of particles with equivalent diameters ranging from 2 to 10 mm. Under top-flooding conditions, the pressure gradients are all smaller than the hydrostatic pressure gradient of water, indicating an important role of the counter-current interfacial drag force. For bottom-flooding with a liquid inflow velocity higher than about 2.7 mm/s, the pressure gradient generally increases consistently with the vapour velocity and the fluid-particle drag becomes important. The system pressures (1 and 3 bar) have negligible effects on qualitative behaviour of the pressure gradients. The coolability of debris beds is mainly limited by the counter-current flooding limit (CCFL) even under bottom-flooding conditions with low flow rates. The system pressure and the flow rate are found to have a distinct effect on the dry-out heat flux.Different classical models have been used to predict the pressure drop characteristics and the dry-out heat flux (DHF). Comparisons are made among the models and experimental results for DHF and pressure drop characteristics. Considering the overall trend in prediction of DHF and two-phase pressure drop, it was observed that none of the models could provide accurate predictions for both DHF and pressure drop under top- and bottom-flooding conditions. This implies that developments of more accurate models are needed including the effects of non-uniform particle sizes and the multidimensional nature of particulate debris beds, which are not reflected so far in these models.     

弥散型燃料板辐照肿胀行为的有限元分析

采用颗粒复合材料的细观力学研究思路,针对弥散型燃料板,选择一种代表性单元作为研究对象,用与裂变密度和燃耗相对应的、不断增大的裂变压力模拟燃料颗粒裂变对铝基体所产生的力学贡献,对研究对象进行了热力耦合分析.考察了裂变压力、温度应变和燃料颗粒沿厚度非均布对颗粒肿胀的影响,并对弥散型燃料板基体的米塞斯应力分布情况进行了分析.研究结果表明,颗粒百分比含量越高,颗粒肿胀的速度相对越快;随着裂变压力增大,颗粒肿胀的速率急剧增大,且颗粒不再是圆形;在正常工作和选取的参数条件下,温度应变对总变形的影响不大;当裂变压力不大时,非均匀分布对颗粒肿胀几乎没有影响,随着裂变压力的增大,非均匀分布的颗粒肿胀明显高于均布情况,而且越来越明显;基体的应力非球对称分布.     

UN核芯TRISO燃料颗粒破损概率模型研究

TRISO燃料颗粒由核芯和4层包覆层组成,具有良好的裂变产物包容能力。TRISO燃料颗粒破损概率是表征TRISO燃料事故安全特性的关键参数。本文基于修正的PANAMA破损概率计算方法,在考虑UN核芯裂变气体释放导致的气体内压以及内外致密热解炭层辐照蠕变和收缩作用的基础上,开发了UN核芯TRISO燃料颗粒压力壳式破损概率计算方法,并采用IAEA基准题6和基准题9对模型进行了验证;基于开发的UN核芯TRISO颗粒破损概率计算方法,采用随机抽样统计方法分析了事故工况下UN核芯和包覆层设计参数（包括包覆层尺寸及密度）对UN核芯TRISO燃料颗粒破损概率的影响。研究结果显示,疏松热解炭（Buffer）层设计参数是影响TRISO颗粒破损概率的关键因素,可通过降低Buffer层尺寸及密度分布设计标准偏差的方法降低UN核芯TRISO燃料颗粒的破损概率。     

An interdiffusional model for prediction of the interaction layer growth in the system uranium–molybdenum/aluminum

The codes PLACA and DPLACA, elaborated in this working group, simulate the behavior of a plate-type fuel containing in its core a foil of monolithic or dispersed fissile material, respectively, under normal operation conditions of a research reactor. Dispersion fuels usually consist of ceramic particles of a uranium compound in a high thermal conductivity matrix. The use of particles of a U–Mo alloy in a matrix of Al requires especially devoted subroutines able to simulate the growth of the interaction layer that develops between the particles and the matrix. A model is presented in this work that gives account of these particular phenomena. It is based on the assumption that diffusion of U and Al through the layer is the rate-determining step. Two moving interfaces separate the growing reaction layer from the original phases. The kinetics of these boundaries are solved as Stefan problems. In order to test the model and the associated code, some previous, simpler problems corresponding to similar systems for which analytical solutions or experimental data are known were simulated. Experiments performed with planar U–Mo/Al diffusion couples are reported in the literature, which purpose is to obtain information on the system parameters. These experiments were simulated with PLACA. Results of experiments performed with U–Mo particles disperse in Al either without or with irradiation, published in the open literature were simulated with DPLACA. A satisfactory prediction of the whole reaction layer thickness and of the individual fractions corresponding to alloy and matrix consumption was obtained.     

基于甲酸的硝酸亚铈微波脱硝前驱体的制备北大核心CSCD

以硝酸亚铈的硝酸溶液为原料,微波加热得到块状的固体产物,加入甲酸后继续加热可生成分散性良好的粉末状前驱体,通过X射线衍射分析(XRD)确定各步产物,并对煅烧前和煅烧后产物的粒度和形貌进行表征。结果表明,微波加热后得到块状的二水合硝酸亚铈,甲酸与其在加热条件下反应,生成棒状结构的甲酸铈。当甲酸浓度为16~20 mol/L、HCOOH/Ce摩尔比为18时,可得到粒度较为均匀的粉末。分别煅烧16 mol/L和20 mol/L甲酸制备得到的甲酸铈,煅烧后得到的二氧化铈与煅烧前的颗粒具有相同的形貌,粒径分布区间相似,但会生成一部分粒径小于0.30μm的细颗粒。     

杂质与粉末粒度对钛酸锶源芯性能的影响

90Sr放射源是一种能量较强的β放射源,在诸多领域有着广泛的应用。由于钛酸锶陶瓷源芯的性能对90Sr源的安全性和制造工艺难度有着重要的影响,因此需要研究杂质含量和粉末颗粒度对钛酸锶陶瓷源芯力学性能和烧结稳定性的影响规律。采用正交实验方法,将钙、钡、锆杂质含量和粉末粒度作为四个因素,按照正交表L9(3^4)设计对照实验。对不同杂质含量以及不同粉末粒度条件下压制烧结出的钛酸锶源芯的密度、尺寸、抗压强度和硬度进行测试。结果表明,在固定的冷压烧结工艺参数条件下,原料颗粒度和杂质元素锆的含量对钛酸锶陶瓷源芯性能的影响明显,原料最佳粒度为10μm,锆含量必须控制在1%以下,杂质元素钙和钡的影响较小,其含量只需分别控制在2%和4%以内即可。研究结果可为制定钛酸锶原料的纯化制备工艺以及钛酸锶源芯的冷压烧结工艺提供参考。     

Dust Charging and Dynamics in Tokamaks

Fundamental characteristics of charging of a dust particle and its dynamics in SOL/divertor plasma in tokamaks are studied. According to the OML (orbit motion limited) theory, the charging process is extremely faster, with a charging time of nanoseconds, than the dynamics process of the dust particle in SOL/divertor plasma, with a characteristic time of milliseconds, which means that the local charge state can be taken as the equilibrium charge state. It was clarified that the equilibrium charge Zd,eq can be determined in the form of Zd,eq / Rd Te, which is a function of both the normalized relative speed of plasma ion flow with respect to the velocity of the dust particle and the plasma temperature ratio. After the investigation of dominant forces acting on dust particles, the friction forces due to the plasma ion absorption and ion Coulomb scattering are found of the same order for the case of low relative speed. The critical radius of a dust particle, for which the gravity is larger than the friction forces due to plasma ions, is obtained.     

Dynamic heating and thermal destruction conditions of quartz particles in polydisperse plasma flow of RF-ICP torch system

Numerical simulation of turbulent mixing process of polydisperse quartz particle (particle size distribution in the range of 0.1–0.4 mm) flow with Ar and Ar-H₂ plasma generated by radio frequency inductively coupled plasma (RF-ICP) torch has been made. An approximate two-stage approach has been proposed to calculate the spatial–temporal distributions of temperature and resulting thermal stress in quartz particles during dynamic heating in polydisperse plasma flow. The influence of working gas compositions, particle size distributions, injection angle and flow rate of carrier gas on the thermal destruction conditions of quartz particles has been determined under different particle feed rates. It is found that all the solid quartz particles (0.1–0.4 mm) could be thermal destructed without overheating in RF-ICP torch system, when the hydrogen volume fraction in working gases is more than 1.5%–2% and particle feed rate is in a certain range. The values of the maximum and minimum feed rates have been determined under different hydrogen volume fractions. An optimal particle injection angle and flow rate of carrier gas is found around 50°–60° and 160–220 slpm, under which the value of maximum equivalent thermal stress in quartz particles is highest.     

Post-irradiation examination of uranium-7 wt% molybdenum atomized dispersion fuel

Two low-enriched uranium fuel plates consisting of U-7wt%Mo atomized powder dispersed in an aluminum matrix, have been irradiated in the FUTURE irradiation rig of the BR2 reactor at SCK•CEN. The plates were submitted to a heat flux of maximum 353 W/cm² while the surface cladding temperature is kept below 130 °C. After 40 full power days, visual examination and profilometry of the fuel plates revealed an increase of the plate thickness. In view of this observation, the irradiation campaign was prematurely stopped and the fuel plates were retrieved from the reactor, having at their end-of-life a maximum burn-up of 32.8% ²³⁵U (6.5% FIMA). The microstructure of one of the fuel plates has been characterized in an extensive post-irradiation campaign. The U(Mo) fuel particles have been found to interact with the Al matrix, resulting in an interaction layer which can be identified as (U,Mo)Al₃ and (U,Mo)Al₄. Based on the composition of the interaction layer it is shown that the observed physical parameters like thickness of the interaction layer between the Al matrix and the U(Mo) fuel particles compare well to the values calculated by the MAIA code, an U(Mo) behavior modeling code developed by the Commissariat à l’énergie atomique (CEA).     

Aerosol retention in the flooded steam generator bundle during SGTR

A steam generator tube rupture in a pressurized water reactor may cause accidental release of radioactive particles into the environment. Its specific significance is in its potential to bypass the containment thereby providing a direct pathway of the radioactivity from the primary circuit to the environment. Under certain severe accident scenarios, the steam generator bundle may be flooded with water. In addition, some severe accident management procedures are designed to minimize the release of radioactivity into the environment by flooding the defective steam generator secondary side with water when the steam generator has dried out.To extend our understanding of the particle retention phenomena in the flooded steam generator bundle, tests were conducted in the ARTIST and ARTIST II programs to determine the effect of different parameters on particle retention. The effects of particle type (spherical or agglomerate), particle size, gas mass flow rate, and the break submergence on particle retention were investigated.Results can be summarized as follows: increasing particle inertia was found to increase retention in the flooded bundle. Particle shape, i.e., agglomerate or spherical structure, did not affect retention significantly. Even with a very low submergence, 0.3 m above the tube break, significant aerosol retention took place underlining the importance of the jet-bundle interactions close to the tube break. Droplets were entrained from the water surface with high gas flow rates carrying aerosol particles with them. However, compared to particle retention in the water close to the tube break, the effect of droplet entrainment on particle transport was small.     

Fast Transport of Colloidal Particles through Quartz-Packed Columns

Abstract

The migration of polystyrene latex particles (diameter = 310, 170 and 40 nm) through columns packed with 30 um quartz powder was experimentally studied at low eluant velocity at pH 6.0 + 0.1. The average velocity of polystyrene latex particle was found to be larger than that of HTO. The ratio (separation factor) of the average velocity of polystyrene latex particle to that of HTO increased with increasing the size of polystyrene latex particle and with decreasing the concentration of sodium chloride added under the experimental conditions. This reveals that the colloid migration process in the geological media is similar to that occurring in hydrodynamic chromatography. A model proposed for hydrodynamic chromatography was used to calculate the separation factor in which the potentials from the double layer force and from the van der Waals force were taken into account. The separation factor, calculated with the model by using the measured surface potentials of polystyrene latex particle and quartz powder and by using Hamaker constant evaluated from the literature, was found to agree well with the experimental result.     

UN核芯TRISO包覆燃料颗粒性能分析

为分析致密热解碳层、内压等因素对TRISO包覆燃料颗粒热-力学性能的影响,基于多物理场耦合软件COMSOL建立了以UN为核芯的TRISO包覆燃料颗粒三维热-力学耦合模型,并通过IAEA CRP-6基准题进行了验证。利用本文模型对稳态运行及反应性引入事故（RIA）工况下典型TRISO包覆燃料颗粒的性能进行了分析,结果表明,正常运行工况下SiC层能维持结构完整性,但IPyC层存在失效风险,需进一步优化TRISO包覆燃料颗粒的设计方案,而RIA工况下热膨胀是造成TRISO包覆燃料颗粒发生结构失效的主要原因。该模型能对轻水堆运行环境下的TRISO包覆燃料颗粒进行复杂的多物理场耦合性能分析,为进一步优化FCM燃料元件设计打下基础。     

CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristics are investigated through the CHF experiments and CFD analysis.CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. The test section geometry was identical to that of commercial PWR fuel assembly not including the heated length (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement using mixing vanes under higher mass flux (1400 kg/m² s) and lower pressure (15 bar) conditions is larger than the CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux (300-1000 kg/m² s) conditions and the hybrid vane performed best near the PWR operating conditions.The detailed flow characteristics were also investigated by CFD analysis using the same conditions as the CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of the CFD analysis in the boiling analysis was confirmed by comparing the average void fractions of the analysis and the experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of the lateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the void fraction in the near wall was decreased and that of the core region was increased resulting in the void fraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply to the wall and consequently the CHF increased. For the quantification of the void flatness, an index was developed and the applicability of the index in the CHF assessment was confirmed.