补偿中子测井中与温度相关轻水热中子散射数据库的研制与应用

补偿中子测井受到环境温度的影响,利用数值模拟计算来进行修正,但目前的理论计算程序MCNP缺乏水的精细温度相关的热中子散射数据库。为了解决蒙特卡罗模拟热中子散射S(α,β)模型只能求解特定温度条件下中子输运问题的局限性,基于热中子散射的S(α,β)原理,采用内插法得到不同温度下水的频谱分布、振动;在最新的ENDFB-VII.1数据库上,利用NJOY99程序制作了ACE格式的轻水热中子散射截面数据库。利用系列次临界基准题对数据的准确性进行了验证,不同数据库之间的计算结果及基准题的结果符合得很好。自制的数据应用于测井仪器中的温度效应修正,取得了较好的效果。     

中子输运方程基于界面修正的源迭代并行计算方法

中子输运方程的计算量非常大。在现有的计算机条件下，进行精密物理的数值模拟所需要的中子计算仍是非常的费时间和费内存的，不采用并行计算是难以承受的。并且，由于中子输运隐式离散纵标方法引起的数据强相关，以及计算过程必须严格沿中子运动方向进行(否则会出现计算不稳定)，因此会出现相当严重的算法同步的问题，使得隐式格式在大型并行计算机上实施时所能得到的并行度十分有限，严格限制了其实现具有高并行度的迭代计算的可能性。因此，对中子输运方程隐式差分格式进行并行改造是十分必要的。     

引入源-样品距离影响的主动中子多重性质量反演公式北大核心CSCD

主动中子多重性计数测量方法是常用的核材料质量无损测量方法,已广泛应用于核材料衡算、核安保测量与军控核查等领域。我们通过对JMCT中子-光子输运程序的二次开发,实现了对经典点模型铀样品质量估算实验的数值模拟,并提出了改进的铀样品质量计算公式。该算法可以显著降低本实验中源-样品耦合与源中子反照等作用对铀样品质量估算精度的影响。建立了主动中子多重性计数测量探测系统模型和32个铀样品半球壳模型,模拟得到了与铀样品距离不同的DT源和AmLi源主动中子多重性计数,利用数值模拟手段检验了质量估算算法的有效性。数值模拟结果表明,改进的铀质量估算算法可以使质量估算的平均偏差率降低到10%以下。     

水冷氚增殖包层的中子学模拟

建立起锂球壳模型,用三个不同的聚变评价中子数据库--FENDL2.1、FENDL3.0和JEFF3.2分别进行了中子输运模拟,比较了三个数据库的模拟结果。再对水冷增殖包层分别建立一维中子学模型和三维中子学模型,进行中子输运模拟。分析模拟结果表明,选择FENDL3.0作为水冷增殖包层三维中子学模拟的数据库;水冷增殖包层一维中子学模拟优先考虑柱壳模型模拟;水冷氚增殖包层的三维中子学模拟所得氚增殖率TBR能满足氚自持要求;而且外包层的TBR贡献是主要的。     

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建立起锂球壳模型,用三个不同的聚变评价中子数据库——FENDL2.1、FENDL3.0和JEFF3.2分别进行了中子输运模拟,比较了三个数据库的模拟结果.再对水冷增殖包层分别建立一维中子学模型和三维中子学模型,进行中子输运模拟.分析模拟结果表明,选择FENDL3.0作为水冷增殖包层三维中子学模拟的数据库;水冷增殖包层一维中子学模拟优先考虑柱壳模型模拟;水冷氚增殖包层的三维中子学模拟所得氚增殖率TBR能满足氚自持要求;而且外包层的TBR贡献是主要的.     

提高超高能中子计算效率的蒙特卡罗抽样技巧

利用蒙特卡罗方法模拟中子与中子碰撞及超高能中子的产生与输运,原有抽样方法需要模拟大量样本方能使与超高能中子有关统计量的计算误差达到要求.针对ICF聚变靶中子输运问题的特点,发展一种"加权赌分裂抽样方法",以增加重要区域的抽样数、减少非重要区域的抽样数,同时通过权修正保证计算结果无偏.典型模型数值模拟计算结果显示,该方法能够有效增加聚变区的中子抽样数、增加中子相互碰撞数,使超高能中子通量计算误差显著降低,达到提高超高能中子计算效率的目的.     

氩-碳-硅等离子体热力学性质和输运系数计算

计算了广温度范围(300—30000 K)和广压力范围(0.1—10 atm, 1 atm=101.325 k Pa)下,不同混合物比例、碳和硅蒸气浓度的局域热力学平衡(LTE)和化学平衡(LCE)的氩-碳-硅等离子体组分、热力学性质和输运系数.等离子体气相平衡组分使用质量作用定律计算,同时凝聚相组分采用相平衡的方法计算.输运系数的计算包括黏度、电导率和热导率,使用拓展到高阶近似的Chapman-Enskog方法.采用文献中较新的数据得到了较为准确的碰撞积分,导出了Ar-C-Si等离子体的输运系数.结果表明,在相变温度以下,凝聚态物种的引入导致Ar-C-Si等离子体的热力学性质、输运系数与纯Ar等离子体接近,在相变温度点则会产生不连续点.压力、碳/硅蒸气浓度和比例对等离子体热力学性质和输运系数具有较大影响.最终计算值与文献数据对比符合良好,有望为氩-碳-硅等离子体传热流动数值模拟提供基础数据.     

基于VENUS-III国际基准模型的JMCT程序验证

基于自主研制的三维中子-光子耦合输运蒙特卡罗通用程序JMCT（J Monte Carlo Transport Code）,采用连续点截面,对国际基准屏蔽VENUS-III模型开展精细建模和中子输运临界及屏蔽计算.临界计算得到系统k_eff、重要区域的通量及能谱.结果表明,JMCT和MCNP程序的重要区域体通量计数吻合较好,偏差均在1%以内.深穿透屏蔽计算采用外源模式,点探测器计数,JMCT计算值与实验测量值偏差在15%以内,满足屏蔽设计对误差的要求.初步验证了JMCT程序临界及屏蔽计算的可用性.     

以中子作外源的假想核弹头主动探测

通过对不同入射能量的中子在假想的核弹头中的输运计算,分析了弹头中的裂变材料芯部的核裂变、各层对中子的吸收和(n,xn)反应以及出壳中子随角度的变化,论证了以外中子源(不随时间变化的外源)对弹头作主动探测的可行性.通过对14 1MeV脉冲中子源和产生的缓发中子在模型中的输运计算,分析了用脉冲外中子源产生的缓发中子探测核弹头的可行性.     

基于神经网络的粒子输运问题高效计算方法

蒙特卡罗方法是求解粒子输运问题的有力工具之一,其局限性在于为达到精度要求需模拟大量粒子,计算耗时长,这阻碍了该方法的进一步应用,尤其在需快速响应的情形.本文结合神经网络和若干蒙特卡罗方法基本原理发展了一种计算方法,能够实现源分布可变,几何、材料和目标计数不变的中子输运问题的快速准确求解.首先,为高效生成用于神经网络训练的数据,利用重要性原理实现在同样模拟次数基础上有效扩充训练数据集容量,在一定程度上克服了使用蒙特卡罗计算获取训练数据耗时长的缺点.进而,基于目标计数是源分布与重要性函数乘积积分的事实,设计了利用神经网络实现快速输运计算的策略.该网络的输入是中子源项,输出是目标计数,在几何、材料和目标计数固定的情况下,该神经网络可重复使用,根据新的源项快速准确得到目标计数.本文所提出方法的原理和框架同样适用于其他种类粒子的同类型输运问题.基于若干基准模型的验证表明,训练得到的神经网络能在不到1 s的时间内得到目标计数,且与蒙特卡罗大样本模拟得到基准结果的平均相对偏差均低于5%.     

Simulation of Space Shuttle neutron measurements with FLUKA

FLUKA is an integrated particle transport code that has enhanced multigroup low-energy neutron transport capability similar to the well-known MORSE transport code. Gammas are produced in groups but many important individual lines are specifically included, and subsequently transported by the main FLUKA routines which use a modified version of EGS4 for electromagnetic (EM) transport. Recoil protons are also transported by the primary FLUKA transport simulation. The neutron cross-section libraries employed within FLUKA were supplied by Giancarlo Panini (ENEA, Italy) based upon the most recent data from JEF-1, JEF-2.2, ENDF/B-VI, JENDL-3, etc. More than 60 different materials are included in the FLUKA databases with temperature ranges including down to cryogenic temperatures. This code has been used extensively to model the neutron environments near high-energy physics experiment shielding. A simulation of the Space Shuttle based upon a spherical aluminum equivalent shielding distribution has been performed with reasonable results. There are good prospects for extending this calculation to a more realistic 3-D geometrical representation of the Shuttle including an accurate representation of its composition, which is an essential ingredient for the improvement of the predictions. A proposed project to develop a combined analysis and simulation package based upon FLUKA and the analysis infrastructure provided by the ROOT software is under active consideration. The code to be developed for this project will be of direct application to the problem of simulating the neutron environment in space, including the albedo effects.     

Structure and proton conduction in CsDSO4

Caesium hydrogen sulphate is one of the most extensively studied superprotonic conductors with hydrogen bonds. A first order phase transition, from the low conductivity to the high conductivity phase, takes place at 414 K. The crystallographic structure of both phases has been established using high-resolution neutron powder diffraction. Quasielastic neutron scattering has provided information about the spatial and temporal characteristics of proton transport. In the present paper we report the results of reverse Monte Carlo modelling of neutron diffraction (total scattering) data which makes it possible to obtain the diffusion pathways for protons. The results are in good agreement with the qualitative model for proton transport proposed previously; in particular we show clearly that the proton motion is highly correlated with the rotation of the sulphate groups. In the low temperature phase we identify a weak correlation between protons and oxygens on the next-nearest-neighbour sulphate group which increases with temperature and probably drives the phase transition.     

基于热质运动概念的普适导热定律

根据爱因斯坦的质能等效关系式,热能具有的等效质量称为热质,从而在固态和气态介质中分别建立了声子气质量和热子气质量的概念.应用牛顿定律建立了含有驱动力、阻力和惯性力的热质(声子气或热子气)运动的动量守恒方程.由于热量在介质中的传递本质上就是热质(声子气和热子气)在介质中的运动,所以热质动量守恒方程就是普适的导热定律,能够统一描述各种条件下的导热规律.当热流密度不是很大从而热质惯性力可以忽略时,热质动量守恒方程就退化为傅里叶导热定律,这表明傅里叶导热定律是特殊条件下的导热定律,对于微纳尺度条件下的导热,热流密度可以极高,由速度空间变化引起的惯性力不能忽略,在稳态导热情况下也将出现非傅里叶导热,此时在计算或者实验中不能用热流密度除温度梯度求导热系数.在超快速加热条件下,必需考虑惯性力,与基于CV导热模型的波动方程相比,普适的导热定律增加了因速度空间变化引起的惯性力项,所以在介质中热波叠加时不会出现产生负温度的非物理现象,表明基于热质运动概念的普适导热定律更为合理. 关键词： 傅里叶导热定律 普适导热定律 热质运动 非傅里叶导热     

Isothermal reduction kinetics of nickel oxide using hydrogen: Conventional and Weibull kinetic analysis

The powder sample of nickel oxide was synthesized by sol-gel procedure. The isothermal reduction of nickel oxide using hydrogen was investigated by thermogravimetric analysis at five operating temperatures: 245, 255, 265, 275 and 300 °C. The kinetic triplet (E_a, A and f(α)) was determined using conventional and Weibull kinetic analysis. Both the kinetically procedures show that the reduction process considered can be explained with a two-step kinetic model. It is established that at lower temperatures (245 °C?T?255 °C), the reduction process considered is governed by two-parameter Šesták-Berggren autocatalytic model (first step) and at higher temperatures (T?265 °C), the reduction process is governed by Fn reaction model with different values of parameter n (second step). In this paper, the complex manner of dependence of the Weibull shape parameter (β) on temperature is established. With alterations of Weibull shape parameter from lower temperatures (β>1) to higher temperatures (β<1), it was concluded that isothermal reduction process of NiO using hydrogen can be described by a multistep reaction mechanism. These results are confirmed by the evaluated density distribution functions (ddf) of apparent activation energies (E_a), which show variations in basic characteristics at lower and higher operating temperature regions. Also, in this paper, it was shown that the shape parameter (β) of Weibull distribution function can represent the behaviour index, which indicates the kinetic pattern of the mechanism controlling the process studied.     

Neutron Transmission of Single-crystal Sapphire Filters

An additive formula is given that permits the calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections as a function of sapphire temperature and crystal parameters. We have developed a computer program that allows calculations of the thermal neutron transmission for the sapphire rhombohedral structure and its equivalent trigonal structure. The calculated total cross-section values and effective attenuation coefficient for single-crystalline sapphire at different temperatures are compared with measured values. Overall agreement is indicated between the formula and experimental data. We discuss the use of sapphire single crystal as a thermal neutron filter in terms of the optimum cystal thickness, mosaic spread, temperature, cutting plane and tuning for efficient transmission of thermal-reactor neutrons.     

基于量子修正的石墨烯纳米带热导率分子动力学表征方法

本文提出了基于量子修正的非平衡态分子动力学模型,可用于石墨烯纳米带热导率的表征.利用该模型对不同温度下,不同手性及宽度的石墨烯纳米带热导率进行了研究,结果发现:相较于经典分子动力学模型给出的热导率随温度升高而单调下降的结论,在低于Debye温度的情况下,量子修正模型的计算结果出现了反常现象.本文研究还发现,石墨烯纳米带的热导率呈现出明显的边缘效应及尺度效应:锯齿型石墨烯纳米带的热导率明显高于扶手椅型石墨烯纳米带;全温段的热导率及热导率在低温段随温度变化的斜率均随宽度的增加而增大.最后,文章用Boltzmann声子散射理论对低温段的温度效应及尺度效应进行了阐释,其理论分析结果说明文章所建模型适合在全温段范围内对不同宽度和不同手性的热导率进行精确计算,可为石墨烯纳米带在传热散热领域的应用提供理论计算和分析依据.     

Natural convection of water-copper nanoliquids confined in low-porosity cylindrical annuli

Natural convection in cylindrical porous annuli saturated by a nanoliquid whose inner and outer vertical radial walls are respectively subjected to uniform heat and mass influxes and out fluxes is studied analytically using the modified Buongiorno-Darcy model (MBDM) and the Oseen-linearization technique. Nanoliquid-saturated porous medium made up of water as base liquid, copper nanoparticles of five different shapes, viz., spheres, bricks, cylinders, platelets and blades, and glass balls porous material is considered as working medium for investigation. The thermophysical properties of nanoliquid -saturated porous medium is modeled using phenomenological laws and mixture theory. The effect of various parameters and individual effects of five different shapes of copper nanoparticles on velocity, temperature and heat transport are found. From the study, it is clear that the addition of a dilute concentration of nanoparticles increases the effective thermal conductivity of the system and thereby increases the velocity and the heat transport, and decreases the temperature. In other words, the heat transport is more in the case of heat and mass driven convection compared to purely heat-driven convection. Among the five different shapes of nanoparticles, blade-shaped nanoparticles facilitate the transport of maximum temperature compared to all other shapes. Maximum heat transport is achieved in a shallow cylindrical annulus compared to square and tall circular annuli. The increase of the inner solid cylinder’s radius is to decrease heat transport. The results of the KVL single-phase model are obtained from the present study by setting to zero the value of the nanoparticles’ concentration Rayleigh number. Also, neglecting the curvature effect in the present problem, we obtain the results of the rectangular enclosure problem.     

Effects of double stratification and heat flux damping on convective flows over a vertical cylinder

Unsteady free convection flows of viscous fluids over a vertical circular cylinder are investigated by taking in consideration thermal and mass stratification and the thermal memory effects. The mathematical model of thermal transport is based on the fractional generalized Fourier's law for thermal flux with the kernel of power-law kind. In this model the histories of the temperature gradient influence the thermal and mass transport process and the fluid motion. On the cylinder's surface the temperature (or the thermal flux) and solute concentration are constant. Solutions in the transformed domain for the perturbation temperature and concentration and fluid velocity are determined using the Laplace transform coupled with the classical method for the ordinary non-homogeneous differential equations. The inverse Laplace transforms are obtained numerically by employing the Stehfest's algorithm. Solutions for the case corresponding to classical Fourier's law are obtained as particular case of general solutions by taking the memory parameter equal to zero. The influence of the thermal memory and of thermal and mass stratifications is numerically and graphically analyzed by using the software Mathcad 15.