氟盐冷却球床堆堆芯燃料球离散模拟

发布时间：2018-01-05 14:32

本文关键词：氟盐冷却球床堆堆芯燃料球离散模拟　出处：《中国科学院研究生院(上海应用物理研究所)》2017年博士论文　论文类型：学位论文

【摘要】：氟盐冷却球床堆(PB-FHR)是一种新型的第四代核反应堆,采用氟盐作为一回路冷却剂和高温气冷堆的燃料球作为燃料,具有较好的安全性和经济性。与气固系统中颗粒-流体耦合运动特性不同,堆芯内部固液两相密度接近且燃料球尺寸在厘米量级。低密度比导致堆积构建的计算非常耗时,大的颗粒半径又不符合传统DEM-CFD耦合方法所要求的流体计算网格足够大的假设。针对堆芯内部燃料球-冷却剂的耦合运动特性,本论文提出了适用于PB-FHR堆芯燃料球的离散分析方法并初步构建了三步式分析流程,包括颗粒堆积的形成,堆积包络面信息的提取和两相耦合运动的计算。首先是初始燃料球堆积的构建。PB-FHR堆芯内部由于颗粒-流体密度相近,净浮力只有重力加速度的约十分之一,小的净浮力导致堆积中小的颗粒间重合量和很小的离散元模拟时长。另外,反应堆安全分析中需要考察不同孔隙率的燃料球堆积对后续物理热工设计的影响。针对这三个问题,本文提出了圆柱容器内生成单粒径颗粒堆积的两阶段重力落球算法(TGM算法),可以在同一套程序框架内提升计算效率,控制颗粒间的平均重合量并且可以在大范围内调控堆积孔隙率。其次是燃料球堆积的信息提取。生成颗粒堆积之后,只用整体的平均孔隙率来评估堆积质量是不全面的,往往还需要进行单颗粒尺度上的信息提取来描述堆积特征。为此本论文提出了基于单个颗粒尺度的Voronoi-Delaunay剖分算法(SVD算法)。算法可以在单个颗粒的尺度上提取堆积内部任意颗粒的配位数,堆积内空洞拓扑的检测,三维孔隙率分布及包络面的提取等等。目前论文主要研究了工程中关心的装料后燃料球包络面的问题,通过提取振动后颗粒堆积的包络面,从拓扑演化的角度揭示了近壁面颗粒结晶化是导致振动密实效应的原因之一。最后是计算颗粒-流体间的耦合运动。针对传统DEM-CFD算法要求流体计算网格足够大这个限制,本论文提出了针对一般流体域的基于双网格的两相耦合算法(PB-FHR DEMCFD算法)。在颗粒和流体部分分别各自引入一套粗网格,粗的颗粒计算网格和流体计算网格之间的信息交换通过插值来完成。在此之上颗粒和流体部分均需维护流体压力,流体速度,孔隙率以及颗粒速度这四个场量来完成两相耦合力的计算。基于双网格的耦合算法解除了流体计算网格和颗粒直径之间的耦合,适用于复杂流场的模拟。通过对气固系统中三个典型基准算例的校核证明了算法对传统气-固系统的适用性。为了进一步考察算法对非规则区域流固强耦合工况的适用性,本论文搭建了缩比实验台架来仿真低密度差的强耦合工况,聚丙烯塑料球和水分别代替燃料球和冷却剂,模拟和实验结果非常吻合,初步证明了该算法对堆芯燃料球运动仿真的准确性。
[Abstract]:Fluorine salt cooled ball bed reactor (PB-FHR) is a new type of 4th generation nuclear reactor. Fluorine salt is used as primary coolant and fuel ball of high temperature gas cooled reactor as fuel. It has good safety and economy. It is different from the gas-solid system in the particle fluid coupling motion characteristics. The solid-liquid two-phase density in the core is close and the size of the fuel sphere is in the order of cm. The calculation of the stacking structure due to the low density ratio is very time-consuming. The large particle radius does not accord with the assumption that the fluid computational grid is large enough for the traditional DEM-CFD coupling method. In this paper, a discrete analysis method for PB-FHR core fuel spheres is proposed and a three-step analysis process is constructed, including the formation of particles. The extraction of packing envelope information and the calculation of two-phase coupled motion. Firstly, the initial fuel sphere stacking. PB-FHR core core due to similar particle-fluid density. The net buoyancy is only about 1/10 of the acceleration of gravity, and the small net buoyancy results in the amount of overlap between small and medium particles and the very small simulation time of discrete element. In the reactor safety analysis, it is necessary to investigate the influence of fuel ball accumulation with different porosity on the subsequent physical thermal design. In this paper, a two-stage gravity ball dropping algorithm is proposed to generate single particle stacking in a cylindrical container, which can improve the calculation efficiency within the same program framework. The average overlap between particles is controlled and the accumulation porosity can be controlled on a large scale. Secondly, the information of fuel ball accumulation is extracted. It is not comprehensive to use the overall average porosity to evaluate the stacking quality. It is often necessary to extract information from single particle scale to describe the stacking feature. In this paper, a single particle scale based Voronoi-Delaunay algorithm is proposed. The algorithm can extract the coordination number of arbitrary particles in a single particle on the scale of a single particle. The detection of the cavity topology, the three-dimensional porosity distribution and the extraction of the envelope surface, etc. At present, the paper mainly studies the problem of the fuel ball envelope surface which is concerned in the engineering. The envelope surface of vibrating particles was extracted. From the perspective of topological evolution, it is revealed that the crystallization of particles near the wall is one of the causes of vibration compaction. Finally, the coupled motion between particle and fluid is calculated. According to the traditional DEM-CFD algorithm, fluid computation is required. The grid is large enough for this limitation. In this paper, a two-phase coupling algorithm based on two-grid is proposed for general fluid domain. A set of coarse meshes are introduced into the particle and fluid parts, respectively, in the PB-FHR DEMCFD algorithm. The information exchange between coarse particle computing mesh and fluid computing grid is completed by interpolation. On this basis, both the particle and the fluid part need to maintain fluid pressure and fluid velocity. Porosity and particle velocity are the four fields to complete the calculation of the coupling force of two phases. The coupling algorithm based on double grids is used to decouple the coupling between the fluid computational grid and the particle diameter. The applicability of the algorithm to conventional gas-solid systems is proved by checking three typical benchmark examples in gas-solid systems. In order to further investigate the fluid-solid coupling conditions in irregular regions, the proposed algorithm is suitable for the simulation of complex flow fields. The applicability of. In this paper, an experimental bench was built to simulate the strong coupling of low density difference. Polypropylene plastic ball and water replace fuel ball and coolant respectively. The simulation results are in good agreement with the experimental results. The accuracy of the algorithm for the motion simulation of the core fuel ball is preliminarily proved.
【学位授予单位】：中国科学院研究生院(上海应用物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TL351.1

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