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摘要
高温气冷堆（High Temperature Gas-cooled Reactor，HTGR）作为第四代先进核能系统的代表，采用各向同性（Tri-isotropic，TRISO）的包覆核燃料颗粒，可以有效阻挡裂变产物，目前广泛使用流化床-化学气相沉积（Fluidized Bed-Chemical Vapor Deposition，FB-CVD）技术制备TRISO包覆核燃料颗粒。在FB-CVD方法制备TRISO颗粒的过程中，反应气体发生裂解，在流化颗粒的表面沉积得到所需包覆层。反应器内的气固接触效率直接决定了包覆的速率，因此优化反应器设计对提高包覆层的质量有着重要意义。一种带有中心筒的底喷式流化床（简称Wurster流化床）得到了广泛的应用，且已成功用于颗粒的喷雾包覆，通过中心筒的内喷雾和反应器壁面的加热，使颗粒快速完成喷雾-干燥-包覆。基于此技术理念，在FB-CVD技术制备核燃料包覆颗粒的过程中，引入内加热外循环分区式流化床。通过中心筒的内加热，使气相裂解反应集中在中心筒内包覆区发生，颗粒穿过包覆区后在外低温区完成循环往复。此设计使颗粒和气流形成了规则有序的定向流场，有效控制颗粒的循环时间和包覆区停留时间，进而实现气体裂解过程和颗粒循环过程的解耦。为了更精确地获取颗粒流化行为的相关参数，和包覆区内的颗粒停留时间与流场特性，本研究采用CFD-DEM（Computational Fluid Dynamics Discrete Element Method）方法，对内加热外循环分区式流化床中的颗粒流化行为进行数值模拟。分析了包覆区内的流场、浓度场及床内的颗粒运动行为，揭示了颗粒包覆过程中的关键参数及其影响机制。模拟结果表明，CFD-DEM方法能够有效捕捉颗粒与流体的相互作用，为实验研究提供了理论支持，同时为优化流化床的工艺参数和提高颗粒的包覆效率提供了重要参考。
Abstract
High Temperature Gas-cooled Reactor (HTGR), as a representative of the fourth generation of advanced nuclear energy systems, uses Tri-isotropic (TRISO) coated nuclear fuel particles, which can effectively block fission products. Fluidized Bed-Chemical Vapor Deposition (FB-CVD) technology is widely used to prepare TRISO-coated nuclear fuel particles. In the process of preparing TRISO particles by FB-CVD method, the reaction gas is cracked and the required coating layer is deposited on the surface of the fluidized particles. The gas-solid contact efficiency in the reactor directly determines the coating rate, so it is important to optimize the reactor design to improve the quality of the coating layer. A bottom spray fluidized bed (Wurster fluidized bed for short) with a central cylinder has been widely used, and has been successfully used in the spray coating of particles, through the inner spray of the central cylinder and the heating of the reactor wall, the particles can quickly complete the spray - drying - coating. Based on this technical concept, a zoned fluidized bed with internal heating and external circulation was introduced in the process of preparing nuclear fuel coated particles by FB-CVD technology. Through the inner heating of the central cylinder, the gas phase cracking reaction is concentrated in the coating area of the central cylinder, and the particles pass through the coating area and complete the cycle in the outer low temperature area. This design makes the particles and air flow form a regular and orderly directional flow field, effectively control the circulation time of particles and the residence time of the coating area, and then realize the decoupling of the gas cracking process and the particle circulation process. In order to obtain the relevant parameters of particle fluidization behavior and the particle residence time and flow field characteristics in the coating area more accurately, the Computational Fluid Dynamics Discrete Element Method (CFD-DEM) was adopted in this study. The particle fluidization behavior in a zoned fluidized bed with internal heating and external circulation was numerically simulated. The flow field, concentration field and motion behavior of particles in the coated area were analyzed, and the key parameters and their influencing mechanisms were revealed. The simulation results show that CFD-DEM method can effectively capture the interaction between particles and fluids, which provides theoretical support for experimental research, and provides an important reference for optimizing the process parameters of the fluidized bed and improving the coating efficiency of particles.
| 关键词 | 核燃料包覆颗粒，流化床设计，CFD-DEM，包覆模拟 |
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| Keywords | coated nuclear fuel, fluidized bed design, CFD-DEM, simulation of coating |
