高速气流冲击法制备U3Si2－Al复合粉体的研究

In the 1980s, high enriched uranium（HEU）was successfully replaced by low enriched uranium (LEU) as fuel in research and test reactor in some countries. Dispersion-type fuel, consist of small particles embedded into a non-fissile and ideally inert matrix material, which guarantees adequate irradiation behavior of the fuel were used in research reactors and high-flux reactors. The fundamental idea of dispersion-type fuels is to isolate the fuel particles such that the fission gases released during irradiation can be accommodated locally within the matrix. As a result the centerline temperatures in the fuel are low and the fuel zone can contain fission products or to support swelling of the fuel itself without additional gap or void between the fuel and the cladding. Aluminum-based dispersion fuels are the most common type in use, for aluminum is both widely available and characterized by a very high thermal conductivity. Because of the large difference in density between the U3Si2 (12 g/cm3) and aluminum powder (2.7 g/cm3), the distribution of the U3Si2 particles in aluminum matrix is difficult to be guaranteed during the blend time and the transfer process to die for compaction, so that maintaining the homogeneity of U3Si2 fuel in fuel zone is difficult. Due to this condition, other parameters in compact production become more sensitive to changes. According to the problem, a dry powder coating method named high speed air flow impact method is put forward in the project. The approach is required to disperse the guest particle agglomerates during processing so that they may adhere to the host particles. There are several strong actions between the raw powder particles involving strong impaction，compression，friction，shearing and collision，which were generated by the high speed air flow in the system and will make the fine particle coat on the core particle in a short time. Based on the particle design and the improvement of U3Si2-Al mixing powder using conventional mixed method, hybridization（HYB） is used to fabricate shell-Al/core-U3Si2 coated particles with different conditions in this project. The possibility of creating U3Si2-Al coated particles approved by theoretical calculation and simulation. The effects of different conditions on the HYB product are analyzed and the most appropriate condition is summarized. The mechanism for the coating reaction is investigated as well as the properties of the blended, coated and blended-coated powder. Theories and practical references on the U3Si2-Al coated particles will be established, which can be provided for the preparation of other dispersion-type fuels like UMo according to the research in this project.

在高通量动力堆、研究堆所采用的金属基弥散型燃料元件中，燃料颗粒在基体中能否均匀分布是燃料元件在运行过程中是否存在局部高温以及裂变气体引起的燃料元件肿胀程度的关键因素，是元件寿命和反应堆能否安全运行的关键。本项目针对目前弥散型燃料元件制备中因U3Si2颗粒和Al粉的密度差异，机械转动混合不均匀以及在转移过程中易分离的问题，探索采用高速气流冲击制备铝粉包覆的U3Si2金属陶瓷复合粉体的方法。项目将从复合粉体粒子设计的角度出发，建立合适的碰撞模型；通过实验研究包覆气氛、转速、处理时间、投料量、处理温度等运转处理条件对复合粉体性能的影响因素，确定最佳工艺条件；研究复合粉体中U3Si2与铝粉的界面特征，揭示包覆机理；考察复合粉体的流动性，对高速气流冲击法所得的复合粉体在弥散型燃料元件的适用性作出科学评价。通过本项目的基础研究，还将为其它弥散燃料（UMo等）的制备提供理论和实践参考。

在弥散型核燃料元件中，核材料U3Si2多为不规则多面体形状颗粒，且与基体Al粉的密度差别较大，目前采用的球磨干法混合很难实现核燃料颗粒在Al粉中均匀分散，造成燃料元件中核材料不能被有效隔离及不均匀分布，从而影响到反应堆的安全性。.为了探索一种可实现燃料颗粒被基体材料有效隔离的混料方法，解决干法转动混合的不均匀分散以及转移过程中燃料颗粒与基体金属粉体的分离，本项目以具有相同颗粒度分布的ZrO2颗粒代替U3Si2在HYB设备内进行高速气流冲击的ZrO2-Al包覆试验，研究了包覆气氛、设备转速、处理时间、投料量、处理温度等参数对复合粉体性能的影响因素，确定了最佳工艺条件。试验结果表明高速气流冲击可以实现密度、颗粒度差别较大的金属粉体对陶瓷粉体的有效包覆。本项目还通过数学建模，研究了包覆机理，并将机械力包覆的方法用于合金材料的混料工艺，研究了合金的性能。.通过本项目的研究，建立了高密度脆性陶瓷材料与金属粉体机械力包覆制备复合粉体的方法，可用于U-Zr、U-Al、U-Mo等弥散型核燃料的生产，实现核燃料颗粒被基体材料的有效隔离，进而降低燃料元件在运行过程中的局部高温以及裂变气体引起的燃料元件肿胀程度，提高反应堆运行的安全性，并为容事故核燃料（ATF燃料）的研制提供支持。