我国坚持包括核安全在内的总体国家安全观。近地面核爆炸、反应堆堆芯熔化事故等会产生高温冲击条件并向环境释放大量放射性粒子。准确描述这些粒子的形成过程和反应机理，对核事故应急、爆后核取证等具有重要意义，但目前相关认识仍不清晰，模型存在矛盾，实验研究不完善。项目使用非放射性物质替代钚等放射性物质，对冷却等离子体射流中粒子形成过程开展实验研究，关键是模拟爆后等离子体冷却过程，难点是分解研究各机理和因素。利用常压电感耦合等离子体在真空中膨胀产生迅速冷却等离子体射流，兼具温度冷却和气流冲击，采用模拟和实验方法表征射流流场。选择湿法连续和干法脉冲进样，对金属硝酸盐、金属、固体标准物质等的产物粒子进行收集分析，寻找其理化特征（粒度分布、形态、化学组成、溶解度）与实验条件、物质性质的关系，考察氧化反应、土壤基体和未熔颗粒的影响。项目结果可以为检验相关模型提供实验数据，还能为吸入研究等提供粒子发生表征方法。

At present in China, much attention is being paid to nuclear safety according to the overall national security concept. In scenarios of near-ground nuclear explosion, nuclear reactor melting accident and so on, high temperature shock conditions were produced followed by release of large amounts of radioactive materials into environment in the form of radioactive particles. Accurate description of particle formation process and reaction mechanism under high temperature cooling condition after explosion is essential in post-detonation nuclear forensic analysis, severe nuclear accident emergence management, radioactive cleanup and safety assessment. However, the description is not clear, and the model needs to be verified. The experimental studies in the literature are limited. .The project aims to conduct experimental study on particle formation processes in a rapidly cooling plasma jet by using non-radioactive materials as surrogates of plutonium, fission products and activation products. The key lies in the production of conditions similar to the plasma cooling process after explosion, while the difficulty is how to investigate the separate effect of each mechanisms and factors. A rapidly cooling plasma jet combining temperature cooling and shock effects is produced by expanding an atmospheric Inductively Coupled Plasma (ICP) through a differentially pumped interface and then into the vacuum environment in a laboratory-designed reactor. The temperature and velocity profiles inside the plasma jet are characterized by comparing the results from Computational Fluid Dynamics simulation and results from in-situ, spatially resolved optical emission diagnosis and pressure probe measurement. Metal nitrate solutions are nebulized and introduced into the ICP torch. The product particles are collected by carbon film or silicon wafer and transferred for further analysis by electron microscope and Laser-Ablation ICP-MS (LA-ICP-MS) to obtain their physiochemical properties including morphology, size distribution, oxide state, and chemical composition. After dispersing these collected particles in solutions, the solubility of radionuclides in the particle is investigated by monitoring the solution concentration using ICP-MS. The remaining suspension is nebulized and collected by a cascade impactor. The size-dependent elemental and isotopic compositions in the particles are determined by ICP-MS which indicates radionuclides fractionation. Furthermore, the effects of chemical reaction, soil matrix and un-melting droplets are studied by introducing matrix gas, silicon dioxide particles and large rare-earth-element powders into the sample solution or plasma, respectively..Direct introduction of laser ablated samples could avoid interferences of water or oxygen from solution, and meanwhile mitigate particle agglomeration, which facilitates to distinguish the effects of nucleation/condensation and chemical reaction. The ablated metal particles are introduced into the ICP torch, and the collected product particles are characterized by SEM/TEM and ICP-MS. Then, the U3O8-matrix standard materials are ablated to investigated the radionuclides fractionation in individual particle by LA-MC-ICP-MS. Oxygen flow is added into the plasma to study the effect of oxidation reaction. By varying the pressure in the reactor and finally the gas velocity, its effect on the physicochemical properties of product particles is also investigated..By using the innovative experimental methods and comprehensive characterization of product particles, the project will give us new insight into the particle formation process in the rapidly cooling plasma jet. It not only provide experimental data for verification of the relevant model, but also has guiding significance for plasma-based analytical chemistry and nanoparticle synthesis. Moreover, aerosol used in aerosol dynamic study and inhalation safety assessment could be generated by the methods developed in this project.