类TRISO型颗粒散裂靶的中子学优化研究

Accelerator Driven Sub-critical Systems (ADS) is considered to be a promising approach for nuclear waste transmutation for its powerful transmutation ability, high system safety and high resource utilization. The spallation target is the neutron production component, and its neutronics performance is the key issue for the design of the entire ADS. However, due to the difficulties in heat removal and radiation damage, the choices of target materials are quite limited. As a consequence, it is quite challenging to increase the neutron yield and to harden the neutron energy spectrum. Therefore, in order to get more neutrons, the optimization of spallation target is very important. This project intends to use the TRISO coated particle as the target material to simulate the laterally leaking neutron yield, neutron energy spectrum and spatial distribution by using the Monte Carlo methods. Ensuring that the coated particle does not failing, we propose to optimize the thickness and the porosity of the kernel region, the composition and thickness of the cladding to increase the laterally leaking neutron yield and to flatten the spatial distribution, and finally to reduce the power peak factor of the sub-critical reactor. Through the research of this topic, It will provide an alternative with obvious advantage in neutronics performance for the material choices of granular target, which is very important for the design of the entire spallation target and ADS system.

加速器驱动的次临界系统（ADS）以较强的嬗变能力、较高的系统安全性和较高的资源利用率等优点，被国际上公认为是最有前景的乏燃料嬗变新型核能系统。散裂靶作为外源中子产生装置，其中子学性能关系到整个ADS系统的设计。但目前由于热移除和辐照损伤等问题的限制，靶材料的选择相当有限，已经很难大幅提高中子的产额和硬化中子能谱。因此，为了得到很强的中子，散裂靶的优化显得非常重要。本项目拟通过蒙卡程序模拟研究类TRISO包覆颗粒作为流化固体颗粒靶材料时，靶侧壁出射中子的产额、能谱和空间分布。在保证元件不失效情况下，优化核芯区的厚度和孔隙率、包覆层的组分和厚度使得散裂靶侧壁泄露中子产额提高的同时，展平出射中子的空间分布，进而降低次临界包层的功率峰因子。通过本课题的研究，将为新型流化颗粒靶的材料选择提供一个中子学优势明显的备选方案，对整个靶和ADS系统的设计都具有非常重要的意义。

目前由于靶中热移除和材料辐照损伤等问题的限制，固态靶材料的选择相当有限，已经很难大幅提高中子的产额和硬化中子能谱。然而，对于流化固体颗粒靶，由于其热移除能力强，靶本身的辐照损伤小等优点，因此对靶材料的选择范围将大大扩展。借鉴TRISO类型的核燃料包覆颗粒的经验，采用铀化物为核芯的TRISO类型的包覆颗粒为流化固体颗粒靶材料，以期来提高散裂靶的泄露中子产额、硬化出射中子能谱、约束散裂和裂变放射性气体和固体产物等。本项目从铀化物中子特性分析出发，选择了中子产额高、平均中子能量高的UC为核芯材料，研究了类TRISO颗粒和多孔UC结构的包覆颗粒为靶材料时的靶中子学特性。同时耦合IAEA-ADS基准题的均匀化堆芯对不同靶参数下出射中子价值和源效率进行了研究，给出了驱动相同的堆功率情况下，需要的最小质子束流强。同时研究了包覆颗粒上的能量沉积和气体积累对包覆颗粒破损率的影响。最终选取了两种优化的类TRISO颗粒靶材料和靶尺寸。本项目的研究将对ADS流化固体颗粒靶的材料选择和靶方案优化提供一个中子学和后处理优势明显的候补方案。