核临界安全事故效应评估中的高效蒙卡输运方法研究

Criticality safety accidents evaluation is very important for the safety of nuclear facilities. CFD/MC coupling provides a quantitative and precise solution for this purpose. However, the procedure of a criticality safety accident is extremely complex which generally results in fine, dense and movable CFD meshes. Traditional MC methods are not able to adapt to this kind of meshes, thus CFD/MC coupling will work with low efficiency and precision. In this project the characteristics of the mechanics meshes will be pointed out, and the MC method based on these meshes will receive a systematic research. In order to represent the meshes of different types, the method of modeling mechanics meshes in MC codes will be given in a uniform way. The neutron free path sampling method based on DT (delta-tracking) algorithm will be studied, including the new ST (surface-tracking)/DT combined method, the improved neutron position localization method and the technique of sampling free path in movable material. The KDE (Kernel Density Estimation) method will be introduced to investigate the DT-based flux tally algorithm in fine and dense meshes. Finally a systematic MC neutron transport method for the mechanics meshes in dynamic coupling of neutronics and thermal-hydraulics will be established, and also the corresponding simulation code will be developed which can be applied to analyze the criticality safety accidents in order to study the physical rules of the accidents in depth.

核临界安全事故效应评估对核装置的安全性具有重要意义，CFD/MC耦合模拟为此提供了一种定量、精确的解决方案。然而事故过程中系统内部状态极端复杂，CFD力学网格往往会有细、密、动的特点，传统MC方法无法与此类网格很好地匹配，将影响耦合模拟的效率和精度。本项目拟针对力学网格的三大特点，对MC输运算法作专门的系统性研究。针对不同网格类型，建立统一的力学网格蒙卡建模方法；开展基于DT（delta-tracking）方法的自由程抽样算法研究，包括新的ST（surface-tracking）/DT耦合方法、改进的中子点定位算法及移动介质中的自由程抽样算法；结合KDE（Kernel Density Estimation）技术，研究细密网格中基于DT的高效通量统计方法；最终建立一套专门针对动态核热耦合力学网格的蒙卡中子输运方法，研制程序，并应用该程序研究核临界安全事故的演化规律，深化对事故过程的物理认识。

核临界安全事故是指意外的中子自持链式裂变反应导致的放射性事故，事故效应评价对核科研生产的可持续发展具有重要意义。核临界安全事故涉及力学、中子物理学过程的复杂相互作用，对瞬态演化规律的定量分析需借助CFD/MC程序的动态全耦合模拟。本项目针对事故过程等极端条件下CFD网格的特点，以提高核热耦合模拟的精度和效率为目标，基于MES程序开展了相应的中子输运方法研究。设计了统一网格几何接口，研究了针对力学网格的中子几何跟踪算法及通量、反应率统计方法，研制了MES/CFD核热耦合程序。为进一步提升MES程序的建模精度和模拟效率，研制了高速运动介质内的自由程抽样功能、中子-光子联合输运功能、燃耗计算功能、并行点燃耗方程求解功能及改进仿真流程的随机中子动力学模拟功能。MES与CFD程序的动态全耦合模拟能力可为核临界安全事故演化规律研究提供有效的技术支撑，目前已在项目成员的科研任务中发挥重要作用。