基于连续变化介质中子输运与统计的蒙卡热工耦合方法研究

The tightly coupled solution of different physics including neutronics, thermal-hydraulic and fuel performance and so on, is the important trend of precise simulation of nuclear reactors. When Monte Carlo particle transport code was coupled with other codes such as thermal-hydraulic code, the physical field property varies continuously with space position, which requires the mesh partition of different codes according to their features. However, the fine mesh partition on Monte Carlo code leads to considerable increase of memory consumption, simulation time and data exchange, as well as difficulties in mesh mapping. Aiming to solve the difficulties of Monte Carlo neutron transport and mesh partition in the continuously varying physical field, the Functional Expansion Tally (FET) method and Continuously Varying Material Tracking (CVMT) method were developed for three-dimensional reactor core simulation. Armed with these developed methods, a novel neutronics and thermal-hydraulic coupling method was proposed based on feedback with continuous distribution. The proposed method can avoid fine mesh partition on the fundamental geometry of Monte Carlo code, with only few parameters exchanged, which improves the efficiency, flexibility and versatility of coupling. The Monte Carlo/thermal-hydraulic coupling system were also developed independently in this research, which was applied to the realistic coupling analysis of PWR assembly and fusion reactor blanket. The research achievement can also be extended to “neutronics/thermal-hydraulic/fuel performance” coupling, and provide the advanced method and tools for multi-physics coupling analysis. We believe this project has both significant academic meanings and great application values.

中子物理-热工水力-燃料性能等物理过程的紧密耦合求解，是核反应堆精确模拟的重要发展趋势。蒙卡输运程序与热工水力等程序耦合时，由于物理场参数随空间位置连续变化，因此不同程序需要根据其特点进行网格离散。然而，对蒙卡几何进行细网划分所需的内存、模拟时间和数据传递量较大，程序间网格对应也比较困难。本课题针对连续变化物理场蒙卡网格离散及中子输运的困难，研究适用于三维反应堆的函数展开计数（FET）方法与连续变化介质输运（CVMT）方法，在此基础上开发一种新的基于连续分布反馈的蒙卡-热工耦合方法。该方法可避免对蒙卡基础几何的细网划分，只需传递少量耦合参数，提高了耦合的效率、灵活性和通用性。课题将自主开发蒙卡物理-热工耦合模拟平台，并实际应用于压水堆组件和聚变堆包层的耦合分析。研究成果可拓展到“物理-热工-燃料”耦合模拟，为核反应堆的多物理耦合分析提供先进计算方法和工具，具有重要的学术意义和实际应用价值。

中子物理-热工水力-燃料性能等物理过程的紧密耦合求解，是核反应堆精确模拟的重要发展趋势。蒙卡输运程序与热工水力等程序耦合时，由于物理场参数随空间位置连续变化，因此不同程序需要根据其特点进行网格离散。然而，对蒙卡几何进行细网划分所需的内存、模拟时间和数据传递量较大，程序间网格对应也比较困难。本课题针对连续变化物理场蒙卡网格离散及中子输运的困难，研究了函数展开计数（FET）方法与连续变化介质输运（CVMT）方法，建立了压水堆燃料棒轴向和径向功率的FET方法，提出了FET的展开阶数选择策略。结果表明，FET与传统网格计数方法的结果吻合良好，同时FET的数据量约为网格计数的20%，时间约为网格计数的65%。提出的选阶策略有助于确定FET方法的最佳阶数，不仅保持了精确的通量分布趋势，而且获得了更为平滑的分布。通过对光程法及Delta-tracking法的研究，实现了基于蒙卡方法的CVMT方法。为了解决传统光程法需要已知截面变化函数的缺点，采用了离散数值积分的方法对光程法进行改进，研究论证了线性近似法和二次函数近似法的可行性。通过不同光程法及Delta-tracking法的计算结果对比，线性近似法更适用于三维实际反应堆计算，是模拟连续变化介质粒子输运的有效方法。在此基础上，开展了蒙特卡罗中子输运程序RMC与有限元程序COMSOL的网格映射与耦合研究，实现了基于连续分布的物理-热工耦合方法，并应用于压水堆UO2燃料棒、FCM弥散燃料等模型的“中子物理-热工-燃料性能”耦合模拟，与栅元映射法相比，采用连续分布的物理-热工耦合方法可以得到连续性更好的结果，克服了栅元映射法在栅元划分不够精细时的温度跳跃、结果出现误差等缺点。本研究为核反应堆的多物理耦合分析提供了高效、灵活、准确的计算方法和工具，具有重要的学术意义和实际应用价值。