Abstract In the whole-core heterogeneous transport calculations with massive parallel, the computational cost of the Coarse Mesh Finite Difference (CMFD) method might become the most time-consuming part if directly using the traditional CMFD method. To accelerate the CMFD calculations, a new three-level CMFD method based on the loosely coupled parallel strategy is developed in the high-fidelity neutronics code NECP-X. Two aspects are considered to accelerate the CMFD calculations, one is reducing the CMFD matrix size and another is minimizing the number of outer iteration numbers. The three-level CMFD acceleration method includes: (1) the multi-group pin-level CMFD (MG-Pin-CMFD) for accelerating transport calculations, in which the cross sections and currents are obtained from the 2D-1D transport calculation; (2) the one-group pin-level CMFD (1G-Pin-CMFD) for accelerating the MG-Pin-CMFD calculations, and the 1G-Pin-CMFD is established by the group-condensed cross sections, flux and currents from the MG-Pin-CMFD; (3) the one-group assembly-level CMFD (1G-ASY-CMFD) for accelerating 1G-Pin-CMFD, and the assembly-wise parameters are from the spatial homogenization of the 1G-Pin-CMFD calculations. The three-level CMFD method can reduce the outer iterations of CMFDs, such as MG-Pin-CMFD and 1G-Pin-CMFD. In every level of CMFD, the group-wised CMFD linear system is constructed based on the loosely coupled spatial domain decomposition, and the serial Generalized Minimum RESsidual (GMRES) method is implemented to solve the linear system instead of the parallel GMRES method to minimize the communications. A set of benchmark problems are tested and the results show good performance and good accuracy in transport calculations.

中文翻译：

---

一种新的基于松耦合并行策略的三级CMFD方法

摘要 在大规模并行的全核异构传输计算中，如果直接使用传统的CMFD方法，粗网格有限差分(CMFD)方法的计算成本可能成为最耗时的部分。为了加速CMFD计算，在高保真中子学代码NECP-X中开发了一种基于松耦合并行策略的新的三级CMFD方法。考虑两个方面来加速 CMFD 计算，一是减小 CMFD 矩阵大小，二是最小化外部迭代次数。三级CMFD加速方法包括：（1）用于加速输运计算的多组引脚级CMFD（MG-Pin-CMFD），其中横截面和电流由2D-1D输运计算获得；(2) 用于加速 MG-Pin-CMFD 计算的一组引脚级 CMFD (1G-Pin-CMFD)，1G-Pin-CMFD 是由来自MG-Pin-CMFD；（3）用于加速1G-Pin-CMFD的一组组装级CMFD（1G-ASY-CMFD），组装参数来自1G-Pin-CMFD计算的空间均匀化。三级CMFD方法可以减少CMFD的外层迭代，如MG-Pin-CMFD和1G-Pin-CMFD。在CMFD的每一层，基于松散耦合空间域分解构建分组的CMFD线性系统，并采用串行广义最小RESsidual（GMRES）方法来求解线性系统，而不是并行GMRES方法，以最小化通讯。