A new three-level CMFD method based on the loosely coupled parallel strategy

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.