An embedded coupling design and development of NECP-Bamboo2.0 and START for PWR whole-core pin-by-pin analysis

In reactor design and safety analysis, the interaction between neutronics and thermal-hydraulics is of significant importance. As an alternative and improved two-step method, the pin-by-pin scheme requires pin-wise thermal-hydraulics feedback to improve the resolution of 3D whole-core analysis. In this work, we implement an embedded coupling of the subchannel code START with the whole-core pin-by-pin calculation system NECP-Bamboo2.0 following the master–slave approach. The 2D lattice code Bamboo-Lattice2.0 in NECP-Bamboo2.0 provides the pin-wise homogenized few-group constants for the coupling system, and the 3D whole-core pin-by-pin code Bamboo-Core2.0 is coupled as the master code with a modified MPI-based START. Bamboo-Core2.0 retains its neutronics module and multi-physics coupling strategy. In contrast, START is only embedded as a slave module into the master. Both of them share the same MPI-based parallelism strategy with a block-based domain decomposition approach. Therefore, the coupling code developed in this paper has a high-level global coupling efficiency on a multi-process platform. The data exchange between neutronics and thermal-hydraulics adopts a direct block-to-block model, thus requiring no additional data interface. The coupling code is verified using the VERA#6 3D single-assembly benchmark problem and the mini-core problem based on the VERA#4 benchmark. The numerical results demonstrate that the coupling code possesses good parallel efficiency and computational precision. Compared with the single-channel model, the subchannel model can simulate the mass/momentum/energy exchange between channels accurately, and thus a more continuous coolant temperature distribution can be obtained. Meanwhile, the subchannel model is also able to reduce the maximum pin-wise coolant temperature, fuel temperature, and power peak, while the eigenvalue can be increased by about 10 pcm for the steady-state problems used for verification in this paper.