Development and steady-state validation of the fine sub-channel code CUNLUN

[Background] The safety assessment of reactor cores and their components relies on thermal-hydraulic analysis. System codes are suitable for system-level analysis, sub-channel codes provide more detailed thermal-hydraulic parameter distributions but struggle to capture complex three-dimensional flows, and Computational Fluid Dynamics (CFD) codes offer high accuracy but are limited by computational cost and modeling complexity. Existing thermal-hydraulic analysis methods, due to insufficient accuracy and applicability, fail to meet the requirements of modern reactor design and safety analysis. [Purpose] This study aims to develop a sub-channel analysis method based on fine-grid control volumes, which incorporates a more complete mixing grid model and a fully three-dimensional numerical solution approach for steady-state thermal-hydraulic analysis of pressurized water reactor (PWR) cores. [Methods] Firstly, the traditional sub-channel control volumes were further subdivided into more than