With the great improvement of computer performance, analyzing the complex flow and heat transfer phenomenon by coupling CFD and neutronics has attracted lots of attentions nowadays. The study aims to investigate the neutron diffusion coupling calculation based on the UDF and UDS functions of FLUENT and its application analysis on fast reactor. The neutron diffusion equation is defined based on the UDF (User Defined function) and UDS (User Defined Scalar) functions of the FLUENT. The neutron diffusion equation is solved iteratively by using the solver of the FLUENT based on the finite volume method. At the same time, the mass, momentum and energy equations are solved iteratively. At each iteration, the power distribution (neutron flux distribution) obtained by the iteration of the neutron diffusion equation is transferred to the thermal-hydraulics calculation and is used as the heat source term. At the same time, the temperature distribution obtained from the thermal-hydraulics calculation is transferred to the neutron diffusion calculation and the macroscopic cross sections of the materials are corrected to realize the coupling calculation of the neutron diffusion and the thermal-hydraulics under the same solver of the FLUENT. Through the modeling and calculation of the 5×5 PWR assembly model and the hot assembly of a modular lead-cooled fast reactor (M2LFR-1000). It is proved that this method is feasible to realize the neutron diffusion and thermal-hydraulics coupling and the data transfer is correct. And the thermal hydraulics characteristics (the maximum fuel temperature and the maximum cladding outer surface temperature) of the M2LFR-1000 are all within the corresponding thermal‐hydraulics design limits.
