Abstract Current generation of codes applied to Light Water Reactors (LWR) are based on 3D neutronic nodal methods coupled with one or two phase flow thermal–hydraulic system or subchannel codes. Multi-scale and multi-physics problems reveal significant challenges while dealing with coupled neutronic/thermal–hydraulic solutions. In addition, spatial meshing and temporal schemes are crucial for the proper description of the non-symmetrical core behavior in case of plant transient e.g. Main Steam Line Break (MSLB). This paper describes the coupling approach between the 3D neutron diffusion code COBAYA3 and the sub-channel code SUBCHANFLOW within the NURESIM platform making use of the unique functionalities for code coupling e.g. the automatic mesh superposition of the involved domains. The details of the implemented coupling schemes for steady-state and transient simulations are introduced. The main results of the validation of COBAYA3/SUBCHANFLOW based on the TMI-1 MSLB and the Kalinin-3 Cooling Transient benchmark are also presented. Finally, a discussion of the prediction capability of COBAYA3/SUBCHANFLOW based on the analysis of different benchmarks is provided.