Development of 3D transient neutronics and thermal-hydraulics coupling procedure and its application to a fuel pin analysis

•A three-dimensional multi-group multi-region neutron diffusion solver has been developed and verified.•A three-dimensional fine-mesh neutronics and thermal-hydraulics coupling procedure has been developed for both steady-state and transient calculations and verified.•The solution of the coupled problem is implemented in the solver with the same time step size and the same set of meshes.•The operator-splitting semi-implicit (OSSI) method and the fixed-point implicit (FPI) method have been applied to the coupling procedure.•The FPI coupling method outperforms the OSSI coupling method due to its accuracy and stability.