Design of the next generation Fast Breeder Reactors has been commenced with the main target of enhanced safety and improved economy. Nuclear heat generated in the fuel subassembly of fast reactors is removed by circulating sodium through the core using centrifugal pumps. Primary sodium pumps (PSP) used are large capacity pump and the design of these pumps are different from traditional pumps. Though many works are reported for performance prediction of centrifugal pumps, most of these works are carried out in a steady state fashion and only a few works are reported where the large sodium pump is modeled. In this work, transient CFD simulation of hydraulics of sodium inside PSP is carried out using sliding mesh approach. Computational domain consisting of impeller and diffuser fluid volumes along with inlet and outlet fluid volumes, are modelled accurately. Instead of traditional frozen rotor formulation or mixing plane approach, relative mesh motion between impeller and diffuser is simulated using sliding mesh method, to capture the rotor-stator interactions. Performance characteristics is predicted at the rated speed. Also, the transient evolution of head developed is also predicted for different points on the characteristics curve. The head oscillation is found to be ±0.12% for the design flow rate and this oscillation increases when one moves away from best efficiency point on the characteristics curve.