The fluid flows going through the Canada Deuterium Uranium (CANDU) moderator inlet diffuser assembly consist of a pipe flow, a curved pipe flow, and an impinging jet. For predicting the velocity profile at the diffuser outlet faces, a computational fluid dynamics (CFD) analysis has been performed to simulate the internal flow in the diffuser assembly. For the validation of a CFD code, some experimental data were chosen for each flow, and various turbulence models were examined. The shear stress transport turbulence model was proven to be the most appropriate for a prediction of the impinging jets and to give better predictions for a curved pipe flow compared to the standard k-[curly epsilon] turbulence model. As a result of the investigation, detailed velocity profiles and turbulent parameters at the real diffuser outlets were obtained, which can be applied as an inlet boundary condition for the CANDU moderator analysis.