The flow conditions and the tracks of sludge particles around the sludge collector in a typical U-tube recirculating steam generator are studied by a Computational Fluid Dynamics method. To evaluate the main factors affecting the deposition process of sludge particles, a Lagrangian Particle Tracking scheme is employed to account for the momentum transfer between the continuous phase of fluid and the discrete phase of particles. It shows that the performance of a sludge collector depends heavily on the transportation effects of flow field outside the sludge collector. The results for different particle diameters indicate that the particles with a larger diameter are more likely to be captured by the sludge collector. By increasing the inlet and outlet area on the top plate, the sludge collector performance can be obviously elevated with a limit of avoiding particle re-entrainment phenomenon.