The paper is devoted to the analysis of the importance of fluid-dynamics phenomena in the neutronic simulation of fluid-fuel multiplying nuclear systems. The motion of the delayed neutron precursors has important effects on both steady-state and transient situations. In this paper the role of the motion is studied by assuming that the coupled neutronic-fluid-dynamics model is simplified, introducing different velocity fields as input data for the delayed neutron precursor balance equations. Significant effects are evidenced for steady-state spatial distributions and integral parameters, such as reactivity and effective delayed neutron fractions. Full time-dependent evaluations are also performed to investigate the response in different system configurations to various transient initiator perturbations.