<p>The three-dimensional flow structure in narrow gaps between the rods of nuclear fuel assemblies (FAs) has a decisive impact on the heat and mass transfer coefficients, which, in turn, govern the FA performance. In this study, instantaneous three-dimensional velocity fields in a peripheral subchannel model of a fuel assembly were measured for the first time using time-resolved tomographic particle image velocimetry (TR-Tomo PIV). The experiments were performed at Reynolds number Re = 2100. The TR-Tomo PIV measurements enabled characterization of the three-dimensional flow structure and dynamics in a narrow gap of the peripheral subchannel. Based on the obtained data, the vorticity vector components were evaluated throughout the entire flow volume. It is shown that the key mechanisms, responsible for generating extreme values of streamwise vorticity, are the transverse flow oscillations. These oscillations are quasi-periodic (Sh = 0.091) and alternate with stabilization phases, characterized by a straight-through flow.</p>