This research aims to experimentally investigate the critical flow velocity of light-water coolant in a reactor parallel-plate fuelassembly. The critical flow velocity is the speed at which rectangular fuel-plates will buckle and collapse onto each other as a result of flow-induced vibration and consequent asymmetric pressure distribution. Although fuel plates do not rupture during plate collapse, the excessive permanent lateral deflection (buckling) of a plate can cause flow blockage in the reactor core, which may lead to over-heating. This is an important consideration in reactor core designs with parallel plate fuel assemblies. The Replacement Research Reactor (RRR) currently under construction at the Australian Science and Technology Organisation (ANSTO) is of such a design. A simple physical model of a parallel-plate fuel-assembly composed of two parallel plates was constructed and tested in a closed-loop water tunnel (figure 1). Plate vibration was measured at low flow speeds and the critical flow velocity was recorded. Test results show plate collapse occurring, in 25°C light water, at an average flow velocity range of 11.9 - 12.0m/s. For the first time, cavitation was observed as a result of leadingedge deformation during plate collapse. The experimental results attained support Miller’s [4] critical-velocity calculation for plate collapse. However, it must be stressed that the flow characteristics of the RRR are significantly different to the results reported here due to the presence of a lateral-support comb at the RRR fuel assembly inlet. This comb greatly reduces any vibrations and increases the critical velocity for the fuel assembly.