Currently used correlations to predict the onset of nucleate boiling heat flux in research reactor channels determine in terms of local channel pressure and wall superheat. Recent experiments show that these correlations may over- or underestimate by as much as a factor of 5 in thin rectangular channels for low-velocity upward flows. Such flow conditions are encountered in the natural convection cooling of research reactors with plate-type fuels. A set of experiments are performed to quantify the effect of channel flow velocity and gap size (in the ranges of 2 to 14 cm/s and 2 to 4 mm, respectively) on for upward flow in rectangular channels. An adjustable gap between two internally heated aluminum blocks forms the flow channel. Other controlled variables are channel mass flow rate, heat generation rate in the aluminum blocks, and coolant temperature at the channel inlet. The shape of the power distribution along the channel walls (truncated cosine), channel height (642 mm), width (73 mm), and surface roughness simulate operating conditions in research reactors using plate-type fuels. The experimental results show that (a) both channel gap size and flow velocity are important parameters in determining under low-velocity, upward flow conditions and (b) currently used correlations yield the upper and lower bounds on under these conditions. A new correlation is proposed that predicts the experimental results within 13% for flows with Re <700 (Re based on channel gap) and that is valid in the 1.40- to 1.46-atm pressure range.