A theory is given for the thermal-hydraulic phenomena during uncovery of a flow channel. This is relevant to a reactor core under typical small break or operational transient conditions. A distinct equivalent collapsed liquid level and a two-phase-mixture level are defined in the model. The former represents the liquid inventory in the channel, while the latter characterizes the heat transfer regimes. The definition of these levels are coupled through the mass and energy conservation equations, and the constitutive relations for void fraction and net vapor generation location. Analytical solutions are obtained for the transient variation of both the collapsed liquid and the two-phase mixture levels. The analyses have been compared with existing single-tube data with uniform heat flux, and rod bundle experiments with an axial power profile and inlet feedwater flow. The results demonstrate the potential of the present model for application to reactor conditions.