[en] 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 operation-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. Analytic solutions are obtained for the transient variation of both the collapsed liquid and the two-phase mixture levels