[en] An important problem in the safety analysis of light water nuclear reactors (LWRs) is the flooding phenomenon or the countercurrent flow limit, which may be encountered during the reflood process of the reactor, following a hypothetical loss-of-coolant accident (LOCA). The present work consists of an experimental and analytical study of an air-liquid/countercurrent flooding limits in a vertical tube. The analysis is based on interfacial instability and kinematic wave theory. The experiments include effects of fluid properties and entry geometry. The theoretical results can be adjusted to fit the data as well as available correlations (e.g., Wallis) by an appropriate choice of constants