A mathematical thermo-hydraulic model of liquid-gas flows has been built. It was applied to the description of convection in vertical nonheated channels, that is as applied to the common draught section of a vessel-type boiling reactor of the NDHP type. The radial distribution of the coolant velocity in the vertical channels was found in the framework of the developed model. It is shown that the reverse flows occur in the channels under some conditions. 49 refs.; 11 figs.