[en] Insufficient heat transfer (second-order burnout) in a direct-flow steam generator has been studied. Vapor contents for which the insufficient heat exchange arises, wall temperature oscillations and heat exchange in the hypercrisis range have been determined. The experimental data have been analyzed for pressures from 7 to 18 MPa, water flow rates from 350 to 1000 kg/cmxs, heat fluxes from 0.2 to 0.6 MW/m². The steam generating wall temperature oscillations have been investigated on a direct-pipe model of a direct-flow steam generator of the ''pipe-in-pipe'' type heated by sodium. No unique dependence of the oscillation intensity on the specific heat flux has been established. A simple formula to calculate hypercrisis heat transfer coefficients with an accuracy of 25% has been obtained