Some disturbances in operation can result in the separation of primary coolant into two phases, thus establishing a water level so that the cladding is cooled only with steam. A calculation model was developed to examine such cases. In comparing the calculation results with the measurement results, the measurements confirmed the calculations. The measurements taken at low pressures covered only the examination of the drying process. For a reactor, as an example, examinations were carried out with the calculation model to determine the extent of cooling of the core in such cases. The extent of cooling was found to be sufficient to avoid an intensive zirconium reaction if the closed primary circuit is cooled through the steam generators.