[en] An equipment with the size in linear scale of one tenth of the steam generators for the Westinghouse 900 MWe nuclear power plants is utilized to study the swell and shrink effects of the water level. The cyclic feature of the level swell and shrink occurring in the low power operation of the nuclear power plants is realized by a sequence of the steam dump valve control. Analyzing the results of the experiments, we found that a simple mathematical model, based on the amount of steam generated during the depressurization, provides a good approximation for predicting the swell and shrink of the measured water level