Equipment that is one-tenth the size of the steam generators for the Westinghouse 900-MW(electric) nuclear power plants is used to study the swell and shrinkage of the water level. The cyclic aspect of level swell and shrinkage occurring during low-power operation of the nuclear power plants is realized by sequential steam dump valve control. Experimental results show that a simple mathematical model based on the amount of steam generated during depressurization provides a good approximation for predicting level swell and shrinkage. Steam generation also causes water movement between the downcomer area and the inner part of the vessel, the effect of which during the initial steam dump period is estimated and applied to adjust this model.