A computerized system that assists pressurized water reactor (PWR) nuclear plant operators in controlling the saturation margin is described. This system continuously calculates and provides the temperature or pressure margin from saturation, and indicates the temperature and pressure values used for its calculation. The reactor core heatup and cooldown rates are also continuously provided by this computer-based system. The saturation curve is represented by a polynomial approximation as a function of temperature or pressure. Twenty-one analog signals of a PWR power plant are acquired by the system for calculating the margin and temperature variation rates. The man/machine interface is done through the visualization panel, which displays the above-mentioned values. Compared to the temperature and pressure values displayed in the visualization panel, the error of margin is always <0.5%.