Abstract: Malfunction of steam generator water level control is the main cause of unexpected shutdowns in nuclear power plants. Particularly at low powers, it is a difficult task due to shrink and swell phenomena. In order to avoid the affect caused by spurious water level, a special discriminator was designed. Parameters of the controller with a simplified steam generator model were tuned offline by the genetic algorithm. Control effect with a discriminator was contrasted to that without a discriminator. The results show that the discriminator shortens control period and reduced overshoot.