Transient analysis and dynamic modeling of the steam generator water level for nuclear power plants

Natural circulating steam generators are extensively applied in pressurized water reactor units, where the water level control is a tough challenge. During transients, the water level may change drastically, and its reverse dynamic characteristics may even cause safety concerns. This article studies the dynamic effects of several crucial impact factors on the level, including their gains, time constants, and coupling relationships. Considering the impact and the inter connections of these factors, a dynamic distributed parameter mechanism model for realtime water level estimation is constructed. It consists of three sub-models: the close-loop control system model, the mass and heat transfer model, and the momentum transfer model. Solving these models, one may yield the flow rates of the inputs and outputs of the downcomer channel. Then, the level is determined employing the dynamic conservation for the feedwater chamber. Pseudo-online simulation studies are carried out with the measurements from an operating 1000 MW unit during the full load rejection test.