Logic modification to avoid ECCS lineup in the primary heat transport system during thermal shrinkage transients of 220 MWe PHWR nuclear power plant

Operational transients involving large thermal shrinkage in the primary heat transport (PHT) volume of a pressurized heavy water reactor nuclear power plant may result in the undesirable lowering of PHT system pressure to a very low value. During some of the reactor trip incidents as reported from the operating units, the minimum PHT pressure obtained have lead to unwanted lineup of emergency core cooling system. A reduction of the dip in the PHT system pressure during such incidence can be achieved by increasing the net feed to the PHT volume, i.e., by increasing the holdup inventory in a fixed PHT boundary. Another option to maintain the PHT system pressure high during such transients is to keep the PHT temperature at a higher value. This can be achieved by removing less heat from the primary coolant. To reduce the heat transfer from the primary coolant to the secondary coolant in the steam generator (SG), the steam generator pressure controller (SGPC) set point has to be increased during such incidences. This will lead to a higher steam pressure and temperature in the SGs, thus reducing the temperature difference and heat transfer between primary and secondary coolant. Analysis has been carried out using the integrated system process dynamics analysis code. This code incorporates the mathematical models for the primary and secondary heat transport systems, these models are based on coupled solutions of unsteady state mass, momentum and energy conservation equations. The present paper deals with the details of the different mitigation schemes considered along with the results arrived at.