A PWR hybrid computer model for assessing the safety implications of control systems

Abstract

As part of the ORNL study of safety-related aspects of control systems, a hybrid computer model was developed to trace the dynamic impact of single and multiple component failures on the control system and remainder of the plant. Since the thrust of this program is to investigate control system behavior, the controls are modeled in detail to accurately reproduce characteristic response under normal and off-normal transients. The balance of the model, including neutronics, thermohydraulics and component submodels, is developed in sufficient detail to provide a suitable support for the control system. The model is being used primarily to address mild to moderate transients that can occur at least partially under action of the non-safety control system. Attention initially focused on overfill events that assumed single or multiple failures of feed valves or the generator low and high level setpoints and trips that regulate the valves. In general, these calculations showed that for single-generator overfeed, water inventory in the affected generator increased to a sufficiently high level to saturate the generator fluid, quench superheat, and inject water into the steam line. Overcooling of the primary side was modest. Other events studied with the model include: insufficient main feedwater cooling induced by a steam generator high level setpoint failing low, potentially drying out the generator; secondary side depressurization induced by turbine bypass valves failing open in loop A or in combination with loop B, at low and high power levels; and steam generator tube ruptures in combination with overcooling incidents.