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Monitoring and Control of IRIS Helical Coil Steam Generators

International Reactor Innovation and Secure (IRIS) is one of the next generation nuclear plant designs with significant advances in safety, operation performance, and proliferation resistance. The Helical Coil Steam Generator (HCSG) constitutes a critical contributor to the integral primary system design of the IRIS reactor. However, the integral design features and the thermal-hydraulic characteristics of boiling and superheating of the secondary water inside helical coiled tubes bring about the instrumentation difficulties of measuring some of the key state variables in the HCSG and the possibility of instabilities at certain operating conditions. Therefore, it is necessary to develop an efficient control strategy and a reliable monitoring and fault diagnosis system for the IRIS reactor design [1]. A high fidelity model is developed for the HCSG control design and monitoring. For each of the three anticipated heat transfer regimes on the secondary side, two nodes are used to consider the axial temperature changes for the primary fluid, the tube metal, and the secondary side, respectively. Effective heat transfer coefficients are derived to describe the heat transfer between the tube metal and the primary fluid and the secondary fluid. A moving boundary model is used to characterize the change of energy distribution in the tube metal when the heat transfer regime changes on the secondary side. The developed model is able to give accurate steady state results and generate correct transient responses for hot leg temperature disturbances and feed water flow disturbances. A model-based controller is designed for the HCSG system over the entire operation regimes. In the HCSG control strategy, the power match follows the program of power demand-feed water flow-reactor core power, and the unpredictable disturbance is rejected through the HCSG pressure control. In order to address the system nonlinearity, multiple linear models for typical operating conditions are derived for controller design. The gain switch can be smoothly obtained by choosing appropriate weighting functions to combine the controller output of the locally linear models. The performance of the designed controller has been demonstrated on the developed HCSG simulation model for automatic power changes. Robust dynamic parity space approach has been developed for the fault detection and isolation of instrument faults considering their importance in monitoring the stability of the HCSG system [2]. Because a dynamic model is used, the fault detection module will not trigger false alarms during normal operation transients. The uniqueness of the developed model-based diagnostic method is that the fault isolation can be achieved by following a predetermined logic without the need of fault information. The fault diagnosis of steam pressure sensor fault and feed water flow meter sensor fault, even with process disturbance and measurement noise, has been demonstrated using the developed HCSG simulation model. Reference [1] B. R. Upadhyaya and K. Zhao, “Robust Techniques for Monitoring and Fault Diagnosis of IRIS Helical Coil Steam Generators,” GENES4/ANP2003, Kyoto, Japan, September 2003.