Estimation of SG TSP Blockage: Innovative Monitoring Through Dynamic Behavior Analysis

EDF operates a fleet of 58 Pressurized Water Reactors (PWR). The “health” of the Steam Generators (SGs) is an essential element contributing to the overall thermal efficiency of a PWR, and finally to the availability of the unit. Among the health issues that may affect SGs, secondary-side corrosion products transport in PWRs may lead to many problems: various contaminants, both particulates and dissolved species, will unavoidably accumulate and concentrate in the Steam Generator. One consequence is the fouling of the heat transfer and support structure interfaces within the SG on the secondary side, especially the U-tubes (fouling deposits on the outer walls of the U-tubes), and the tube support plates (TSPs) that support the U-tubes. The accumulation of the corrosion products may lead to 3 main safety risks that must be monitored: fluid-elastic instability of tubes in flow-accelerated areas, a reduction in SG water mass inventory and an increase in the risk of water level oscillation. It has also significant performance issues because of the decision to power derate of some EDF PWRs. Thus, a global strategy to monitor the fouling and TSP blockage issues and to schedule preventive and curative actions has been designed and is under deployment by EDF nuclear operator. This dedicated periodic test relies on the recording of the following measurements in stabilized configuration: steam pressure, feedwater flowrate and temperature, primary circuit temperatures, SG blowdown flowrate and SG water level (wide and narrow range). A more precise monitoring of potential TSP blockage situations would be an interesting help to operation and maintenance strategies: deposit build-up in TSP foils could be minimized, preventive chemical cleaning operations could be scheduled and a more efficient fleet wide SG Management Program (SGMP) could be designed in accordance with secondary side deposit issues. Consequently, EDF R&D is experimenting a new method based on modeling dynamics behavior of SGs to assess a spatially distributed estimator of the TSP blockage ratio. This method, based on a 1D physical model of the SG that simulates the complex dynamics of the two-phase flow phenomena inside the SG, consists in computing the wide range water level responses according to various configurations during a particular transient which is particularly sensitive to this phenomenon. The TSP blockage ratio estimator is then obtained by comparing the computed response curves to those measured on-site. This new method has the potential advantages of being fully non-invasive, of providing a quarterly update of the TSPs blockage estimator, and of requiring no additional measurements by processing available plant data. It is also capable of estimating the efficiency of a chemical cleaning after restarting the plant and checking the evolution and kinetics of eventual TSP re-blockage.