Development of a Gamma Radiotracer Method for the Analysis of Clogging in Steam Generators With COLENTEC Loop

The tube support plate (TSP) clogging is a complex phenomenon that can occur in the steam generator (SG) of pressurized water reactors (PWRs) and induce several consequences, such as thermohydraulic flow changes, vibrational or oscillatory risks, mechanical resistance of SG tubes, and internal structures or impact on water inventory. The deposit at the origin of this issue is mainly composed of iron oxides (Fe
₃
O
₄
). In addition to observations and feedback on real reactors since 2014, an experimental installation dedicated to preventive studies, COLmatage ENTretoise Etudes Cinétiques (COLENTEC) loop, has already provided a large number of data on the physicochemical properties of the deposit formed on a TSP in specific thermohydraulic and chemical conditions. This loop has been recently improved to allow the injection of a gamma radiotracer
⁵⁹
Fe inside the circuit and follow the presence of
⁵⁹
Fe, especially on the TSP. For that purpose, three dedicated gamma-measuring stations are used to follow online the presence of
⁵⁹
Fe and the kinetic of deposition during the test in order to determine the impact of parameters (such as pH, temperature, and fluids velocity) without opening or dismounting the test section. This article presents the qualitative results acquired online from these three gamma detectors during the first active test. It becomes possible to detect and monitor the active iron remaining in solution in the pipes, the activity along the straight tubes, and the global accumulation of active iron. In complement of the first kinetics observations, it then details the iterative modeling method implemented, allowing the analysis and discrimination of the observed effects. The iterative method starts with the modelization of the simplest low-background gamma-measuring station to quantify the initial injection of
⁵⁹
Fe in the experimental loop. Then, the modelization of the second station aiming at a section of the loop provides a first estimation of the residual
⁵⁹
Fe deposit along the straight tubes. Finally, a detailed model of the main TSP measuring station allows determining the clogging amount. The optimization of the model and sensitivity studies of the main parameters are reported. The iterative method makes it possible to quantify and discriminate the main clogging effect from secondary phenomena taking place in the specifically selected thermohydraulic and chemical conditions. Therefore, it improves the understanding of clogging phenomena and will lead to better management of SG maintenance in PWR with next active tests to come.