Strengthening Model of Particle Deposits for Tube Support Plate Flow Blockage in Steam Generators

International audienceCorrosion product deposits in the secondary side of nuclear power plant Steam Generators may result in Tube Support Plate flow blockage, and tube fouling. Tube fouling is a deposit on the tube surfaces which is penalizing for the heat exchanges between the primary and the secondary circuits. Tube Support Plate flow blockage is a deposit at the inlet of Tube Support Plate flow holes which can induce high velocity zones and transverse velocities in the secondary flow, then flow induced vibrations and tube cracks in some cases. Indeed three significant primary to secondary leaks occurred at one nuclear plant in France between 2004 and 2006. The thermal-hydraulic and vibration studies confirmed a cracking mechanism caused by flow induced vibrations resulting in a circumferential crack. Those studies pointed out the role of an important flow blockage distribution at the upper Tube Support Plate as an aggravating factor.In order to simulate those two phenomena in the whole Steam Generator, a solid deposit growth model on the sec- ondary side of Steam Generators has been developed by the EDF R&D Division. This model is implemented in the frame of THYC, which is the EDF’s reference code for the modelling of two-phase thermal-hydraulic phenomena at the subchannel scale. A subchannel scale is a mesoscale which has the advantage to calculate thermal-hydraulic flows and deposition in whole nuclear components with reasonable CPU times. The deposit model aims to predict the localization and the growth rate of deposits in order to simulate tube fouling as well as Tube Support Plate flow blockage at this scale. This work specifically describes the efforts performed by the EDF R&D Division to develop and implement models for Tube Support Plate flow blockage.A new deposit process, based on Tube Support Plate flow blockage studies, has been developed and implemented in the model, and is presented in this work. It can be defined by two main steps : particle deposition, and strengthening process called “flashing” due to soluble species precipitation in the pores of the particle deposit. This process is calibrated on blockage rates observed in Steam Generators by using an inverse method also developed in this work. The relevance of this process is tested by comparing the simulation results to the actual levels of flow blockage observed in some nuclear plants. Two dominant trends are showed in this work : the flow blockage is more important on the hot leg than on the cold leg and at the top than at the bottom of Steam Generators. Moreover the flow blockage distribution at the upper Tube Support Plate has the special feature to be more important at the periphery than at the center. These results are in agreement with observed data in some nuclear plants.A sensitivity analysis of soluble species solubility, and more specifically magnetite solubility, has been performed. The “flashing” phenomenon allows one to underline the magnetite solubility dependence of flow blockage phenomenon. A reduction of magnetite solubility in Steam Generators seems to be an interesting remedy for reducing the Tube Support Plate flow blockage phenomenon, and more specifically for Tube Support Plates at the top of Steam Generators. Ac- cording to the deposit process developed in this work, this reduction would be more effective in cold leg than in hot leg. In practice, such a magnetite solubility reduction could be obtained by increasing the pH of the secondary circuit. This pH elevation is one of the remedy considered at short term on EDF fleet for its potential benefit on both tube fouling and Tube Support Plate flow blockage. Indeed the operational pH of secondary circuits were planned to raise at certain nuclear plants. This remedy is in agreement with the results obtained in this work

Fractal description of fouling deposits in boiling heat transfer modelling

International audienceA novel methodology is developed for predicting the thermal impact of fouling in Steam Generators (SG). The originality of this methodology is to resort to fractal and statistical theories to depict the porous structure of the deposits. The proposed Statistical Fractal methodology (SF) accounts for the heat transfer driven by the liquid-vapor phase change inside the deposits. It simulates the complex intricate networks of sinuous open pores of different scales, with liquid inflows (capillaries) and vapor outflows (steam-chimneys). The multi-layered representation of fouling deposits allows to mimic aging mechanisms such as densification which occur during SG operation.The SF predictions are consistent with experimental data. The deposit thickness and the profile of porosity are found to be the most influential fouling properties on the heat exchange. The methodology is capable to simulate the experimentally observed heat transfer enhancement for thin and porous deposit as well as the heat exchange decline for thick and dense deposit

Deposit Model for Tube Support Plate Blockage in Steam Generators

International audienceCorrosion product deposits in the secondary side of nuclear plant steam generators may result in tube support plate blockage, and tube fouling. In order to improve our understanding of those two phenomena, a solid deposit growth model has been developed by the R&D Division of EDF. The validity of this model is tested by comparing the results of simulations to the actual levels of tube support plate blockage observed in some nuclear plants. The limits of the model and the strategy to improve it are discussed

CFD Analysis of Singular Pressure Losses due to Tube Support Plate Flow Blockage in Steam Generators

International audienceCorrosion product deposits in the secondary side of nuclear plant steam generators may result in tube support plate flow blockage, and tube fouling. As deposition phenomena are very sensitive to parameters such as pressure and velocity, notable enhancement of the deposit models may be obtained from a better knowledge of local thermal-hydraulic profiles. Tube support plate flow blockage can be induced by the sudden contraction due to the broach holes. Recirculation zones and sudden pressure drops are expected. These physical phenomena can encourage specific particle deposition mechanisms or the precipitation of soluble species. The “flashing” deposition mecha- nism for instance shows high sensitivity to pressure drops. CFD calculations may capture and quantify the local and sudden change of thermal-hydraulic parameters at the inlet of broach holes. In this paper, CFD calculations are performed by EDF’s open-source Code Saturne CFD code. The initial concern of this study is to better understand local thermal-hydraulics. It does not cover the modelling of particle deposition or precipitation of soluble species. Plant observations such as TV inspections of steam generators have allowed to determine best estimate profiles for tube support plate flow blockage. Several blockage rates and fluid velocities are investigated. The results are compared to experimental pressure loss data. The global trends are in agreement with experimental results. The limits of this initial CFD calculation series and the strategy to model the effect of local recirculation on deposition are discussed