Tube Support Plate Research Articles

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Sensitivity Study & Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetite permeability.The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non- axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN. Here, the specificity of the TSPs and their clogging justifies qualifying the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main features of the simulation module in CIVA, its experimental validation, and the main trends in the distortion of the simulated signal as a function of variation in the clogging description.

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Numerical Tools and Experimental Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetic permeability. The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non-axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN, related for example to the inspection of the U-bend tubes or of a friction wear under the anti-vibration bars. Here, the specificity of the TSPs and their clogging justifies further investigations between CEA and IRSN to qualify the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main characteristics of the simulation module in CIVA.

Improving the Predictivity of a Steam Generator Clogging Numerical Model by Global Sensitivity Analysis and Bayesian Calibration Techniques

In pressurized nuclear reactors, steam generators are massive tubular heat exchangers transferring heat from the primary to the secondary fluid to produce the steam needed by the turbines. After several years of operation, because of deposit, their tube support plates (TSPs) can undergo clogging that may cause important economic and safety issues in case of nonpreventive actions. To understand and predict this phenomenon, several nondestructive examinations can generally be gathered at various times during the heat exchanger operation. A numerical mechanistic model has been recently developed and implemented in a dedicated computer code. The objective of this work is to improve the modeling of the clogging phenomenon to increase the predictive capability of the computer code. A global sensitivity analysis, based on Sobol’ indices, is first performed by the use of a metamodel that is learned on several runs of the computer code. Such an analysis, cast under a physical perspective, helps the identification of the most influential physical parameters and paves the way to a better understanding of TSP clogging. A Bayesian calibration of an epistemic calibration model parameter is then applied to fit the simulation results to experimental data. The additional information coming from the experimental data is then transferred to the calibration parameter with a mathematical model (artificial neural network). The resulting hybrid model thus compensates some lacks of the initial physical model on the considered data set.

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ). 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> Fe inside the circuit and follow the presence of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> Fe, especially on the TSP. For that purpose, three dedicated gamma-measuring stations are used to follow online the presence of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> 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.

Simulation of eddy current testing of steam generator tubes with wear and secondary side magnetite deposits

The steam generator (SG) tubes of pressurised water reactors are prone to degradation and are periodically inspected using eddy current testing (ECT) techniques. SGs provide a safety barrier between the radioactive primary side and the non-radioactive secondary side. Historically, a major degradation issue was stress corrosion cracking of tubes made of alloy 600. In modern SGs, better resistance to stress corrosion cracking is observed with the replacement of alloy 600 by alloy 690. However, this alloy does not prevent tubes from wear at support locations or wear resulting from fretting with loose parts. Long-term operation of SGs also leads to non-volatile species accumulation in the secondary side of the SGs, mainly magnetite. Magnetite deposits may build up as a dense and solid material on top of the tubesheet (hard sludge) and also in the flow slots of tube support plates, therefore modifying the general flow pattern and velocity of the secondary fluid. This has detrimental effects on the safe operation of SGs. The clogging-up ratio of the flow slots therefore needs to be assessed. The development of an ECT simulation model to address tube wear and magnetite deposits in SGs is presented. This model is based on the CIVA NDT platform. A bobbin coil signal is simulated and its performance for tube wear and magnetite deposit evaluation is discussed.

The Fouling Behavior of Steam Generator Tube at Different Positions in the High-Temperature Water

The fouling behavior of a steam generator (SG) tube was investigated at different positions after 500 h of immersion in high-temperature water. A triple-layer structure of fouling appeared at both the crevice position and the free span position, namely, the large, dispersedly distributed deposition layer on the top; the small and faceted outer layer; and the relatively continuous inner layer. There was no obvious positional effect on the thickness of the inner layer. However, in the crevice position, the density of the deposited particle and the thickness of the outer layer was much higher than those of the free span position. The tube support plate (TSP) made of 410 stainless steel contributed significantly to the fouling behavior of the SG tube in the crevice between the SG tube and the TSP.

Fusion Images of Versatile Array Sensors for Multiobject Detection

Regular inspections of steam generator tubes (SGTs) to evaluate structure degradation and sludge deposition are essential for maintaining safety and efficiency of power plants. However, with the interference of complex structures such as tube support plates (TSPs), simultaneously detecting multiple objects is still a challenging problem. This article proposes a novel integrated versatile probe for multiobject inspection. The probe consists of a low-frequency operating region (LFOR) with tunnel magnetoresistance (TMR) array sensors and a high-frequency operating region (HFOR) that uses array coils as the pickup sensors. After frequency optimization and image calibration, the signals of three types of objects are concentrated in three different modalities of the probe’s output. Then, features for position of TSP, profile, and location of sludges are extracted from the image of LFOR. A feature extraction algorithm based on convolution and pattern search is developed to extract defects’ features from the quadrature component image of HFOR. Finally, a hierarchical independent decision-level data fusion scheme is adopted to integrate and visualize the recognition and discrimination of multiple objects in an RGB image. Combinations of machined defects, sludges, and TSP mockup on an SGT are experimentally tested by the probe. Experimental results show that the TSP and sludges are detected accurately. Most of the defects, including the defect embedded under the TSP, can be identified and localized with a high intersection of union (IoU) of 0.92. The TSP, sludges, and defects are clearly visualized in the fused image. For complex scenarios with multiple objects superimposed, the fused image shows each type of objects clearly.

Magnetic Imaging of Sludge Outside Steam Generator Tube With Array Magnetoresistance Sensors

The water impurities and corrosion products of the nuclear power plant structure form sludge outside the steam generator tube (SGT), which decreases the energy exchange efficiency resulting in tremendous economic loss and may lead to serious defects posing a threat to the safe operation. Thus, it is critical to efficiently inspect the deposition of sludge outside SGT. This work reveals that the sludge sample of a nuclear power plant has little conductivity and relative permeability about 2.88. Based on the electromagnetic properties of the sludge, we proposed a novel probe with array tunnel magnetoresistance (TMR) sensors for inspection of magnetic field disturbed by the presence of sludge outside the SGT. The principle of the method and the parameters of the probe were studied using a 3D finite element method (FEM) model. It is seen that the signal amplitude of the offset sensor configuration is stronger than that of the probe with non-offset sensor setup and the offset design avoids the disadvantage of non-sensitive to symmetry sludge. Further, it is observed that the sludge inspection performance is better at a lower excitation frequency, i.e. 1 kHz. The TMR sensors provide superior image quality at the low excitation frequency. A prototype probe is manufactured and tested. Sludge samples prepared according to the realistic fouling scenarios in steam generator were tested by the probe. The experimental results validate the feasibility of the probe and demonstrate the advantage of enhancing signal to noise ratio (SNR) of imaging by suppressing the background signal of tube support plate structure under the low excitation frequency.

Measurement of pressure loss throughout a clogged steam generator tube support plate in two-phase flow

Two-phase flow head losses across tube support plates in U-bend tube steam generators are classically evaluated from single phase head losses duly modified with a correction factor to account for two-phase effects.This communication reports upon a series of experiments to feed head loss models for clogged tube support plates under two-phase flow. By using a classical approach, a single phase head loss model is used together with a two-phase flow multiplier. The experiments cover a large span of clogging levels, up to the unrealistic limit case of 95% of clogging.Singular head loss due to the tube support plate is measured under single-phase flow for clean and clogged plates. The head loss factor is hence evaluated and compared to the prediction of some available models. Then the same experiment is repeated under two-phase flow condition. By combining the previously measured single-phase factor with the two-phase flow measurements, recommendations for predicting two-phase flow multiplier with existing models are given.

Numerical simulation of the effects of trefoil tube support plates on the flow and heat transfer characteristics of a steam generator

A three-dimensional coupled model of heat transfer tube and trefoil tube support plate (TSP) is developed based on a full-size steam generator (SG). The CFX thermal phase change model is adopted to numerically investigate the flow and heat transfer behaviors of the secondary side fluid. And the effects of trefoil TSPs and inlet subcooling on the thermal-hydraulic characteristics of tube bundle are obtained. The calculation model reasonably reveals the distributions of the flow and heat transfer parameters in the SG, especially in trefoil TSP regions. Based on the across flow velocity and density of the secondary side fluid in U-bend region, the most severe flow-induced vibration damage is predicted at the angles of 110° on the hot leg and 60° on the cold leg, respectively. Due to the effects of trefoil TSPs, vortexes are induced, the outer wall temperature in the circumferential direction shows periodic characteristic in TSP regions, which increases the possibility of aggressive solutes deposition and U-tubes rupture. In addition, inlet subcooling of the secondary side fluid has significant effects on the SG. As inlet subcooling decreases, the heat transfer is enhanced. The numerical simulation results obtained may provide guidance to facilitate the optimal design and in-depth thermal-hydraulic analysis of TSPs and SGs.

Two-phase flow-induced instability and nonlinear dynamics of a rotated triangular tube array in parallel direction

A mathematical model for a rotated triangular tube array with 7 rigid-body degrees of freedom subject to two-phase flow and loose support was presented to study the two-phase flow-induced instability and nonlinear dynamics in the parallel direction. To verify the correctness of this model, an experiment was carried out to obtain the Hopf bifurcation velocities of fluidelastic instability of a rotated triangular tube array with the pitch-to-diameter ratio is 1.48 for five void fraction conditions. Then, considering the effects of the impact stiffness forces between the tube bundles and the tube support plates, the Hopf bifurcation and the nonlinear dynamics of a rotated triangular tube array within 0%, 20%, 40%, 60%, and 80% void fraction conditions were calculated. The numerical results show that when the flow pitch velocity increases beyond the Hopf bifurcation velocity, the periodic motion occurs. At a sufficiently high flow pitch velocity, some complex motions of the tube bundles such as the multi-frequency motion, the chaotic motion may occur. And, the effects of the void fraction on the bifurcation characteristic of the tube bundles were also investigated.

Numerical investigation on Hopf bifurcation and post-instability of tube bundles subjected to two-phase cross-flow and loose support

This paper investigated the Hopf bifurcation and post-instability of tube bundles subjected to two-phase cross-flow and loose support. A nonlinear dynamic model of a single tube in tube bundles considering the effects of two-phase flow force and impact force between the tube and tube support plate was established. Based on this model, a numerical simulation program was written to calculate the bifurcation diagrams, limit cycle motions, and quasi-periodic motions of the tube. The numerical results showed that a supercritical Hopf bifurcation of a single flexible tube in a tube array at the original stable equilibrium occurs at the Hopf bifurcation velocity within void fraction ranging from 0% to 80%. And, the vibrations of the tube for the high void fraction condition are much more complex than those for the low void fraction which could be the reason for the early damage of the tube bundles in a steam generator.

Experimental study on cross flow induced vibration response of four-span straight tube bundle with TSP or AVB supports

Flow induced vibrations in steam generator and other heat exchangers can result in tube failure and lead to large economic losses. Tube supports such as tube support plates (TSP) or anti-vibration bars (AVB) are commonly used to suppress the flow induced vibration. Knowledge on the tube dynamic characteristics and instability phenomena under these support conditions is therefore essential, when fluid flows across the multi-span tubes. In this paper, flow induced vibration of a four-span rotated triangular tube array with a pitch-to-diameter ratio of 1.4 under three sets of AVB or TSP supports is experimentally studied. Part of the middle segment of the tube is subjected to cross flow. The vibration displacements and frequency spectra of a tube on first row are analyzed. It was found that the vibration mode for four-span tube bundle with TSP is different from that of the tube supported by AVB when fluid elastic instability (FEI) happens. The effective FEI critical velocities for the tube with TSP and that with AVB is 6.8 m/s and 3.0 m/s, respectively. Five correlations for predicting the onset of FEI of the tube bundle tested are compared with experimental results. Weaver and El-Kashlan correlation (1981b) is best. The effective FEI critical velocities predicted by Weaver and El-Kashlan correlation (1981b) are 15% and 27% higher than the experimental results for tube with TSP and tube with AVB, respectively. The vortex shedding responses are analyzed for the two tube-support structures. The Strouhal number for non-resonant vortex shedding response is calculated and compared with expectations from literature. The turbulent buffeting response for tube under different supports are compared with each other and discussed.

Design of Automatic TV Inspection Equipment for CPR1000 Steam Generator 9th Tube Support Plate

With steam generator running more and more years, sludge will accumulate on the surface of 9th tube support plate, causing broached holes clogging and heat transfer efficiency decreasing. In order to know the situation of sludge deposit and provide evidence of judging whether soft chemical cleaning is necessary, the 9th tube support plate automatic TV inspection equipment is required and needs to be developed based on the upper structure of CPR1000 steam generator. In this paper, the application surroundings and requirements were analyzed. The inspection method, general design, magnetic crawler, probe characteristics and mockup tests were described, especially how to insert the probe into bundles to achieve inspection scope was introduced.

Modeling the influence of tube support plates on flow boiling and dryout of once-through steam generators

Considering the tube support plates installed among the tube bundle of actual once-through steam generator, the mathematical model suitable for smooth channel was modified to numerically simulate the two-phase flow and heat transfer and dryout under three structures (no tube support plates, tube support plates with and without gaps). The results show that the slip ratio increases rapidly when fluid flows through the tube support plates, and then decreases rapidly under the strong disturbance caused by the turbulence transition at the downstream position of tube support plates. The maximum slip ratio for the case of TSPs with gaps is lower than that for the other two cases, which is conducive to prevent interphase slipping. The turbulence transition and great steam quality caused by heat transfer enhancement at the downstream position of tube support plates both have positive effects on tearing the annular liquid film, resulting in the dryout position being at the upstream position of that for the case of no tube support plates. The damage of dryout deterioration can be relieved significantly when considering tube support plates, and the corresponding rising amplitude of wall temperature at dryout position reduces from 300 K for the case of no tube support plates to 200 K. With the development of flow and heat transfer, the non-uniform levels of circumferential wall temperature for the cases of tube support plates with and without gaps show completely opposite trends.

Non-planar responses of cantilevered pipes conveying fluid with intermediate motion constraints

In this paper, the nonlinear responses of a loosely constrained cantilevered pipe conveying fluid in the context of three-dimensional (3-D) dynamics are investigated. The pipe is allowed to oscillate in two perpendicular principal planes, and hence its 3-D motions are possible. Two types of motion constraints are considered. One type of constraints is the tube support plate (TSP) which comprises a plate with drilled holes for the pipe to pass through. A second type of constraints consists of two parallel bars (TPBs). The restraining force between the pipe and motion constraints is modeled by a smoothened-trilinear spring. In the theoretical analysis, the 3-D version of nonlinear equations is discretized via Galerkin’s method, and the resulting set of equations is solved using a fourth-order Runge–Kutta integration algorithm. The dynamical behaviors of the pipe system for varying flow velocities are presented in the form of bifurcation diagrams, time traces, power spectra diagrams and phase plots. Results show that both types of motion constraints have a significant influence on the dynamic responses of the cantilevered pipe. Compared to previous work dealing with the loosely constrained pipe with motions restricted to a plane, both planar and non-planar oscillations are explored in this 3-D version of pipe system. With increasing flow velocity, it is shown that both periodic and quasi-periodic motions can occur in the system of a cantilever with TPBs constraints. For a cantilevered pipe with TSP constraints, periodic, chaotic, quasi-periodic and sticking behaviors are detected. Of particular interest of this work is that quasi-periodic motions may be induced in the pipe system with either TPBs or TSP constraints, which have not been observed in the 2-D version of the same system. The results obtained in this work highlight the importance of consideration of the non-planar oscillations in cantilevered pipes subjected to loose constraints.

Optical Fiber Demodulation System with High Performance for Assessing Fretting Damage of Steam Generator Tubes.

In order to access the fretting damage of the steam generator tube (SGT), a fast fiber Fabry-Perot (F-P) non-scanning correlation demodulation system based on a super luminescent light emitting diode (SLED) was performed. By demodulating the light signal coming out from the F-P force sensor, the radial collision force between the SGT and the tube support plate (TSP) was interrogated. For higher demodulation accuracy, the effects of the center wavelength, bandwidth, and spectrum noise of SLED were discussed in detail. Specially, a piezoelectric ceramic transducer (PZT) modulation method was developed to get rid of the interference of mode coupling induced by different types of fiber optics in the demodulation system. The reflectivity of optical wedge and F-P sensor was optimized. Finally, the demodulation system worked well in a 1:1 steam generator test loop and successfully demodulated a force signal of 32 N with a collision time of 2 ms.

Fiber Fabry-Perot Force Sensor with Small Volume and High Performance for Assessing Fretting Damage of Steam Generator Tubes.

Measuring the radial collision force between the steam generator tube (SGT) and the tube support plate (TSP) is essential to assess the fretting damage of the SGT. In order to measure the radial collision force, a novel miniaturized force sensor based on fiber Fabry-Perot (F-P) was designed, and the principle and characteristics of the sensor were analyzed in detail. Then, the F-P force sensor was successfully fabricated and calibrated, and the overall dimensions of the encapsulated fiber F-P sensor were 17 mm × 5 mm × 3 mm (L × W × H). The sensor works well in humid, high pressure (10 MPa), high temperature (350 °C), and vibration (40 kHz) environments. Finally, the F-P force sensors were installed in a 1:1 steam generator test loop, and the radial collision force signals between the SGT and the TSP were obtained. The experiments indicated that the F-P sensor with small volume and high performance could help in assessing the fretting damage of the steam generator tubes.

CFD evaluation on the thermohydraulic characteristics of tube support plates in steam generator

Abstract The integrity and thermal hydraulic characteristics of steam generator are of great concern in the nuclear industry. The tube support plates (TSP), one of the most important components of the steam generator, not only support the heat transfer tubes, but also affect the flow dynamic and thermal hydraulic characteristics of the secondary-side flow inside the steam generator. Different working conditions, ranging from single-phase adiabatic condition to two-phase high-void boiling condition, are simulated and analyzed. Calculated void fraction, under simple geometry, agrees well with the experiment data whilst the simulated heat transfer coefficient is tremendously close to the empirical correlation. Temperature, void fraction, and velocity distributions in different locations show reasonable distribution. The simulation results indicate that TSP can enhance the heat transfer in the secondary side of the steam generator. On the top of TSP, with the increase in cross-section flow area, the back-flow phenomenon occurs, which might lead to the contamination of precipitation.

Modeling the thermal stress of heat transfer tubes with tube support plate gaps in a steam generator

This work presents a three-dimensional “unit pipe” model with and without gaps between tubes and associated tube support plates in a steam generator. The model is used to simulate the thermal–hydraulic characteristics of the steam generator and obtain the distributions of key parameters of the heat transfer tubes, the primary and secondary sides. Using this model, we pass fluid calculation results to the structure through a flow-heat-solid coupling in workbench, conduct thermal stress calculations of the heat transfer tubes, and analyze the influence of gaps on the system’s thermal–hydraulic characteristics and resulting thermal stress. The results indicate that there is a rapid change in the secondary side’s velocity and heat transfer coefficient at the tube support plates, and that the steam quality and fluid velocity in the gaps is higher and lower, respectively, than in the flood holes. Moreover, the resulting thermal stress experienced by the heat transfer tubes shows a periodic fluctuation around their circumference at the tube support plates, while the thermal stress without gaps at the tube support plates is greater than the thermal stress seen with gaps, and its circumferential volatility is approximately twice the volatility with gaps present.