Thermal Fatigue Assessment Research Articles

Thermal fatigue analysis of structures subjected to liquid metal jets at different temperatures in the Gen-IV nuclear energy system

In the Generation-IV (Gen-IV) nuclear reactor system, the liquid metal cooled fast reactor is regarded as a promising reactor type, especially the sodium-cooled fast reactor (SFR). It is necessary to investigate the thermal striping of liquid metal jets and the thermal fatigue behavior of adjacent structures at the core outlet of a liquid metal cooled fast reactor where coolant at different temperatures is mixed and causes thermal fatigue of adjacent structures. In this work, a fluid-structure coupling model and thermal fatigue assessment methodology are proposed and employed to study the thermal fatigue of structures subjected to liquid mental jets at the core outlet of the SFR. The temperature field and velocity distribution of the fluid, as well as the deformation and thermal stress of structures are obtained. The transient load and thermal fatigue damage of the structure at different locations are also calculated and analyzed. The fatigue damage factor of the structure is less than 1.0. The max deformation is predicted and a relatively large von Mises stress is located in the junction of central column and control rod guide tube, as well as the area of geometric structure mutation.

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Investigation of cracking behaviors in divertor armor-blocks under cyclic loading

Optimal design of materials and configurations is necessary to assure intended functions of divertor module against nuclear fusion plasma. Relevant activities performed not only diverse numerical analyses of armor-blocks and structures but also material property tests of specimens and limited experiments of mock-ups under harsh environments. As recent studies reported remarkable cracks in the tungsten-armored multiple-block subjected to cyclic loading, this research is intended to investigate cracking behaviors and to establish a promising numerical approach for engineering application. First of all, three sets of traction-separation law(TSL) parameters for a tungsten material were assumed based on reference data under a severe heat flux of 20 MW/m2. Extended finite element analyses were carried out mainly for the mock-up that consists of five blocks to determine a proper set of TSL parameters by comparing with experimental observation. Subsequently, parametric analyses were conducted to examine further realistic situations by changing the number of blocks and joining restraints. As results of analyses, crack propagation directions and amounts as well as initiation locations were predicted in the simulated blocks. Different cracking behaviors were also evaluated in relation to the number of applied cycles followed by a discussion of key findings from the thermal fatigue and failure assessment standpoint.

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Thermal Fatigue Assessment for Penetration Flow into a Branch Pipe of Mixing Tee

Heat transfer coefficient suitable for thermal fatigue assessment at a T-junction

Thermal fatigue cracks have been found at T-junctions where high and low temperature fluids flow in. In this study, the heat transfer coefficient at a T-junction was investigated. The obtained heat transfer coefficient was then validated by estimating the temperatures on a pipe inner surface. The temperatures of the fluid and the pipe inner surface were measured by mock-up tests in which high and low temperature water was mixed. The fluid temperature fluctuation near the boundary between the hot jet flow and the cold main flow caused the temperature on the pipe inner surface to fluctuate. The heat transfer coefficient was obtained by the power spectrum method for the pipe inner surface where the temperature fluctuation was relatively large. The estimated heat transfer coefficient was 3.3 times larger than that calculated by the Dittus-Boelter equation. The temperature on the pipe inner surface was successfully estimated using the obtained heat transfer coefficient and the measured fluid temperature in the vicinity of the inner surface. The prediction error was less than ±20% for the temperature fluctuation range.

Stress and Fatigue Analysis of Pressurizer Surge Line under Thermal Stratification

Pressurizer surge line is one of the key equipments of nuclear power plants. The thermal stratification due to the intersection of hot and cold fluids inside the pressurizer surge line may affect the safe operation of nuclear power plant. In order to investigate the stress distribution and fatigue characteristics of surge line subjected to long-term thermal stratified loadings, a mechanical model of the surge line was established. And then, according to different temperature distribution assumptions, thermal stress analysis and fatigue assessment were conducted. The results show that the maximum stress appears under the load condition with maximum temperature difference, and finer temperature distribution can obtain more accurate stress and displacement results. The maximum value of fatigue cumulative coefficient appears at the junction of straight pipe and elbow with large temperature difference.

Assessment of Thermal Fatigue Predictions of Pipes With Spectral Methods

Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment procedure are employed in this paper to deliver more accurate statistics of predicted lives of pipes and their uncertainties under turbulent fluid mixing circumstances. The wide variety of synthetic fluid temperatures, generated with an improved spectral method, results in a set of estimated distributions of fatigue lives through linear one-dimensional (1D) heat diffusion, thermal stress estimates, and fatigue assessment codified rules. The results of the fatigue analysis indicate that, in order to avoid the inherent uncertainties due to comparatively short fluid temperature histories to the estimated fatigue lives, a conservative safe design against thermal fatigue could be attempted with the lower bounds of the predicted life distributions, such as the 5% probability life (5% of samples fail). The impact of the convection heat transfer coefficient on the predictions is also studied in a sensitivity analysis. This represents a detailed attempt to correlate the uncertainties in the physical fluid mixing conditions and heat transfer to the estimated fatigue life using spectral methods.

Study of the quantitative assessment method for high-cycle thermal fatigue of a T-pipe under turbulent fluid mixing based on the coupled CFD-FEM method and the rainflow counting method

With the development of nuclear power and nuclear power safety, high-cycle thermal fatigue of the pipe structures induced by the flow and heat transfer of the fluid in pipes have aroused more and more attentions. Turbulent mixing of hot and cold flows in a T-pipe is a well-recognized source of thermal fatigue in piping system, and thermal fatigue is a significant long-term degradation mechanism. It is not an easy work to evaluate thermal fatigue of a T-pipe under turbulent flow mixing because of the thermal loads acting at fluid–structure interface of the pipe are so complex and changeful. In this paper, a one-way Computational Fluid Dynamics-Finite Element Method (CFD-FEM method) coupling based on the ANSYS Workbench 15.0 software has been developed to calculate transient thermal stresses with the temperature fields of turbulent flow mixing, and thermal fatigue assessment has been carried out with this obtained fluctuating thermal stresses by programming in the software platform of Matlab based on the rainflow counting method. In the thermal analysis, the normalized mean temperatures and the normalized root mean square (RMS) temperatures are obtained and compared with the experiment of the test case from the Vattenfall benchmark facility to verify the accuracy of the CFD calculation and to determine the position which thermal fatigue is most likely to occur in the T-junction. Besides, more insights have been obtained in the coupled CFD-FEM analysis and the thermal fatigue damage assessment, and the normalized thermal fatigue damage rate D∗τ is found to be positively related to the normalized RMS temperature TRMS/ΔT to some extent. The method proposed in this paper is the inheritance and development of the research studying thermal fatigue of the T-junction under turbulent fluid mixing of hot and cold fluid by analysing the temperature fluctuations, and may provide a promising way for the quantitative assessment of high-cycle thermal fatigue of the pipe structure induced by complex flow and heat transfer in practical engineering of the nuclear power plants (NPPs).

Numerical simulation of long-period fluid temperature fluctuation at a mixing tee for the thermal fatigue problem

Thermal fatigue cracks may be initiated at mixing tees where high and low temperature fluids flow in and mix. According to a previous study, damage by thermal fatigue depends on the frequency of the fluid temperature fluctuation near the wall surface. Structures have the time constant of structural response that depends on physical properties of the structure and the gain of the frequency response tends to become maximum at the frequency lower than the typical frequency of fluid temperature fluctuation. Hence the effect of the lower frequency, that is, long-period temperature fluctuation is important for the thermal fatigue assessment. The typical frequency of fluid temperature fluctuation is about St=0.2 (nearly 6Hz), where St is Strouhal number and means non-dimensional frequency. In the experimental study by Miyoshi et al. (2014), a longer-period fluctuation than St=0.2 was also observed. Results of a fluid–structure coupled analysis by Kamaya et al. (2011) showed this long-period temperature fluctuation causes severer damage to piping. In the present study, a large eddy simulation was carried out to investigate the predictive performance of the long-period fluid temperature fluctuation more quantitatively. Numerical simulation was conducted for the WATLON experiment which was the water experiment of a mixing tee performed at the Japan Atomic Energy Agency. Four computational grids were used to confirm grid convergence. In the short time (9s) simulations, tendencies of time-averaged and fluctuated velocities could be followed. Time-averaged temperature distributions were also reproduced, although overestimation appeared near the wall. The fluid temperature fluctuation intensity near the wall surface could be predicted qualitatively, while the peak value was overestimated. From the engineering viewpoint, it was concluded that the numerical simulation provided results that were conservative and on the side of safety. From the grid convergence study, the coarsest grid for which grid convergence was almost attained was selected and the simulation was continued until 100s. Frequency analysis of the fluid temperature fluctuation showed that the long-period fluctuation appeared as well as the well-known typical frequency (St=0.2). This indicated that numerical simulation could reproduce the long-period temperature fluctuation at the mixing tee.

Uncertainties in the thermal fatigue assessment of pipes under turbulent fluid mixing using an improved spectral loading approach

This paper proposes improved thermal fatigue assessment of pipes subjected to turbulent fluid mixing using an improved spectral loading approach. The fluid temperature histories are generated synthetically from the spatially incomplete experimental or very expensive computational data, preserving consistency with the first two statistical moments and power spectral densities of the measured/computed temperatures. This enables a variety of affordable and physically adequate fluid temperature distributions. These have been used in a novel and rather straightforward analysis of the uncertainties involved in the calculated fatigue life times.The proposed thermal fatigue assessment procedure has been fully developed for the equi-biaxial stress fields on the pipe surface. This rather realistic assumption allows for a simple one-dimensional model of the pipe with numerically resolved time-dependent temperatures and analytical expressions for the linear elastic wall thermal stresses varying only in the radial direction. The fatigue lives are predicted for diverse variations in fluid temperatures following the ASME Nuclear Boiler and Pressure Vessels codified rules for varying principal stress direction, Rainflow counting algorithm, linear damage accumulation and the NUREG/CR-6909 design fatigue curve.The results of the proposed method are less conservative than results of similar methods in the literature. This is expected, since the proposed method accounts for a much wider variability and more physical description of the fluid temperatures. Further, the reduction in the conservatism is effectively compensated through the ability to assess the uncertainties inherent in the calculations of fatigue life time. The proposed assessment could be straightforwardly extended to other fatigue design rules and, with the use of finite elements, to possibly nonlinear behavior and discontinuous shapes of the pipes. In this way the proposed improved thermal fatigue assessment of pipes could facilitate further development of currently very scarce and simple design rules for thermal fatigue.

Stress assessment in piping under synthetic thermal loads emulating turbulent fluid mixing

Thermal fatigue assessment of pipes due to turbulent fluid mixing in T-junctions is a rather difficult task because of the existing uncertainties and variability of induced thermal stresses. In these cases, thermal stresses arise on three-dimensional pipe structures due to complex thermal loads, known as thermal striping, acting at the fluid-wall interface. A recently developed approach for the generation of space-continuous and time-dependent temperature fields has been employed in this paper to reproduce fluid temperature fields of a case study from the literature. The paper aims to deliver a detailed study of the three-dimensional structural response of piping under the complex thermal loads arising in fluid mixing in T-junctions.Results of three-dimensional thermo-mechanical analyses show that fluctuations of surface temperatures and stresses are highly linearly correlated. Also, surface stress fluctuations, in axial and hoop directions, are almost equi-biaxial. These findings, representative on cross sections away from system boundaries, are moreover supported by the sensitivity analysis of Fourier and Biot numbers and by the comparison with standard one-dimensional analyses. Agreement between one- and three-dimensional results is found for a wide range of studied parameters. The study also comprises the effects of global thermo-mechanical loading on the surface stress state. Implemented mechanical boundary conditions develop more realistic overall system deformation and promote non-equibiaxial stresses.

The role of the axial heat fluxes in the thermal fatigue assessment of piping

Thermal fatigue is a structural damage of materials induced by the cyclic thermal loads that are frequently generated by the changes of fluid temperature inside of pipes. Among the thermal fatigue assessment methods we find the one-dimensional (1D) approach. Thermal, mechanical and fatigue analyses are performed for the pipe wall assuming that the distribution of temperatures only varies along the wall thickness. On the other hand, pipe regions with higher stress oscillations are those where the fluid temperature changes spatially, meaning cold or hot spots near the pipe surface, and with low frequencies. Spatial fluid temperature differences generate heat fluxes within the pipe wall which can’t be reproduced with 1D methods. For this reason, the present work focuses on understanding the wall temperature distributions for different values of heat fluxes and frequencies of fluid temperature. Due to the implication in wall temperature measurements, the heat fluxes and frequencies effects on temperature readings of wall thermocouples are also investigated.In this paper, the influence of axial heat flux in a pipe wall is studied. The temperature distribution within the pipe wall is analyzed considering a fluid temperature signal in the proximity of the pipe surface with axial temperature dependence. The effect of the temperature fluctuations frequency is also investigated. The two-dimensional finite difference equations for the transient temperature of a pipe wall, as well as one-dimensional equations, have been derived in cylindrical coordinates. The analyses contemplate different values of the heat transfer coefficient between the fluid and wall. The results show that temperature distributions from 1D analyses are more conservative for the calculation of thermal stresses than considering axial heat fluxes. However, surface temperature amplitudes obtained from thermocouples readings may be underestimated if the spatial heat fluxes in the fluid near the pipe surface are not taken into account.

ASSESSMENT OF THERMAL FATIGUE IN MIXING TEE BY FSI ANALYSIS

Thermal fatigue is a significant long-term degradation mechanism in nuclear power plants. In particular, as operating plants become older and life time extension activities are initiated, operators and regulators need screening criteria to exclude risks of thermal fatigue and methods to determine significant fatigue relevance. In general, the common thermal fatigue issues are well understood and controlled by plant instrumentation at fatigue susceptible locations. However, incidents indicate that certain piping system Tee connections are susceptible to turbulent temperature mixing effects that cannot be adequately monitored by common thermocouple instrumentations. Therefore, in this study thermal fatigue evaluation of piping system Tee-connections is performed using the fluid-structure interaction (FSI) analysis. From the thermal hydraulic analysis, the temperature distributions are determined and their results are applied to the structural model of the piping system to determine the thermal stress. Using the rain-flow method the fatigue analysis is performed to generate fatigue usage factors. The procedure for improved load thermal fatigue assessment using FSI analysis shown in this study will supply valuable information for establishing a methodology on thermal fatigue.

European THERFAT project—thermal fatigue evaluation of piping system “Tee”-connections

Thermal fatigue is a significant long-term degradation mechanism in nuclear power plants (NPP). In particular, as operating plants become older and life time extension activities are initiated. Operators and regulators need screening criteria to exclude risks of thermal fatigue and methods to determine significant fatigue relevance. In general, the common thermal fatigue issues are well understood and controlled by plant instrumentation at fatigue susceptible locations. However, incidents indicate that certain piping system Tee's are susceptible to turbulent temperature mixing effects that cannot be adequately monitored by common thermocouple instrumentations. Therefore, a European project on thermal fatigue evaluation of piping system Tee-connections THERFAT has been initiated. In THERFAT, a collation of existing field experience has been conducted leading to a selection of Tee-connections for further investigations. The load determination covers experimental tests and advanced thermo-hydraulic flow simulations. The integrity evaluation work package comprises stress/fatigue analyses and fracture mechanics assessments supported by targeted verification damage tests. Proposals will be made for improved load thermal fatigue assessment procedures, screening criteria to determine lower non fatigue significant threshold values and for establishing a European methodology on thermal fatigue. THERFAT participants No. Name Organisation Country 1 K.-J. Metzner E.ON Hannover, GER 2 C. Faidy EDF Villeurbanne, F 3 J.-A. Le Duff Framatome-ANP Paris la Défense, F 4 O. Braillard CEA St Paul Lez Durance, F 5 C. Cueto-Felgueroso Tecnatom S.A. Madrid, E 6 I. Varfolomeyev Fraunhofer-Gesellschaft Freiburg, GER 7 J. Solin VTT Espoo, SF 8 M. Schippers Framatome ANP Offenbach, GER 9 L. Stumpfrock MPA Stuttgart, GER 10 K.-F. Nilsson EC-JRC/IAM Petten, NL 11 Y. Hytoenen Fortum Nuclear Services Vantaa, SF 12 J. Seichter Siempelkamp Dresden, GER 13 T. Abbas Cinar London, GB 14 S. Figedy VUJE Trnava, SLK 15 P. Carmena Endesa Generacion Madrid, E 16 L. Cizelj Jozef Stefan Institute Ljubljana, SLO

Thermal fatigue assessment of components made with particulate polymer composites

Many appliance materials are made of PMMA/Si acrylic casting dispersion. In these situations, failure can occur by thermal fatigue induced by severe temperature variations such as alternating flows of cold and hot water. This paper is concerned with the numerical analysis of the thermal stresses in three composites with different volume fractions of filler and particle size. Their trade marks are Asterite, Amatis and Ultra-quartz. Cosmos/M finite element method software was used to study the influence of the cold and hot water temperatures as well as the time of interruption of water flow in the transition between hot and cold water on thermal stresses. Residual stresses were measured and superimposed to thermal stress in fatigue analysis. Typical defects in the corner of holes produced by drilling were predicted using experimental fatigue lives and d a/d N curves. Based on predicted defects thermal fatigue assessment of commercially available sinks made with the three materials mentioned earlier was done by taking into account the influence of both cyclic thermal and static residual stresses induced by the manufacturing process.

Fatigue monitoring and life cycle analysis of a recuperative heat exchanger in the primary circuit

The example of a recuperative heat exchanger is used to demonstrate the temperature monitoring and the thermal fatigue assessment of a component in the primary circuit of a PWR. After describing the geometry and the operating conditions, the placement of the thermocouples at locations of interest is shown. The temperature recordings show that the existing operational instrumentation was very slow and the fluid transient were actually much faster than anticipated. With a computer program allowing the reduction from outside wall to fluid temperature, the thermal inertia of the old instrumentation could be quantified. Consequently, old recordings from slow instrumentation could still be used. Thermal finite element computations are presented that yield quite favourable correlations between computations and measurements.